Abstract:
A propeller having one or more blades eccentrically mounted to a shaft dynamically changes the blade pitch to produce free vortices in a fluid. For extracting energy from a moving fluid, the fluid flow acting on the blades rotates the propeller, while the pitch changes create a fluid flow pattern known as a von Karman vortex street. The resulting time averaged flow field distant from the propeller is a wake flow, and the energy of the fluid flow can be efficiently converted to rotation of a shaft driven device. For propulsion, applied shaft rotation and the dynamic pitch change in a fluid together create a flow pattern that is the inverse of the von Karman vortex street. For either energy extraction or propulsion, the propeller is particularly suited for low flow speeds, where the effects of low Reynolds number induced flow separation on blades may make other propellers inefficient.

Description:
BACKGROUND  
       [0001]     Propellers and turbines can transfer shaft power to a fluid or extract power from a moving fluid. (Herein, the term propeller is used in the generic sense to include structures used for propulsion and for extracting energy from a moving fluid.) Currently known propellers generally employ blades that are aerodynamically shaped with cross sections or foils commonly referred to airfoils or hydrofoils depending on the fluid. The foils can produce a force commonly referred to as lift that enables the desired energy transfers. However, the foils also produce drag that transfