Patent Publication Number: US-2012045329-A1

Title: Method for circulation controlled vertical axis and turbines

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/151,367 filed Feb. 10, 2009, entitled “Circulation and Boundary Layer Control Augmented Wind Turbine”. 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/151,341 filed Feb. 10, 2009, entitled “Circulation Control Augmented Wind Turbine”. 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/151,417 filed Feb. 10, 2009, entitled “Control System for a CC-VAWT”. 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/151,391 filed Feb. 10, 2009, entitled “Use of a Constant Blowing Rate Required for the Circulation Control Augmented Vertical Axis Wind Turbine”. 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/159,712 filed Mar. 12, 2009, entitled “Joint Assembly for Fluid Delivery”. 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/159,713 filed Mar. 12, 2009, entitled “Shape Memory Actuators For Air Flow Controllers”. 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/159,714 filed Mar. 12, 2009, entitled “Valve System for Air Flow Control in Airfoils”. 
     The present application claims the benefit of U.S. Patent Application Ser. No. App. No. 61/159,715 filed Mar. 12, 2009, entitled “Drag Reducing Coanda Jets for Airfoils”. 
    
    
     FIELD 
     Embodiments of the subject matter described herein relate generally to a system and method for using circulation control to control the aerodynamic characteristics of airfoils in vertical axis wind turbines. 
     BACKGROUND 
     Wind turbines are a source of renewable and clean energy that can be divided into two major classifications, horizontal and vertical axis. Horizontal Axis Wind Turbines (HAWTs) are similar to propellers except they are driven by the wind. HAWTs are typically located at heights approaching several hundred feet in the air. The majority of maintenance for HAWTs must be performed at these heights, making repairs and maintenance difficult. HAWTs also require being pointed in the direction of the wind for effective operation. Vertical Axis Wind Turbines (VAWTs) have an advantage over horizontal turbines since the most maintenance intensive components (generator, transmission, etc.) are located at the bottom of the turbine shaft nearer to the ground. 
     There are currently two significant design theories implemented in the design of both HAWTs and VAWTs to handle the fatigue and vibration issues associated with the fluctuating loads generated by varying wind conditions, especially wind gusts. The most commonly implemented design theory is a rigid design in which solid connections are made between components to counteract the fluctuating loads. These rigid connections result in localized stress concentrations which require heavier designs at the attachment points to prevent fatigue failure. The second design theory is that of a dynamically soft system in which the connection points are allowed to move via pinned or sliding connections which are then damped to prevent the system from vibrating at its natural frequencies. The use of moveable connections reduces the stress concentrations associated with rigid connections and enables a lighter wind turbine to be constructed with a longer fatigue life. 
     VAWTs do not have to orient in the direction of the relative wind for effective operation. However, a VAWT must adapt to changing and unsteady wind conditions to maximize energy production. Varying the blade pitch for VAWT is one method of controlling aerodynamic forces to compensate for unsteady wind and to maximize the efficiency for generating power. Unlike HAWTs, VAWTs dynamically change the blade pitch for each blade during each rotation to achieve optimum performance. The pitch change, needed during operations at for tip speed ratios (TSRs) λ&lt;5, can approach extremes that are difficult to achieve mechanically. VAWT&#39;s are also not as popular today as HAWTs due to the perceived performance limitations created by the blade moving into the wind during a portion of its rotational path. 
     SUMMARY 
     Presented is a system and method of using circulation control in Vertical Axis Wind Turbines, or VAWTs. Circulation control is used instead of, or in addition to, physically changing blade pitch to control the lift-drag characteristics of the blades of a VAWT. The introduction of circulation control to the turbine blade alters the performance, particularly at low tip speed ratios (λ&lt;5) by maximizing the blades interaction with the wind in favorable locations while minimizing the wind interaction in detrimental locations along the blades&#39; path. Circulation control also improves wind turbine power generation performance over a wide operating range of TSRs, or Tip Speed Ratios. Circulation control is further capable of reducing blade and structure stresses of VAWTs. 
     A Circulation Controlled VAWT, or CC-VAWT, comprises a controller to adjust blowing slots on the airfoil blades. Multiple span-wise independently controlled blowing slots, or Coanda jets, are positioned near the trailing edge of the airfoil for circulation control, and are activated individually or in concert together to modify the lifting force and/or drag characteristics of the airfoil. In some embodiments, suction ports for boundary layer control are positioned near the leading edge of the airfoil. In some embodiments the suctions ports and blowing slots act in concert to achieve the desired local aerodynamic conditions for the turbine. In some embodiments the air flow between the suction ports and blowing slots is accelerated means located within the airfoil itself. The use of various levels of blowing and suction and combinations thereof from suction ports and blowing slots disposed on the surface of the airfoil is generally called circulation control. Modulating the aerodynamic characteristics of the individual blades of the VAWT using circulation control thus results in Circulation Controlled VAWT, or CC-VAWT. The CC-VAWT uses circulation control to adjust the aerodynamic performance of each turbine blade, thus allowing the CC-VAWT to be controlled to maximize power generation over a wide range of wind speeds and environmental conditions, reduce dynamic loads during high wind conditions, and manage unsteady wind conditions. 
     In one exemplary method, at low tip speeds when higher ranges in angle of attack are experienced, the boundary layer suction ports delay the onset of stall, increasing the lift coefficient. In normal wind conditions, blowing slots maintain constant rotation speeds allowing the CC-VAWT to generate power at a desired frequency, such as the same frequency as an existing AC power grid. In another method, use of circulation control also enables the controller to aerodynamically brake the wind turbine, by reducing the amount of energy extracted from the wind at high tip speed ratios (λ&gt;6), allowing for safe operation of the CC-VAWT. In another method, a constant blowing rate methodology can be implemented to simplify design decisions, facilitating implementation of CC-VAWTs in multiple locations each having different environmental conditions. The constant blowing rate can be varied from turbine to turbine resulting in a wide range of blowing coefficients as the wind speed and tip speed ratio are varied. Span-wise variation of the circulation control blowing slots enables the ability to use a constant blowing rate to limit the performance of the system, while managing the stresses in the turbine blades and their attachment points. 
     Valve systems located within the airfoils of the CC-VAWT that are in close proximity to the blowing slots of the trailing edge provide a means for rapid and controllable actuation of the valve system via a solenoid or other actuator. Actuators using shape memory materials have desirable weight-to-force characteristics, fast reaction times, and are capable of exerting sufficient force over a range of motion suitable for opening and closing blowing slots. 
     External air sources are hydraulically or pneumatically connected via conduits in the support structure and connection points. Connection points with integrated ports provide conduits for supplying air directly through the support arms and into the airfoils of a CC-VAWT. CC-VAWT that utilize the dynamically soft design methodology require flexible connections between structural elements and the connected airfoils. Connection points with integrated ports allow air to be supplied to the airfoils directly through the connection points without having to use external bypass hoses. 
     The circulation control system of the CC-VAWT expands the operational wind speed range of VAWTs, increasing the areas upon which wind turbines can be utilized and the percentage of time they are operating. The present invention is described in terms of wind turbines for convenience purpose only. It would be readily apparent to apply this technology to a similar device that operates in any fluid, such as hydro-electric power plants, aircraft and rotorcraft blades, or other aerodynamic or hydrodynamic surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures depict various embodiments of the system and method for using circulation control to control the aerodynamic characteristics of airfoils in vertical axis wind turbines. A brief description of each figure is provided below. Elements with the same reference number in each figure indicated identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number indicate the drawing in which the reference number first appears. 
         FIG. 1   a  is an illustration of a Vertical Axis Wind Turbine; 
         FIG. 1   b  is an illustration of multiple span-wise blowing slots in one embodiment of the circulation control system and method; 
         FIG. 2  is an illustration of a speed (ω) &amp; torque (τ) simplified CC-VAWT controller in one embodiment of the circulation control system and method; 
         FIG. 3  is an block diagram of advanced CC-VAWT controller in one embodiment of the circulation control system and method; 
         FIG. 4  is an illustration of the calculated performance of a CC-VAWT in one embodiment of the circulation control system and method; 
         FIG. 5  is an illustration of the relative velocity and angle of attack additional control capabilities in one embodiment of the circulation control system and method; 
         FIG. 6   a  is an illustration of a 2 zone blowing partition in one embodiment of the circulation control system and method; 
         FIG. 6   b  is an illustration of a 3 zone blowing partition in one embodiment of the circulation control system and method; 
         FIG. 6   c  is an illustration of a 4 zone blowing partition in one embodiment of the circulation control system and method; 
         FIG. 6   d  is an illustration of a 8 zone blowing partition in one embodiment of the circulation control system and method; 
         FIG. 7  is an illustration of the predicted performance of a partitioned CC-VAWT in one embodiment of the circulation control system and method; 
         FIG. 8  is an illustration of a momentum model predictions at solidity σ of 0.05, for the three levels of circulation control augmentation at a Reynolds number of 360,000 in one embodiment of the circulation control system and method; 
         FIG. 9  is an illustration of vortex model predictions at solidity σ of 0.05, for the three levels of circulation control augmentation at a Reynolds number of 360,000 in one embodiment of the circulation control system and method; 
         FIG. 10  is an illustration of a simulated coefficient of performance using a NACA0012 airfoil at Reynolds number of 300,000, for various solidities σ in one embodiment of the circulation control system and method; 
         FIG. 11  is an illustration of Schematic of an 18% Thick Elliptical Airfoil Incorporating Boundary Layer Suction and Circulation Control Blowing on its Upper Surface in one embodiment of the circulation control system and method; 
         FIG. 12  is an illustration of Cross-Sectional Profile of Upper and Lower, Boundary Layer Suction and Circulation Control Blowing Airfoil in one embodiment of the circulation control system and method; 
         FIG. 13  is an illustration of Schematic of the Piston-Type Flow Actuator in one embodiment of the circulation control system and method; 
         FIG. 14  is an illustration of Schematic of the Two Piston-Type Flow Actuator in one embodiment of the circulation control system and method; 
         FIG. 15  is an illustration of Illustration of the Support Arm Piston Air Supply Configuration for a Vertical Axis Wind Turbine in one embodiment of the circulation control system and method; 
         FIG. 16   a  is an illustration of airfoil and one Coanda jet in one embodiment of the circulation control system and method; 
         FIG. 16   b  is an illustration of airfoil and two equal strength Coanda jets producing a Kutta condition in one embodiment of the circulation control system and method; 
         FIG. 16   c  is an illustration of airfoil with two unequal strength Coanda jets creating a variable lift-drag condition in one embodiment of the circulation control system and method; 
         FIG. 17  is an illustration of valve system and actuators positioned within the airfoil in one embodiment of the circulation control system and method; 
         FIG. 18  is an illustration of valve system with an exemplary actuator in one embodiment of the circulation control system and method; 
         FIG. 19  is an illustration an alternative embodiment of the valve system and actuators positioned within the airfoil in one embodiment of the circulation control system and method; 
         FIG. 20  is a chart showing a comparison of force output vs. weight for actuators, shape memory materials, and magnetic solenoids in one embodiment of the circulation control system and method; 
         FIG. 21  is an illustration of exemplary shape memory alloy actuator in one embodiment of the circulation control system and method; 
         FIG. 22  is an illustration of the assembly of the fluid connection device in one embodiment of the circulation control system and method; 
         FIG. 23  is an illustration of male bracket of the fluid connection device in one embodiment of the circulation control system and method; 
         FIG. 24  is an illustration of female bracket of the fluid connection device in one embodiment of the circulation control system and method; 
         FIG. 25  is an illustration of the orientation of the ports in the fluid connection device in one embodiment of the circulation control system and method; 
         FIG. 26   a  is an illustration of an alternative pin assembly in the fluid connection device in one embodiment of the circulation control system and method; 
         FIG. 26   b  is an illustration is an illustration of the pin of the alternative pin assembly in the fluid connection devices in one embodiment of the circulation control system and method; 
         FIG. 27  is an illustration of variation of the blowing coefficient with respect to tip speed ratio per meter span of the turbine blade in one embodiment of the circulation control system and method; 
         FIG. 28  is a top view of a two-bladed vertical axis wind turbine in one embodiment of the circulation control system and method; and 
         FIG. 29  is an illustration of a top view of a symmetrical airfoil blade with alternative blowing slot locations in one embodiment of the circulation control system and method. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is illustrative in nature and is not intended to limit the embodiments of the invention or the application and uses of such embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     The use of circulation control has been applied to fixed wing aircraft since the late 1960&#39;s and early 1970&#39;s. Both, passive and active systems have been investigated. Despite the need to add a system to supply a blowing (or suction) to the blowing slots  102  for an active system, a large increase in lift has been shown. Introduction of a blown jet of air, or any fluid/gas, near a rounded surface alters the interaction between the free stream fluid/gas and the surface/object. Known loosely as flow control, in the form of boundary layer or circulation control, blowing air over the upper surface of the rounded trailing edge augments the lifting capacity of an airfoil. This concept has been shown by Kind [1968], Kind and Maull [1968], and others (including [Myer, 1972], [Englar, 1975], [Englar et al., 1996], and [Englar, 2005], to name a few.) Generally, the techniques disclosed utilize a blowing slot over the upper surface of the rounded trailing edge to augment the lifting capacity of an airfoil. A passive system, such as the use of vortex generators, has been able to provide a smaller increase in lift, but is generally used as methods to delay flow separation at high angles of attack. 
     Referring now to  FIG. 1   a , an exemplary Vertical Axis Wind Turbine, VAWT  10 , is presented. The VAWT  10  comprises a plurality of airfoils  100  or blades  100 , support structures  112  that connect the airfoils  100  to the rotating main support shaft  108 , and a turbine housing  110 . The support structures  112  are illustrated connecting to the airfoils  100  at multiple support structure connection points, or joints, along the airfoil  100 , although any number of joints, including one, are contemplated. The terms airfoil  100  and blade  100  are used interchangeably throughout this specification. The airfoils  100  each have a length called the span  106 . Wind  104  across the span  106  creates lift on the airfoils  100  which is passed through the support structures  112  to the main support shaft  108  in the form of torque  116 , causing the main support shaft to rotate at angular velocity ω  114 , hereafter also referred to as the rotational speed  114 . 
     Circulation Control System 
     Referring now to  FIG. 1   b , circulation control increases the airfoil  100  bound circulation to increase lift. Circulation control is implemented in the embodiment of  FIG. 1   b . using one or more blowing slots  102  in surface of the airfoil  100  to blow a high-velocity jet of air over a rounded surface, inducing the Coanda effect. The use of circulation control enhances the lift produced by an airfoil  100 . Application of circulation control to a VAWT, or CC-VAWT, enables the creation of more lift, resulting in more torque generation from the VAWT. In one embodiment circulation control is used to modulate the aerodynamic characteristics of fixed CC-VAWT turbine blades  100  during operation thus eliminating the need to rotate or pitch the turbine blades  100  during operation. In another embodiment, circulation control is used to enhance the operation of traditional mechanical mechanisms for pitching the turbine blades  100  to maximize performance while minimizing the complexity of the actuators. In one aspect, traditional actuators are used to provide slower, gross movement of the turbine blades  100  while circulation control is used to manage transient conditions and maximize the torque  116  generated by the blades  100 . 
     In one embodiment circulation control is implemented using multiple span-wise blowing slots  102  with independent valve control on the CC-VAWT airfoil(s), for example a NACA0018 airfoil  100  cross-section. This airfoil  100  cross-section is given only as an example and the circulation control strategies can be applied to any aerodynamic shape. In embodiments the CC-VAWT has one or more airfoils  100  incorporating the active circulation control through blowing slots  102 . In embodiments, the blowing slots  102  in each airfoil  100 , or turbine blade are selected by one of ordinary skill in the art to provide the desired performance. The blowing slots  102  in the embodiment depicted are located on the trailing, leading, top and bottom areas of the airfoil  100 . The valve system  1202 , shown in  FIG. 12  and described in detail later, for each blowing slot  102  is located in the vicinity of the blowing slot  102 , and inside of the airfoil  100  or as part of the blowing slot  102  itself. The valve  1204  may be either digital (fully open, or fully closed), analog (any state from fully open to fully closed), or any combination thereof. In embodiments, the valve  1204  is opened or closed by any suitable means whether mechanical, electrical, electro-mechanical, hydraulic, pneumatic, a thermally actuated device, or an equivalent means as would be known in the art. 
     To optimize the turbine performance, the valve  1204  has response time requirements dictated by the maximum rotating speed  114  ω max  and circumference, or radius  312  (R), of the CC-VAWT.  FIG. 3  depicts sensors and parameter inputs to a control system corresponding to these values. The response time of each valve  1204  is rapid enough to allow for multiple openings and closings per revolution, as well as pulsed or frequency controlled blowing. Pulsing the circulation control system in lieu of constant blowing provides the ability to reduce the mass flow rate of air, or other fluids, required to be passed through the blowing slot while maintaining the ability to augment the lift generated, and allow for finer control over the amount of lift force being generated by varying the pulsed frequency, pulse duration, or inter pulse interval of the circulation control blowing. 
     In one embodiment, a turbine blade  100  with independently controllable sites of actuated blowing slots  102  is incorporated on a VAWT. A planer form view of an example blowing slot  102  distribution is shown in  FIG. 1   b . This configuration of blowing slots  102  is for convenience purpose only. In embodiments, the blowing slots  102  are controlled many times during a rotation, shown in the diagram of  FIG. 6 , with different span-wise distributions or patterns, in a single uniform span-wise distribution, or in an always-on or always-off state. A CC-VAWT incorporating the always-on blowing control shows improvement in the coefficient of performance over a standard VAWT of similar geometric and atmospheric specifications, especially at moderately low TSRs  324 , or Tip Speed Ratios shown as a calculated value derived from sensor  310  inputs in  FIG. 3 . In embodiments, the control over the blowing slots  102  is homogenous over the entire span  106  of the blade  100 , but different for each position along the rotational path  602  of the turbine blade  100 . This produces a blade  100  that is either in a high lift (blowing on), standard lift region (blowing off), or reduced lift (blowing on opposite surface)—with blowing slot  102  changes coordinated with the phase of rotation. 
       FIG. 2  depicts a block diagram of a CC-VAWT with integrated controller  202 . The amount of power that a CC-VAWT generates is the product of torque generated (τ)  116  and the rotational speed (ω)  114 , and is limited by a maximum wind energy extraction efficiency commonly known as the Betz Limit. The highest efficiency of extracting the energy available in the natural wind  104 , according to the Betz limit, is a coefficient of performance (C p )  410  of 16/27 (˜0.59). At this theoretical maximum C p    410  the average downstream velocity is ⅓ of the upstream velocity. The addition of circulation control to a VAWT cannot violate the Betz limit, but through the use of the controller  202 , a VAWT approaches this limit at a larger range of wind speeds  308 . 
     In an embodiment, multiple independently span-wise  106  blowing slots  102  are disposed along the span of the blade  100  and controlled to improve performance, manage upper and lower blowing, and reduce blade and structure stress using advanced control techniques. In embodiments, each blowing slot  102  is synchronized with other blowing slots  102  or activated asynchronously for other blowing slots  102  located on the same blade  100  or different blades  100 . One embodiment of the controller  202  is shown in  FIG. 2 . The controller  202  functions on any type of a VAWT and is presented in this disclosure for a straight-bladed Darrieus turbine as an example only. In other embodiments, control with specific modifications is applied to a HAWT or any rotary device which employs circulation control, and requires a different distribution and scheduling of the blowing slots  102 . In other embodiments, the control is to turbines operating in different fluid media such as water. 
     Circulation control maximizes overall power generation, while reducing the blade  100  and structural stresses, improving startup characteristics, and providing the ability to decrease power uptake during excessive wind  104  conditions. In a first mode, circulation control increases performance through scheduling of blowing and increased jet velocity through the blowing slots  102 . This mode increases power generation over a typical VAWT by enhancing the lift force via circulation control. In a second mode, circulation control assists with turbine rotational startup. Achieving a TSR  324  (λ&gt;1) is an issue with some VAWT&#39;s due to a limited and potentially negative torque  116  (τ) generated at low rotation speeds. In this second mode, circulation control assists by boosting the lift coefficient at low wind speeds  308  using a circulation control blown jet. Circulation control is typically more effective with high levels of blowing and low wind speeds  308  according to analytical models. In a third mode, circulation control modifies the configuration of the blowing slots  102  to decrease the lift force, reducing the rotational speeds  114  and/or torques  116  generated at wind speeds  308  that would otherwise be unsafe for operation of the turbine. 
     Referring now to  FIGS. 2 and 3 , block diagrams of a simplified CC-VAWT control system  200  and an advanced CC-VAWT control system  300  are presented. Control systems  200 ,  300  use environmental and performance parameters as well as physical information about the turbine itself to determine when to activate the blowing slots  102 . In various embodiments, sensors  310  provide wind speed  308  (instantaneous and averaged, one, two, and three axes), wind direction  302  (instantaneous and averaged), turbine rotational speed  114  and instantaneous blade rotational position  304 . In some embodiments additional sensor information or calculated values are used such as blade stress and force information (static, continuous, maximums, measured, and/or calculated), pressure and mass flow information about the blowing slot  102  air, blowing slot  102  valve response time, and the pre-determined performance and physical data or parameters about the wind turbine, such as the turbine radius  312 . In embodiments, some or all of these parameters are estimated by the controller  202 . 
     In  FIG. 2 , a block diagram of the simplified CC-VAWT system  200  is presented. An estimator  204  produces desired speed ω ref  and torque τ ref  commands based on the wind velocity  104 . The desired speed ω ref  and torque τ ref  are combined with feedback measurements  206  from the measured output of the VAWT to produce error signals that the controller  202  uses to determine when to activate the blowing slots  102 . This information flow is given as an example, and it should be understood by anyone knowledgeable in the art that in other embodiments, additional information and inputs, such as atmospheric pressure, relative humidity and temperature, can also readily be incorporated into the controller  202 . to achieve the predetermined set point  314   
     In  FIG. 3 , an expanded view of the information flow conducted within the advanced CC-VAWT control system  300  is shown. The advanced CC-VAWT control system  300  breaks apart the functional roles of components of the controller  202  into estimators  318  and a decision matrix  330 , however in various embodiments the controller  202  should be generally understood to encompass a subset of superset of elements of the estimators  318  and a decision matrix  330 . 
     In embodiments of the advanced CC-VAWT control system  300 , sensor  310  inputs are converted to the desired system state variables by suitable state estimators  318  incorporated into the CC-VAWT control system  300 . In embodiments, estimators  318  estimate the virtual angle of attack  320  of the blade  100 , the relative velocity  322  of the blade  100  in relation to the wind  104 , and the tip speed ratio, or TSR  324 . Using these estimates from the estimators  318 , a decision matrix  330  signals the slot controller  332  to activate the appropriate blowing slots  102 . In one embodiment, the decision matrix  330  comprises an upper/lower slot selector  326 , a blow level controller  328 , a slot controller  332 , one or more pre-computed decision tables  316  and a predetermined set point  314  for activating the blowing slots  102 . In the embodiment presented in  FIG. 3 , the upper/lower slot selector  326  of upper or lower blowing slots  102  is based on the estimated angle of attack  320 , and the blow level controller  328  determines the level based on both TSR  324  and inputs from the pre-computed decision tables  316 . In other embodiments, valve  1204  actuations for activating the blowing slots  102  are computed in real time using, for example, a processor adapted for determining when to activate the blowing slots  102  for a dynamic range of conditions and desired power generation from the CC-VAWT. The decision matrix  330  computes the level of the blow level controller  328  and which blowing slots  102  to utilize for desired performance from the CC-VAWT. 
     In embodiments, the decision matrix  330  is based upon any combination of experimental, simulated, and historical performance data of the specific CC-VAWT. Referring now to  FIG. 4 , the performance capabilities of a particular wind turbine at different tip speed ratios  324  and different blowing coefficients, Cμ  412 , is shown graphically. This information is generated using computer performance simulations of the capabilities of a CC-VAWT blade using a chord to radius ratio of 0.05. In this case, the performance characteristics are determined for a NACA0018 airfoil  406  without blowing, a NACA0018 airfoil  408  with a blowing coefficient, Cμ  412 , of 10%  402  and a NACA0018 airfoil  406  with a blowing coefficients, Cμ  412 , of 1%  404 . From these data, a control region  408  is developed for producing a high coefficient of performance, C p    410 , over a wide range of TSRs  324 . 
     The data is used by the decision matrix  330  and augmented with the environmental and performance measurements from the sensors  310  and estimators  318 . The decision matrix  330  determines the blowing and non-blowing state of the circulation control jets, or blowing slots  102 , to obtain a desired goal such as a high coefficient of performance, C p    410 . The decision matrix  330  also adapts to varying situations such as large or small changes in wind speed  308  and wind direction  302 , and blowing slot  102  or valve  1204  failures. 
     Referring now to  FIGS. 3 and 5 , the upper/lower slot selector  326  selects which of the blades&#39;  100  upper and lower (or turbine inner and outer) blowing slots  102  are activated. The virtual angle of attack  320  estimator  318  determines the apparent angle of attack  320  of the blade  100 , with respect to the relative velocity  322  (vector sum of the rotational speed  114  and wind velocity  308 ). To enhance the turbine performance, for a negative apparent angle of attack  320  the lower blowing slot  1208  is used and vice versa for the upper blowing slot  1206 . The apparent angle of attack  320  is determined by the relative velocity  322  estimator  318  and is a function of the wind speed  308  and wind direction  302 , rotational speed  114  and blade rotational position  304 . Also, used to determine the virtual blade angle of attack  320  is the static dimension parameters of the wind turbine, such as the radius  312  and the blade  100  chord  502  and span  106 . 
     In addition to the control of the upper blowing slot  1206  and lower blowing slot  1208  for proper angle of attack  320  selection and to maximize power, circulation control is used to reduce performance. In some cases a reduction in performance, which is a reduction in torque, is beneficial to a wind turbine. Excessive rotational speeds  114  or wind speeds  308  can have the potential to damage a turbine. Circulation control, when used fully or intermittently during rotation or in sections along the blade span  106 , in known wind speeds  308  and rotational speeds  114  can reduce lift produced by the blade  100  and in turn reduce or shutdown power production. In other embodiments, this reduction in power is used to match an electrical or mechanical load being driven by the turbine. 
     Referring now to  FIGS. 6   a ,  6   b ,  6   c , and  6   d , the turbine&#39;s rotation is divided into partitions  2 -A,  2 -B;  3 -A,  3 -B,  3 -C;  4 -A,  4 -B,  4 -C,  4 -D;  8 -A,  8 -B,  8 -C,  8 -D,  8 -E,  8 -F,  8 -G,  8 -H; or collectively, zones. In  FIG. 6   b , a CC-VAWT with one blade  100  rotates through the three zones labeled  3 -A,  3 -B, and  3 -C on a circular path  602 . In various other embodiments, the path  602  of the rotation is broken into any number of zones.  FIG. 7  illustrates the coefficient of performance C p    410  for the three-zone rotation of the turbine of  FIG. 3 . The coefficient of performance C p    410  varies in each states of conditional zone blowing  704 , always on blowing  706 , and no blowing  702 . Using zones provides a method of selecting a desired performance level for the wind turbine, and facilitates controlling the degradation of the performance level between the always on blowing  706  and no blowing  702  states. 
     In another embodiment, reduction of blade stresses or forces on a CC-VAWT is achieved by reducing the lift force in certain sections of the rotational path  602 , depending upon the rotation speed  114 , wind direction  302 , wind speed  308 , and disturbances or changes to the wind speed  308  and wind direction  302 . Parts of the CC-VAWT that benefit from a reduction in stress are determinable by detailed machine analysis, and include such areas as the joint(s) between the blade  100  and the support structure  112 . In addition, the areas of stress reduction include the entire wind turbine, with emphasis on the blades  100 , support structure  112  for the blades  100 , and the main support shaft  108 . The stresses in blades  100  and support structure  112  for the blades  100  are reduced by controlling, reducing or enhancing, the aerodynamic forces that are generated using circulation control. 
     The forces on a blade  100  are not uniform during the rotation of a VAWT which will want to cause the rotating structure to vibrate and or to wobble about the main support shaft  108  of the turbine. Because of this the rotating main support shaft  108  experiences cyclic loading and fatigue. The CC-VAWT with circulation control balances out, or smoothes the forces generated during rotation to reduce this cyclic stress. 
     The power generated by a CC-VAWT may either be used in mechanical or electrical form. This power may be controlled to develop under a constant level of torque  116 , or rotational speed  114 , or in a desired range of these two variables. In one embodiment, electrical power require a constant rotational speed  114  with varying or constant levels of torque  116  in order to generate a constant frequency compatible for insertion of power into a fixed frequency AC electrical power grid. In this embodiment the CC-VAWT controller presides over a power-conditioning unit that handles electrical power conversion and generation, reducing the number of components required to integrate a wind turbine to the electrical grid. 
     In one embodiment, the implementation of the CC-VAWT controller is realized with software running either real-time or scheduled, written in a single or combination of programming languages commonly known in the arts, such as but not exclusively C, C++, JAVA, C#, Visual Basic, Assembly, MATLAB, ADA. In embodiments, the hardware is a PC or micro-controller, or other types of controller/computing hardware. In embodiments, the hardware uses x86, x86-64, RISC, or ARM processors. In embodiments, the hardware uses any number of digital inputs, digital outputs, analog inputs and/or analog outputs. This hardware may also comply with standardized, ad-hoc, or proprietary serial and parallel data transfer methods and protocols. 
     In embodiments, the software of the controller uses Artificial Intelligence (AI), classical control techniques, non-linear control techniques, and/or any combination of control techniques commonly known in the arts. In embodiments, the AI system may be comprised of Fuzzy Logic, Neural Networks, Genetic Algorithms and/or any combination of these methods in any manner. 
     In embodiments, the controller uses a sensor  310  or a plurality of sensors  310  to compute the environmental parameters of wind speed  308  and wind direction  302 , and bases decisions on either instantaneous and/or averaged values. In embodiments, the controller uses one or more filters and/or neural networks to estimate the wind speed  308  and wind direction  302  based upon data from wind speed sensors  308 , such as anemometer(s), wind direction sensors  302 , such as wind vane(s), rotational speed sensor(s)  306 , force sensor(s), on the blade(s)  100 , support structure  112  and rotating main support shaft  108 , a torque sensor(s) located on the main support shaft  108 , and/or power output from turbine. In embodiments, the power levels produced by a particular CC-VAWT are estimated by software to control the blowing slots  102 . In embodiments, the sensors  310  are analog or digital and output the sense on analog, digital, or serial or parallel communication paths. In embodiments, the communication paths may be wired, wireless, or optical. 
     Circulation and Boundary Control 
     The addition of circulation control to the airfoil  100  of a vertical axis wind turbine blade makes a vertical axis wind turbine (VAWT) appear to have a higher solidity factor  1000 , σ, than the physical shape indicates. Referring now to  FIGS. 8 and 9 , performance projections are illustrated for constant blowing coefficient values  802  applied throughout a range of tip speed ratios  324  using the momentum models  800  and vortex models  900 . The momentum models  800  and vortex models  900  are illustrated for blowing coefficients, Cμ  412 , of 0.00, 0.01, and 0.10 used as the constant blowing coefficient values  802 . For each of the constant blowing coefficient values  802 , increasing the blowing coefficient considerably increases the coefficient of performance Cp  410  at tip speed ratios  324  less than six, enabling CC-VAWT at these lower tip speed ratios  324 . 
     Referring now to  FIG. 10 , an illustration of the coefficient of performance Cp  410  for a range of tip speed ratios  324  is presented for a plurality of solidity factors  1000 , σ. Comparing the circulation control performance of  FIGS. 8 and 9 , with the solidity factor  1000 , σ, performance of  FIG. 10 , it is seen that the use of circulation control resembles increasing the solidity factor  1000 , σ. Circulation control augmentation is different than solidity factors  1000 , σ, in that circulation control varies with respect to the blade rotational position  304 , the blowing slot&#39;s  102  span-wise  106  location on the blade  100  and as a function of the wind speed  308 . In circulation control, this variation is achieved through a computer-based controller  202  to optimize and condition the power output. In embodiments, other control methods known in the arts, e.g. mechanical or electronic controller, are implemented in the controller  202 . 
     In embodiments, boundary layer control is used enhance the aerodynamic performance of the wind turbine blades  100 . In embodiments, boundary layer control is used instead of, or in addition to, using the circulation control using blowing slots  102 . Boundary layer control achieves a delay in the separation of the flow of air (i.e., fluid including gas, water, etc) from the surface of the blade  100 , thereby achieving higher angles of attack  320 . In embodiments, boundary layer control is based on either active or passive (powered/unpowered) systems to change the near surface characteristics of the flow of air over an airfoil  100 . 
     A passive system, such as the use of small scale vortex generators, increases the mixing of free stream energy into the boundary layer. This increased mixing adds energy to the flow near the surface of the airfoil  100 , resulting in a delay in the flow separation, i.e., enabling the ability to generate lift at higher angles of attack  320 . An active system is similar to circulation control in that it adds energy to the boundary layer that delays the separation, but does not occur in the vicinity of a rounded trailing edge. Another active boundary layer control technique is to utilize suction to remove the low energy (speed) fluid near the surface of the body. 
     Referring now to FIGS.  11 , 12 ,  13 , and  14 , in embodiments, boundary layer suction is combined with circulation control blowing. In one embodiment, a perforated or porous surface over a portion of the blade  100 , non-dimensionalized with the length of the chord  502  and from 0.05&lt;x/c&lt;0.5, creates one or more suctions ports  1102  that are pneumatically (or hydraulically) connected to the circulation control blowing slot(s)  102 . The circulation control blowing slots  102  are located near the trailing edge from 0.75&lt;x/c&lt;1−D te /2c. The upper bound on the trailing edge blown slot is based on the diameter of the trailing edge, D te , and the chord  502  length of the airfoil  100 , and thus are located the distance equivalent to the trailing edge radius from the trailing edge of the airfoil  100 . 
     The use of a combination of suction ports  1102  and blowing slots  102  is applicable to any airfoil  100  or hydrofoil shape, and is shown on an 18% thick elliptical airfoil for convenience only. The air/hydrofoil, henceforth referred to as airfoil  100 , incorporates a rounded trailing edge, with a diameter between 0.4 inches and 0.6 times the thickness of the airfoil (e.g., if the airfoil is 3 inches thick, the diameter of the trailing edge could be as large as 1.8 inches). The modification of the trailing edge of the airfoil  100  creates a Coanda surface that facilitates the flow control phenomenon, or Coanda effect, being utilized with the circulation control blowing. 
     In the embodiment depicted in  FIG. 11 , the porous surface suction ports  1102  and blowing slot(s)  102  are illustrated in the upper surface of the airfoil  100 . In embodiments the suction ports  1102  and blowing slot(s)  102  are located on the upper surface, the lower surface, or any permutations of upper and lower surfaces of the airfoil  100 . Referring now to  FIGS. 12 and 14 , the airfoil  100  may also be divided into multiple regions (i.e., upper and lower sections) for part or all of the chord  502 . Referring now to  FIG. 12 , in one embodiment a valve system  1202  and associated valve  1204  enables boundary layer suction on the lower surface and circulation control blowing over the upper surface of a rounded trailing edge through the use of a valve system  1202 . By opening and closing the appropriate valves  1204 , air from the upper suction port  1210  is directed to either the upper blowing slot  1206  or the lower blowing slot  1208 , or a combination of the upper blowing slot  1206  and lower blowing slot  1208 . Similarly air from the lower suction port  1212  is directed to either the upper blowing slot  1206  or the lower blowing slot  1208 , or a combination of the upper blowing slot  1206  and lower blowing slot  1208 . 
     The fluid dynamic surface is supported with at least one internal structural element  1108 . In embodiments, the internal structural element  1108  provides rigidity to the blade  100  and is solid (not shown) or porous (shown in  FIGS. 11 and 12 ) depending on its location and orientation. These internal structural elements  1108  may be in the span-wise  106 , chord-wise  502 , or in the thickness direction, as well as in composite directions, combining more than one of the three primary directions. Though illustrated in  FIG. 11  and  FIG. 12  as attaching the interior of the upper surface to the interior of the lower surface, the internal structural elements  1108  are not required to connect opposite surfaces. Referring now to  FIG. 13 , an illustration of a reinforcing internal structural element  1108  that does not connect the two surfaces together is presented. Referring now to embodiments depicted in the cross-sectional illustrations of  FIGS. 11 ,  12 ,  13 , and  14 , the internal structural elements  1108  may also not span the entire length of the airfoil  100  or similar fluid dynamic surface being constructed, and hence sections of the surface may be solid (without the blowing/suction augmentation) and provide additional structural support to the regions where blowing/suction is utilized. 
     In embodiments, the airfoil  100  contains more than one internal structural element  1108 , each of which may or may not contain porous sections. For example, there may be sections of a blade  100  or wing where the augmentation of boundary layer suction and/or circulation control blowing is not desired, thus the porosity is not needed. It may also be desired to separate the upper surface from the lower surface, such that suction/blowing can occur on both the upper and lower surface simultaneously, independently, or in an overlapping manner. For example, during the transition from upper surface to lower surface flow control it may be beneficial to have both systems activated at the same time. The separation of the upper and lower zones of flow control enables the variation in mass flow rates, i.e., the upper surface flow control may be set at a different jet velocity/momentum than the lower surface. The variation in performance can also be achieved by placing a pressure regulator between the suction ports  1102 , blowing slots  102  and the activation system (fan  1104 , piston  1302 , or similar) near the valve  1204  to activate each respective region of the airfoil  100 , hydrofoil, or similar device. 
     In embodiments, the connection between the two active flow control elements, the suction ports  1102  and blowing slots  102 , includes a means to accelerate air, or similar gas or liquid. In embodiments, the means is a fan  1104 , impeller, or other mechanical flow accelerating device placed inside the turbine blade  100 . In one embodiment the fan  1104  is placed near the location of maximum thickness of the blade  100  to provide the greatest area upon which the fluid can be accelerated. The fan  1104  is powered by a motor  1106  and orientated such that air is drawn or forced from the suction ports  1102  toward the circulation control blowing slots  102 . The controller  202  determines when the valves  1204  of the valve system  1202 , and the fans  1104  are activated. The motor  1106  is shown on the right hand side of the fan  1104 , but in alternate embodiments is attached to the left as shown in  FIG. 12  or embedded into the structural element within the airfoil  100  cavity. 
     Referring now to  FIGS. 13 , and  14 , in other embodiments the means to accelerate the air or fluid is a piston  1302 . The piston  1302  provides a pressure gradient pulling the fluid near the suction ports  1102  and sending it out of the blowing slot  102 . In embodiments, the use of a piston  1302  includes mechanisms to relieve pressure when returning to the piston&#39;s  1302  useful position. Referring now to  FIG. 14 , in a first embodiment one or more one-way pressure devices  1402 , for example check valves, release when the piston  1302  is traveling right to left. In a second embodiment, a bypass channel sends the excess pressure either to another section of the airfoil  100  or to the opposite side of the piston  1302 . 
     In one embodiment, a fan  1104  powered by a motor  1106  or similar means, is the supply mechanism to attach two regions of boundary layer suction to two circulation control blowing slots  102 . It is also possible to use a single piston  1302  configuration in this manner. The suction and blowing may be linked either together (i.e., upper-upper) or opposite (i.e., upper-lower, as shown in  FIGS. 12 and 14 ) as well as with both suction ports connected to one blowing slot  102 , or vice versa, and potentially with all four valves  1202  open at once.  FIG. 14  shows a two piston configuration to provide control over the upper-upper and lower-lower linked suction port  1102  and blowing slot  102 . It is also possible to use a two fan  1104  configuration in this manner. 
     Referring now to  FIG. 15 , another potential source of air for either circulation control blowing or boundary layer suction, for applications, such as a vertical axis wind turbine, is to place a piston  1302  in the hollow support structure  112  of the blade  100 . The piston  1302  utilized in this configuration can either incorporate the one-way pressure device  1402  or provide alternating suction and blowing to the blade  100 . In embodiments, this alternating pressure gradient is used in conjunction with a mechanism to select between the blowing slot  102  and the boundary layer suction port  1102  on the augmentation equipped surface. 
     Circulation Control using Coanda Jets 
     Referring now to  FIG. 16   a  and  FIG. 12 , a blowing slot  102  is used to blow a stream of fluid, such as air, over the upper surface of an airfoil  100  having a rounded trailing edge. This blown stream of fluid produces an effect, known as the Coanda  1602  effect, that augments the lifting capacity of the airfoil  100 . Referring again to  FIGS. 12 ,  13 , and  14 , in other embodiments of the present disclosure, a second blowing slot  102  is added to the lower surface of the trailing edge of the airfoil  100 . The addition of the second blowing slot  102  to the trailing edge of the airfoil  100  results in expansion of the lift augmentation capability, allowing the inversion of the direction of the lifting force and/or creating a lower drag scenario without physically altering the airfoil. In one embodiment, the upper and lower blowing slots  102  are separately controllable, allowing the lift performance to be biased in one direction by using different blowing rates in the two slots  102 . For example, on a helicopter main rotor it may be desirable to increase the upward force during part of the blades&#39;  100  rotational path  602  and reduce, but not invert, the force in another portion of the rotation. 
     Referring now to  FIG. 16   b , and continuing to refer to  FIGS. 12 ,  13 , and  14 , in another embodiment, in addition to using a blowing slot  102  to blow a jet over one surface, either upper or lower, air is blown out of both blowing slots  102  simultaneously. If the jet blowing rate out of the two blowing slots  102  are the same then a stagnation point is created slightly downstream of the trailing edge of the airfoil, called a Kutta  1604  condition. A Kutta  1604  condition, when used in lieu of turning the circulation control blowing off, reduces the profile drag of the aerodynamic structure by reducing the size of the wake created by the airfoil  100 . 
     Simultaneously opening the upper and lower blowing slots  102  diminishes the lift enhancing capabilities of the Coanda  1602  jets by producing a Kutta  1604  condition, but this Kutta  1604  configuration enables a drag reduction when compared to the un-blown, rounded trailing edge. Thus, when the lift augmentation is not needed the drag penalty of the rounded trailing edge can be reduced considerably. In a vertical axis wind turbine, or VAWT, for a portion of each blade&#39;s rotational path  602  the addition of lift is not beneficial. In those portions of the rotational path  602 , opening both the upper and lower blowing slots  102  reduces the blade&#39;s  100  drag. Reducing drag on one blade enhances the amount of torque  116  available to the vertical access wind turbine (VAWT) from the other blades  100 . 
     Referring now to  FIG. 16   c , and continuing to refer to  FIGS. 12 ,  13 , and  14 , in other embodiments, variably controlling the blowing rates out of each blowing slot  102  to produce Coanda  1602  jets enables a lower drag scenario as well as lift augmentation capability. This variable lift-drag  1606  condition is shown in  FIG. 16   c  and illustrates the potential to use different blowing coefficients, Cμ  412 , out of each blowing slot to augment the lift created while also providing a reduction in drag. The difference in blowing coefficients, Cμ  412 , on the upper and lower surfaces can be used to augment the lift and drag forces at different levels. 
     There are several potential uses of the combined blowing conditions,  1604 ,  1606 , with regards to an aerodynamic surface, such as an aircraft wing or wind turbine blade  100 . In one embodiment, the equal blowing rate scenario can be used to effectively create a jet thruster to assist in creating a yawing moment in fixed wing aircraft. In another embodiment, the equal blowing rate scenario creates a rotational torque  116  about the main support shaft  108  of a vertical axis wind turbine to help in the start-up of the turbine. 
     In one embodiment, differential blowing is used as a pneumatic control surface, i.e. an aileron for a fixed wing aircraft, to increase and decrease the lift force depending on the input parameters to the circulation control system  200 ,  300 . The ability to adjust the direction of the lift force provides several advantages for the application of circulation control in vertical axis wind turbines. One advantage is to enable an augmented performance profile by enhancing the torque  116  generation or creating an aerodynamic brake by providing a lower torque  116  from the turbine blades than that required by the generator to maintain the operating rotational speed  114 , a net negative torque  116  about the main support shaft  108  of the wind turbine. The lower aerodynamic created torque  116  can be accomplished by either reversing the direction of the force(s) being created and/or altering the schedule of when the blowing slots  102  are activated during a rotation or complete revolution of the turbine. 
     Another advantage in applying the dual directional blowing is the ability to alter the structural loading profile of the turbine blade  100 . As the stress increases the circulation control scheduling can be altered to limit the stresses at specific locations, such as the attachment points of the support structure  112 . 
     Blowing Slots 
     For aircraft applications, circulation control is accomplished by simply pumping air into the wing and thus out of the blowing slot  102  for a length of time. However, for a VAWT the blowing slots  102  are opened and closed in quick succession depending on the instantaneous orientation of the airfoil  100  relative to the wind  104 . Circulation control is adapted for the conditions typical of a VAWT, for example the large blade angle of attack  320  and low tip speed ratios  324  (less than 4) that are typical of VAWT. The circulation control system  200 ,  300  for a VAWT implements a control scheme for controlling the air flow through the blowing slots  102  to generate the maximum power output for the VAWT. The terms blowing slot  102  and air flow slot are therefore used interchangeably in this disclosure. 
     Referring now to  FIG. 18 , in one embodiment, to achieve a suitable response time for controlling the air flow, the valve system  1202  is positioned in the interior of the turbine blade, between span-wise  106  spaced rib element  1702  sections of the turbine blade, dividing the length of the turbine blade  100  into multiple blowing slots  102  between rib element(s)  1702 . Multiple blowing slots  102  enable a higher level of control over the amount of total air flow required. Each of the valves  1204  is modulated between wide open, fully closed, as well as cycling at various frequencies. In one embodiment, a valve  1204  is located within the turbine blade  100 , in close proximity to the blowing slot  102 , and positioned at least 75% of the chord length from the leading edge  1704  of the airfoil  100 . This proximity to the blowing slot  102  and positioning near the trailing edge  1706  of the airfoil  100  permits a rapid response time for controlled opening and closing of the blowing slots  102  to produce a desirable level of performance of the circulation control augmented VAWT. 
     Referring now to  FIG. 18 , in one embodiment, the valve  1204  contains a fixed wall section  1802  that creates a plenum between itself and the blowing slot  102 . In one embodiment, this fixed wall section  1802  is integrated as part of the structure for the turbine blade  100 . In one embodiment, the fixed wall section  1802  supports a sliding plate  1804  that has the ability to slide in the span-wise  106  direction. The sliding plate  1804  and the fixed wall section  1802  have slots  1806 , or a series of holes, milled out of them that are aligned in a manner that allows for full-flow, no-flow and any variable flow condition to be selected between, by sliding the sliding plate  1804  linearly in the span-wise  106  direction. In one embodiment, further enhancement of the circulation control wind turbine is achieved through the use of dual upper blowing slots  1206  and lower blowing slots  1208  placed near both the leading edge  1704  and the trailing edges  1706  of the airfoil  100 . In another embodiment, two separate sliding plates  1804 , one sliding plate  1804  for the upper air flow slot and a second sliding plate  1804  for the lower air flow blowing slot  102 , allow independent control of the air flow blowing slots  102 . 
     Referring back to  FIG. 17 , in embodiments, the valve system  1202  maintains an elevated pressure. For efficiency, a quality seal is established between the sliding plate  1804  and the fixed wall section  1802 , as well as other portions of the airfoil  100  to prevent leakage. Those skilled in the art will be able to maintain tight manufacturing tolerances and apply sealant around necessary joints. The sliding plate  1804  is pressed flush against the fixed wall section  1802 . In one embodiment, the pressure differential between the plenum and air pressure in the blowing slot  102  assists in pressing the sliding plate  1804  against the fixed wall section  1802 . In one embodiment, the circulation control system  200 ,  300  has less than five percent leakage (measured by mass flow of air when closed divided by mass flow of air when fully open), although in other embodiments that circulation control system  200 ,  300  maintains effectiveness with leakage levels as high as 20 percent. 
     Referring to  FIGS. 17 and 18 , the actuation of the sliding plate  1804  is controlled using a solenoid  1808 . In various embodiments, the sliding plate  1804  is actuated by any number of devices including, but not limited to, solenoids  1808 , linear servo motors, shape memory alloy (SMA) devices, piezoelectric actuators and rotary motors coupled with gears and any linkage(s) and mechanism(s). The choice of actuator is largely based on the specific design constraints for a given VAWT, with response time, size and weight being the dominant considerations for choice of actuator. 
     Referring to  FIG. 19 , an alternate embodiment of a valve system  1202  is presented. In embodiments, one or more solenoids  1808  are coupled to a sealing rod  1902  that seals the blowing slot  102 . In these embodiments, the solenoids  1808  retract the linkages  1904  and the sealing rod  1902 , allowing allow air to flow past the sealing rod  1902  and out of the blowing slot  102 . In order to close the blowing slot  102 , the solenoid  1808  pushes the sealing rod  1902  back up against the blowing slot  102  to create a seal. 
     Shape Memory Actuators 
     Circulation control is achieved by selectively opening and closing the blowing slots  102 . The blowing slots  102  are opened and closed using actuators, which in some embodiments are solenoids  1808 . Mechanical cams, solenoids  1808 , and piezoelectric valves can be used to control the flow of air to the blowing slot  102 , for example, by attaching them to shutters, louvers, flaps, valves and other mechanisms. But generally these mechanical and electromechanical means have relatively slow reactions times as well as size and weight considerations that substantially impact any airfoil designs that utilize them. 
     In embodiments, a shape memory actuator is used to selectively open and close a blowing slot  102 . Actuators that are capable of converting thermal energy to mechanical energy in the form of force, displacement or torque are referred to as thermal actuators. Shape memory actuators  2100  are a subset of these actuators that use the shape memory effect to generate the desired force and motion. 
     Referring now to  FIG. 20 , a comparison of the weight-to-force characteristics of common actuators and shape memory actuators is presented. Shape memory actuators  2100  present practical advantages over the more commonly used mechanical or electromechanical actuators such as solenoids and piezoelectrics, especially in devices under 1 g in weight that are capable of generating over 50 N of actuation force. These advantages are due to the characteristics of the shape memory materials used in the actuators. Shape memory actuators  2100  outperform other means of actuation in both the force and range of motion. Shape memory actuators  2100  allow designers the ability to use smaller actuators with an equivalent amount of force, creating a faster reaction time. Shape memory actuators  2100  are not limited to either linear or rotary motion like most other actuators. In one embodiment, the shape memory actuator  2100  is incorporated into the “skin” of the airfoil. In various embodiments, the shape memory actuators  2100  are designed to operate in tension, compression, torsion, and in more complex configurations to achieve three dimensional motion in any combination of direction(s). In various embodiments, the geometric and spatial orientations of the SMA are used to control the actuation characteristics of the SMA. In various embodiments the SMA material is tubular, or has a cross-section of a circle, an ellipse, a rectangle, or any irregular or regular shape. In various embodiments, the multiple SMA wires are bundled together, for example into strands, ropes, arrays or other shapes. In this embodiment, the SMA bundles can be configured to generate substantially continuous motion or generate increased force output. 
     Shape memory materials are a class of “smart” materials that have the ability to store a deformed shape and recover the original shape without affecting the structural integrity of the material. In various embodiments, the shape memory material is NiTi, CuAlNi, CuAl, CuZnAl, TiV, or TiNb. In other embodiments, the SMA is incorporated into a ferromagnetic shape memory alloy (FMAS) composite, for example by layering the shape memory material in grooves or indentations in iron or FeCoV alloys. The shape memory effect is an ability to recover, upon heating, mechanically induced strains, resulting in a transformation to a predetermined position. This effect is thermally driven and hinges on a critical temperature, the transition temperature for polymers and the reverse transformation temperature for alloys. These temperatures vary with the material type and loading of the material. Although the polymers can recover much larger strains than alloys, they generally do not produce enough recovery force to be used for most actuators. On the other hand, when constrained to prevent the shape memory effect, some shape memory alloys can generate stresses up to 700 MPa making them effective as actuators. 
     The shape memory effect occurs in specific alloys because of their ability to transform austenite to martensite (phases of their crystalline structure), a process that naturally occurs in steels and other metals with a carbon content when they are rapidly cooled. However, shape memory alloys are also able to reverse the process, from martensite back to austenite, allowing the alloy to have a memorized “parent” shape. At lower temperatures the alloy can be manipulated because the atoms move cooperatively allowing for variants of the parent phase, but when the temperature is raised above a certain point the martensite becomes unstable and reverse transformation occurs and the alloy reverts back to its parent phase. 
     Shape memory alloys (SMA) have a natural one way actuation; a pre-stretched wire will contract upon heating above the reverse transformation temperature. The wire will not ‘re-stretch’ upon cooling so in order for the alloys to be used for two way actuators they are used in conjunction with an external force that resets the alloy during cooling. Because the wire will not ‘re-stretch’, two main design embodiments are presented for two-way motion shape memory actuators: (1) in one embodiment, a differential method is utilized and (2) in another embodiment a biasing method is utilized. The differential embodiment provides more precise control of motion whereas the biasing embodiment gives more flexibility in the design of the shape memory actuator  2100 . 
     The differential embodiment uses two shape memory elements that are heated separately. Upon heating, one pre-stretched actuator contracts and stretches the other shape memory actuator preparing it to be heated in the return portion of the cycle. In one embodiment of the differential method, ribbons of SMA are placed on either side of a freely rotating pivot point to create two-way differential actuation. 
     Referring now to  FIG. 21 , an embodiment of a shape memory actuator  2100  using the bias method is presented. The bias method uses a force-creating component such as a bias spring  2104 , elastic member, or dead weight to re-stretch the shape memory component  2102 . In one embodiment of the bias embodiment,  FIG. 2  shows the relationship between the load deflection curves and the two-way motion of the shape memory actuator  2100 . At points A and B, the opposing spring forces are equal defining the total compressed length of the shape memory actuator  2100 . The stroke length D is generated as the shape memory actuator  2100  is heated and cooled between these two points. In one embodiment, the shape memory component  2102  is operated under an additional external force, illustrated above as P 1 , and the stroke is proportionally shortened to D 1 . The bias spring  2104  stiffness modifies the temperature response, in particular the transformation temperature, the available force, and the hysteresis. In various embodiments, the bias spring  2104  stiffness can essentially be chosen to be any value since it directly affects the operating characteristics of the shape memory actuator  2100 . However, in one embodiment the bias spring  2104  stiffness is selected to be equal to the stiffness of the shape memory component  2102  at a low temperature. 
     In various embodiments, the temperature of the SMA actuator is controlled. In one embodiment, the SMA actuator is thermally shielded. In another embodiment, the SMA actuator is cooled by a cooling system. In another embodiment, the SMA actuator is air cooled. 
     Joint for Fluid Delivery 
     Circulation control on a wind turbine utilizes air that is pumped in and/or out of blowing slots  102  in the turbine blades  100 . Incorporating circulation control on a rigidly designed turbine, such as a vertical axis wind turbine or VAWT, with rigid solid connections between the support structure  112  and the blade  100  can be implemented by an air, or similar fluid, circulation control system  200 ,  300  that uses the main support shaft  108  and support structure  112  support arms as a conduit for passing air to the turbine blades  100 . Alternatively, an air flow circulation control system  200 ,  300  is contained entirely within the turbine blades  100 .  FIGS. 11 ,  12 ,  13 ,  14 , and  15  are illustrations of fan  1104  and piston  1302  type systems in which the air flow is developed within the blade  100  or support structures  112  instead of being provided from an external source. 
     The use of moveable connections on a dynamically soft turbine reduces the stress concentrations associated with rigid connections of a rigidly designed turbine. Reducing stress concentrations enables a turbine, such as a VAWT, to be constructed that will be both lighter and have a longer fatigue life. However, on a dynamically soft turbine, the sliding or pivoting pinned connection between components creates an impediment to using the turbine support structure  112  members as conduit(s) to pass air into the blade  100 . One solution is to incorporate a “jumper” hose that circumvents air around the pinned connections and pneumatically connects the turbine support structure to the blade  100 . However a jumper hose creates other problems including, but not limited to, the production of unwanted aerodynamic forces. One aspect of the disclosure is the design of a pinned connection which allows any gas or fluid, referred to as air for simplicity, to pass directly through the pinned joint eliminating the need for a bypass hose, or jumper hose, around the pinned connection. 
     Referring now to  FIG. 22 , in embodiments, a three component pinned connection system  2200  comprises an air channel  2202  that supplies air from the circulation control system  200 ,  300  to the blade  100  through the support structure  112  using the air channel  2202 . The three component pinned connection system  2200  comprises a male bracket  2204  attached to either of the structural members, with a female bracket  2206  attached to the other structural member, and a pin  2208  connecting the two brackets  2204 ,  2206  together. A distinguishing feature of this disclosure is that each of the three components has the ability via a port, or similar conduit structure, to allow air or fluid to pass directly through the joint. 
     Referring now to  FIG. 23 , in embodiments the male bracket  2204  comprises a rounded face  2304  adapted to be inserted into a female bracket  2206 , a hole  2302  into which a pin  2208  can be inserted, and a hollow port  2302 . The hollow port  2302  creates part of the air channel  2202  which extends from the male brackets&#39;  2204  connection point  2306  to the support arm support structure  112  through the pin hole  2308  and through the rounded face  2304 . The bracket connection point  2306  can be any number of configurations, from a threaded connection or a flat face which can be either welded or bolted to the support arm, or any similar fastening mechanism(s) or means. 
     Referring now to  FIG. 24 , in embodiments the female bracket  2206  comprises two side flanges  2410  between which the male bracket  2204  can be inserted, and a rounded internal face  2404  to mate up with the rounded face  2304  on the male bracket  2204 . This rounded internal face  2404  may be coated with a sealing gasket made of rubber, Teflon, or any other material capable of maintaining an air-tight, or near air-tight seal between the mating surfaces  2304 ,  2404 . The side flanges  2410  of the female bracket  2206  contain a pin hole  2408  that when lined up with the pin hole  2308  on the male bracket  2204  enable the pin  2208  to be inserted through the three component pinned connection system  2200  assembly. The male bracket  2204  and female bracket  2206  when assembled together with the pin  2208  comprise a joint having a single axis of rotation, or one degree of freedom. 
     Referring now to  FIG. 25 , in one embodiment, the port  2402  is oriented in such a manner that when the centerline of the male bracket  2204  is aligned, positioned at a 90 degree angle to the back surface of the male bracket  2204 , the ports  2302 ,  2402  on both the male bracket  2204  and female brackets  2206  are aligned. 
     Referring again to  FIG. 24 , a port  2402  allows fluid or air to flow through the female bracket  2206  and run through the rounded internal face  2404  to the back side of the female brackets&#39;  2206  connection point. In various embodiments, one of the side flanges  2410  on the female bracket  2206  contains either a slot, pinned, or threaded region for the purpose of attaching to the pin  2208  flange in order to prevent the pin  2208  from rotating within the assembled male bracket  2204  and female bracket  2206 . 
     In embodiments, a series of holes around the pin  2208  allow the pin to rotate while maintaining the fluid connection between the male bracket  2204  and female bracket  2206 . This can also be achieved by making the male bracket  2204  and female bracket  2206  larger than required by the size of the pin  2208 , allowing for the fluid to flow around the pin  2208 , in which case an external seal may be utilized to prevent excessive losses in the system. The female bracket  2206  connection point  2406  is created using any number of configurations, from a threaded connection or a flat face which can be either welded or bolted to the turbine blade, or similar fastening mechanism(s). 
     Referring now to  FIGS. 26   a  and  26   b , in one embodiment, the pin  2208  comprises a solid cylinder encased in a sealant material  2210  which will provide an air tight seal between the pin  2208  surface and surfaces  2304 ,  2404  of the male and female brackets  2204 ,  2206 . On one end of the pin  2208 , a flange with an alignment mechanism  2602  mates with pin alignment mechanism  2604  on the female flange  2410  to prevent the pin  2208  from rotating within the pin holes  2308 ,  2408 . The opposite end of the pin  2208  contains a mechanism for securing  2608  the pin within the pin hole, such as a cotter pin or threads onto which a fastener  2606  can be installed so that the pin  2208  is prevented from losing connection and alignment during operation. The pin  2208  also contains a port  2212 , or series of ports  2212 , through it which are oriented such that when the alignment mechanisms on the pin  2208  and female flange  2410  are mated; the ports  2212  are aligned with the port  2302  on the female bracket  2206 . 
     While the ports  2302 ,  2212  on both the pin  2208  and female bracket  2206  are continuously aligned due to the alignment mechanism, the male bracket  2204  is free to rotate about the pinned  2208  axis for a finite number of degrees while still allowing the fluid access to the pin  2208  and female bracket  2206  ports  2302 ,  2212 . Passage of fluid through the joint is dependent on the angular displacement of the ports  2302 ,  2402 ,  2212  relative to one another and the size of the ports  2402 ,  2304 ,  2212 , with larger ports  2302 ,  2402 ,  2212  permitting larger angular variations. 
     In other embodiments, altering the shape of the ports  2302 ,  2402 ,  2212 , to oval for example, extends the angular displacement while maintaining pneumatic or similar fluid dynamic flow capability. By varying the arc length of the rounded face of the male bracket  2204 , the connection is designed to limit the joint to rotating within a desired range. In embodiments, in addition by varying the arc length on the rounded face of the male bracket  2204  and/or varying the port  2302  diameter, the connection is designed to only allow fluid to pass through the channel  2202  during a desired range of rotation. It is important to note that the port  2302  diameter does not exceed the diameter, height, or width of the bracket  2204 ,  2206  connection point and still maintain a sealed channel  2202  through which fluid can pass. 
     Design equations relating the range of operation of the joint mechanism to the face arc length and radius and port diameter are as follows. 
     Length of curvature of male bracket face for desired range of joint operation (Rd): 
         l=r (π+ R   j )  [1]
         l=length of curvature of male bracket face   R j =desired range of joint operation
 
Range of port hole operation based on port hole diameter.
       

     
       
         
           
             
               
                 
                   
                     R 
                     p 
                   
                   = 
                   
                     4 
                      
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         
                           d 
                           
                             2 
                              
                             r 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
         
         
           
             R p =range of port hole operation 
             d=port hole diameter 
             r=radius of curvature of the male bracket face
 
The maximum port hole diameter as a function of desired range of joint operation.
 
           
         
       
    
     
       
         
           
             
               
                 
                   
                     d 
                     max 
                   
                   = 
                   
                     2 
                      
                     r 
                      
                     
                         
                     
                      
                     
                       sin 
                        
                       
                         ( 
                         
                           
                             π 
                             2 
                           
                           - 
                           
                             
                               R 
                               j 
                             
                             2 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   3 
                   ] 
                 
               
             
           
         
       
         
         
           
             d max =maximum port hole diameter 
             r=radius of curvature of the male bracket face 
             R j =desired range of joint operation
 
Range of operation of port hole
 
           
         
       
    
     
       
         
           
             
               
                 
                   
                     R 
                     p 
                   
                   = 
                   
                     4 
                      
                     
                       
                         sin 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         
                           d 
                           
                             2 
                              
                             r 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   4 
                   ] 
                 
               
             
           
         
       
         
         
           
             d=port hole diameter 
             r=radius of curvature of the male bracket face 
           
         
       
    
     Constant Rate Circulation Control Method 
     In embodiments, two additional blowing schemes are presented. The first blowing scheme implements a constant blowing coefficient and the second blowing scheme implements a constant blowing rate. The proper selection of the blowing coefficients Cμ  412  for use on a CC-VAWT is complex and depends on the physical size of the turbine, the wind speed  308 , rotational speed  114  and the rate at which momentum is introduced from the blowing slot, with a maximum rate of momentum of 30 kg-m/s2 per meter span of the blade  100 . The maximum benefit from an energy perspective has been predicted to occur with a blowing coefficients Cμ  412  of 0.10 or less, thus this value has been used in various embodiments, however other blowing coefficients Cμ  412  are also contemplated. At nominal wind conditions, the blowing coefficients Cμ  412  uses a jet momentum blowing rate of no more than 30 kg-m/s2 per meter in span  106  of the turbine blade  100  utilizing the circulation control blowing. The blowing coefficients Cμ  412  is a design decision to be made based on the environmental conditions of the location wherein said VAWT is to be constructed. Thus, the constant blowing rate is varied from turbine to turbine resulting in a wide range of blowing coefficients Cμ  412  as the wind speed  308  and tip speed ratio  324  are varied. 
     The blowing coefficients Cμ  412 , as defined in Eq. [5], is a function of the jet properties of mass flow rate and velocity as well as the relative velocity  322  of the wind speed  308 , density and area of the turbine blade  100 . Thus, maintaining a constant blowing coefficients Cμ  412  is difficult and can result in large power requirements. In one embodiment of the VAWT, a constant blowing rate of {dot over (m)}V j  is used. But the determination of the most efficient blowing rate is dependent on the wind  104  conditions at the site of the wind turbine and the desired size of the turbine. 
     
       
         
           
             
               
                 
                   
                     C 
                     μ 
                   
                   = 
                   
                     
                       
                         
                           m 
                           . 
                         
                         j 
                       
                        
                       
                         V 
                         j 
                       
                     
                     
                       
                         1 
                         2 
                       
                        
                       ρ 
                        
                       
                           
                       
                        
                       
                         A 
                         w 
                       
                        
                       
                         V 
                         ∞ 
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   5 
                   ] 
                 
               
             
           
         
       
     
     The specification of the constant blowing rate needed for the circulation control augmented vertical axis wind turbine (CC-VAWT) is a design choice based on the environmental conditions and the parameters of the turbine, such as turbine size. Two additional factors, the tip speed ratio  324 , λ, and the turbine rotor solidity factor  1000 , σ, affect the blowing rate requirement. These parameters are chosen by examining several Cp—curves. The non-dimensional parameter of tip speed ratio  324  is the ratio of rotational speed to free stream velocity and impacts the coefficient of performance Cp  410 , of the wind turbine. Referring again to  FIGS. 8 and 9 , performance projections are illustrated for constant blowing coefficient values  802  applied throughout a range of tip speed ratios  324  using the momentum models  800  and vortex models  900 .  FIG. 8  is an example of a predicted non-dimensional performance curve for a vertical axis wind turbine with a solidity factor  1000 , as defined in Eq. [6], of 0.05 for various blowing coefficients, Cμ  412 , based on performance at a specific Reynolds number, Eq. [7], of 360,000.  FIG. 8  shows the performance for the case when the blowing coefficient, Cμ  412 , is maintained at a constant value through the speed range which in one embodiment is a circulation control blowing strategy implemented for the CC-VAWT. 
     
       
         
           
             
               
                 
                   σ 
                   = 
                   
                     Nc 
                     r 
                   
                 
               
               
                 
                   [ 
                   6 
                   ] 
                 
               
             
             
               
                 
                   Re 
                   = 
                   
                     
                       ρ 
                        
                       
                           
                       
                        
                       
                         V 
                         r 
                       
                        
                       c 
                     
                     μ 
                   
                 
               
               
                 
                   [ 
                   7 
                   ] 
                 
               
             
           
         
       
     
     In an alternate embodiment, one tip speed ratio is selected for maximum coefficient of performance or some other criterion of optimal performance, C p    410 , and prescribes the blowing rate required to achieve this optimum blowing coefficient, C μ ,  412 , for example less than 0.20 for reasonable operating conditions and tip speed ratios  324  significantly above one. 
     Wind classifications such as the Beaufort scale, shown in Table 1, determine typical speeds for various wind descriptions and the operational wind speeds of a CC-VAWT. Generally the wind turbine will be shut down, for structural safety reasons, in and above “Strong Gale” wind conditions, while operating in winds in the Beaufort classifications of 2 through 8. To obtain a range of blowing rates for the CC-VAWT, the blowing coefficient of 0.10 is selected at a tip speed ratio  324  of 1.0 and 6.0 and a variety of wind speeds. The three wind speeds that were used are Beaufort classifications 3 (4 m/s), 4 (7 m/s), and 6 (12 m/s). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Beaufort Wind Speed Scale 
               
            
           
           
               
               
               
            
               
                   
                 Wind speed 
                   
               
            
           
           
               
               
               
               
               
            
               
                 Beaufort # 
                 km/h 
                 mph 
                 m/s 
                 Description 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0 
                  &lt;1 
                  &lt;1 
                  &lt;0.3 
                 Calm 
               
               
                 1 
                 1-5 
                 1-3 
                 0.3-1.5 
                 Light air 
               
               
                 2 
                  6-11 
                 3-7 
                 1.5-3.3 
                 Light breeze 
               
               
                 3 
                 12-19 
                  8-12 
                 3.3-5.5 
                 Gentle breeze 
               
               
                 4 
                 20-28 
                 13-17 
                 5.5-8.0 
                 Moderate breeze 
               
               
                 5 
                 29-38 
                 18-24 
                  8.0-10.8 
                 Fresh breeze 
               
               
                 6 
                 39-49 
                 25-30 
                 10.8-13.9 
                 Strong breeze 
               
               
                 7 
                 50-61 
                 31-38 
                 13.9-17.2 
                 High wind 
               
               
                 8 
                 62-74 
                 39-46 
                 17.2-20.7 
                 Fresh Gale 
               
               
                 9 
                 75-88 
                 47-54 
                 20.7-24.5 
                 Strong Gale 
               
               
                 10 
                  89-102 
                 55-63 
                 24.5-28.4 
                 Whole Gale/Storm 
               
               
                 11 
                 103-117 
                 64-72 
                 28.4-32.6 
                 Violent storm 
               
               
                 12 
                 &gt;118 
                 &gt;73 
                 &gt;32.6 
                 Hurricane-force 
               
               
                   
               
            
           
         
       
     
     The blowing rate, {dot over (m)}V j  of Eq. [5], requirements are determined for the median wind velocity of 7 m/s, which at a tip speed ratio  324  of 1.0 and a chord  502  length of 0.2 m results in a jet velocity of 63.7 m/s and a 1.7 kg-m/s 2  per meter blowing rate. Similarly, specifying a blowing coefficient of 0.1 to occur at a tip speed ratio  324  of 6 results in a jet velocity of 222.9 m/s and 30 kg-m/s 2  per meter. Thus, the maximum value for the blowing rate is 30 kg-m/s 2  for every meter in span  106  of the blade  100 , for example a 3 meter tall blade  100  requires no more than 90 kg-m/s of air, or similar gas or liquid. 
     Referring now to  FIG. 27 , an illustration shows the influence that tip speed ratio  324  has on the blowing coefficients, Cμ  412 , when using a constant jet momentum rate. It is important to note that a change in the length (or span  106 ) of the blade  100  requires a change in the total jet momentum rate. 
     Circulation Control to Regulate Environmental Effects Method 
     One benefit of an active system is the ability to alter the effectiveness of the augmentation based on wind speed  308  and blade direction. Thus, the circulation control lift increase can be reduced for higher wind speeds, providing a lower torque  116  and thus providing a way to limit the rotational speed  114  of the system. Both, active and passive circulation/flow control systems can be utilized to change the aerodynamic coefficients of a lifting surface and thus alter its performance. The power generated by a wind turbine is related to the rotational speed  114  and torque  116  at the main support shaft  108 . By favorably altering the lift coefficient of the turbine blades  100  to increase the torque  116  being supplied to the turbine main support shaft  108 , a larger generator and/or a larger gear ratio can be used to increase the electrical power generated. The augmented torque  116  generated, particularly at lower speeds, could also be used to extend the operational wind speed range of the turbine by enabling the production of power at a lower wind speeds  308 . The maximum safe wind speed  308  can also be increased by removing the augmentation, resulting in a reduction in the torque  116  that is generated. An alternative modification to the turbine would be to reduce either the chord  502  of the turbine blade  100  or the radius  312  of the turbine while maintaining an equal power output in currently used systems with circulation control augmentation. 
     The addition of a feedback control system allows the turbine to respond to changes in wind speed  308 , mitigating the effects of wind  104  gusts, to maintain a relatively constant torque  116  and/or rotational speed  114  to the generator main support shaft  108 . Providing a constant rotational speed  114  to the generator decreases the fluctuating stress in the major components (transmission, generator, etc), increasing the expected life of the respective parts. The connection of the CC-VAWT to an existing electrical grid is also made easier with the constant shaft speed because the controller can be programmed such that the specified frequency (i.e., 50/60 Hz) of AC power can be generated. 
     Referring now to  FIG. 28 , one embodiment of the circulation control augmented wind turbine, or CC-VAWT, is a structure having the solidity factor  1000 , σ, as defined in Eq. [6], based on the number of blades  100 , N, the blades&#39;  100  chord  502  length, c, and the turbine radius  312 , r, of less than 0.30 and incorporates at least one blowing slot  102  located either near the trailing edge  1706  (location to chord  502  length ratio (x/c)&gt;0.75) or in front of the location of maximum thickness (0.20&lt;x/c&lt;0.50 typically) on either the upper or lower (or inner and outer) surface of the turbine blade  100 . The addition of a second blowing slot expands the augmentation capabilities of the circulation control system.  FIG. 28  shows a two-bladed  100  wind turbine for convenience only, circulation control augmentation can be applied to a wind turbine with any number of blades  100 . 
     The cyclic use of circulation control applied to each blade  100  as it goes around its rotational path  602  alters the interaction of the wind turbine with the naturally occurring wind  104 . The optimum and most efficient amount of augmentation applied to the blades  100  is also dependent on the wind speed  308 , V. In embodiments, presented are several strategies for cyclic application of circulation control to the blades  100  of a vertical axis wind turbine. Referring also to  FIG. 6   a , a first embodiment employs a strategy of cyclic blowing on one span-wise  104  distributed blowing slot  102  location that is utilized when the blade  100  is in the downwind half of the profile, and no blowing during the upwind half of the profile. 
     Referring now to  FIG. 29 , a top view of one embodiment of a symmetric airfoil blade  2900  is presented indicating alternative blowing slot  102  locations. In alternate embodiments, the airfoil  100  could be cambered. In particular, the symmetric airfoil blade  2900  comprises a single upper blowing slot  1206 , on the outer surface  2902  and near the trailing edge  1806  of the symmetric airfoil blade  2900 , that is downwind of the wind direction  302 , V. In an alternative embodiment, a single lower blowing slot  1208  on the inner surface  2904  of the symmetric airfoil blade  2900  near the trailing edge  1706  is presented. 
     In another embodiment, the blowing scheme is to use two different blowing slots  102 , an upper blowing slot  1206  on the outer surface  2902  and near the trailing edge  1806  of the symmetric airfoil blade  2900 , and a second lower blowing slot  1208  on the inner surface  2904  of the symmetric airfoil blade  2900  near the trailing edge  1706 . The use of the second blowing slot  102  is most useful for force augmentation with a symmetric airfoil blade  2900  shape due to the uniform force augmentation in both directions (inward and outward). This scheme uses the upper blowing slot  1206  of the outer surface  2902  during a portion of the rotational path  602  of the symmetric airfoil blade  2900  (while the second lower blowing slot  1208  is not used), and then the lower blowing slot  1208  of the inner surface  2904  is used (while the first upper blowing slot  1206  is not used) during the remainder of the blades&#39; rotational path  602 ; essentially inverting the lift force, providing more control over the instantaneous torque  116  being produced. 
     The upper blowing slot  1206  and lower blowing slot  1208  are used as needed for efficient and maximum performance of the wind turbine. For example, in one embodiment, the upper blowing slot  1206  on the outer surface  2902  is used in the upwind (into the wind  104 , V) portion of the symmetric airfoil blade&#39;s  2900  rotational path  602  while the second lower blowing slot  1208  on the inner surface  2904  is used in the downwind (with the wind  104 , V) portion of the symmetric airfoil blade&#39;s  2900  rotational path  602 . In an alternative embodiment, the upper blowing slot  1206  is used in the downwind portion of the path  602  of the symmetric airfoil blade&#39;s  2900  rotational path  602  and the second lower blowing slot  1208  is used in the upwind portion of the symmetric airfoil blade&#39;s  2900  rotational path  602 . In still another embodiment, both the upper blowing slot  1206  and lower blowing slot  1208  are used to maximize performance, such as in high winds  104  when extra control of the symmetric airfoil blade  2900  is required. 
     In other embodiments, a pair of secondary blowing slots  2902 ,  2904  disposed in front of the location of maximum thickness  2906  on either the outer surface  2902  or inner surface  2904  of the symmetric airfoil blade  2900 . These secondary blowing slots  2902 ,  2904  are used in a similar manner as the upper blowing slot  1206  and lower blowing slot  1208  such that each secondary blowing slots  2902 ,  2904  can be used independent of or in conjunction with the other secondary blowing slots  2902 ,  2904 . Further, the secondary blowing slots  2902 ,  2904  on a symmetric airfoil blade  2900  expands the augmentation capabilities of the wind turbine when used in concert with the upper blowing slot  1206  and lower blowing slot  1208  as described above. 
     In yet another embodiment, the symmetric airfoil blade  2900  may have one or more blowing slots (not shown) near the leading edge  1704  of the blade, wherein such blowing slots  102  may be on the outer surface  2902  or the inner surface  2904  of the symmetric airfoil blade  2900 . In an embodiment, these blowing slots  102  are similar to the blowing slots  102  disclosed in U.S. patent application Ser. No. 11/387,136 (which is incorporated in its entirety by reference), and where there is a small step in the blade  100  surface near the jet that is before the maximum thickness  2906 . 
     The use of circulation control for vertical axis wind turbines adds the complexity of cycling the blowing rate. The optimal performance, based on the power generation over a range of wind speeds, of the turbine requires the varying of the aerodynamic performance characteristics of the blade  100  depending on the blade rotational position  304  relative to the wind  104 , and the rotational speed  114  of the turbine. Using the non-dimensional rotational speed, or tip speed ratio  324 , λ, as defined in Eq. [8] a preliminary analysis was conducted of the performance alterations that circulation control provides to a wind turbine. Applying a circulation control blowing rate to the blade of a VAWT results in an increase in the coefficient of performance, C p    410 , which is a measure of the energy extracted from the wind, which cannot exceed the theoretical upper limit of 16/27≅0.59, the Betz limit. 
     
       
         
           
             
               
                 
                   λ 
                   = 
                   
                     
                       ω 
                        
                       
                           
                       
                        
                       r 
                     
                     
                       V 
                       ∞ 
                     
                   
                 
               
               
                 
                   [ 
                   8 
                   ] 
                 
               
             
           
         
       
     
     For this analysis the turbine blade rotational path  602  was divided in half with the blowing on the inner surface  2904 , near the trailing edge  1706 , of the turbine blade  100  when the blade  100  is on the half of the turbine away from the wind  104  (zone  2 -B of  FIG. 6   a ) and on the outer surface  2902  of the blade  100  when in the half of the turbine nearest the wind  104  direction (zone  2 -A of  FIG. 6   a ) at a solidity factor  1000 , σ, of 0.05 and a Reynolds number, Re, as defined in Eq. [7] of 360,000. 
     Comparing the blowing coefficients of 0, 0.01, and 0.10 as shown in  FIG. 8  and  FIG. 9 , it is seen that increasing the blowing coefficients, Cμ  412 , considerably increases the coefficient of performance, C p    410 , at tip speed ratios  324  less than six, improving operation at lower tip speeds. By comparing the circulation control performance to the influence of solidity factors  1000 , σ, in  FIG. 10 , it is seen that the use of circulation control resembles increasing the solidity factor  1000 , σ. Closer inspection of  FIG. 10  reveals that as the solidity factor  1000 , σ is increased, by increasing either the number of blades  100  or the size of the blades  100 , or reducing the radius  312  of the wind turbine, up to a 6 of 0.4, the maximum coefficient of performance, C p    410  is increased and occurs at a lower tip speed ratio  324 . However, at higher tip speed ratios  324 , the performance of low solidity factors  1000 , σ, becomes better than at high solidity factors  1000 , σ. Thus, a design decision is required to determine the preferred solidity factor  1000 , σ, and tip speed ratio  324 . For a conventional VAWT the solidity factor  1000 , σ, cannot be adjusted during the operation of the wind turbine, whereas for a CC-VAWT a change in the circulation control blowing parameters results in an apparent solidity factor  1000 , σ, change. Circulation control allows adjustment of the performance of the turbine to achieve the highest possible coefficient of performance, C p    410  at a variety of tip speed ratios  324 , which is a function of the rotational speed  114  and wind speeds  308 ; and with a rapid response control scheme, the ability to adjust performance for gusting winds  104 . At high tip speed ratios  324  the turning on of the circulation control system  200 ,  300  will reduce the power extracted from the wind  104 , allowing for safer operation at higher wind speeds  308  than conventional wind turbines. 
     Referring again to  FIGS. 6   a ,  6   b ,  6   c , and  6   d , additional configurations of dividing the blade path  602  into regions or zone results in more efficient performance of the circulation control system  200 ,  300  by using circulation control only when the performance enhancement in lift increases the torque generated by the turbine.  FIGS. 6   a ,  6   b ,  6   c , and  6   d  illustrate four potential configurations, the two division section already analyzed, and three, four, and eight divisions per revolution. In embodiments, with faster response times, the blade path  602  is further divided to optimize the performance of a circulation control augmented, vertical axis wind turbine, resulting in near-continuous control by the circulation control system  200 ,  300 . 
     In embodiments, in addition to varying the circulation control performance with the blade rotational position  304 , the blowing coefficient, Cμ  412 , is varied with the span  106  of the turbine blade  100 . Distributing the blowing in the span-wise  106  direction enables the ability to operate with a portion of the blade  100  making a larger contribution to the forces than other portions of the blade  100 . This allows the circulation control system  200 ,  300  to reduce the stress on the three component pinned connection system  2200  and/or to mitigate the harmonic vibration of the blade  100  near its natural frequency. In embodiments where a constant blowing rate is used for the circulation control system  200 ,  300 , then fractions of the maximum performance can be achieved by activating an equivalent fraction of the blowing slots  102 . 
     CONCLUSION 
     While various embodiments have been described above, it should be understood that the embodiments have been presented by way of example only, and not limitation. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the subject matter described herein and defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.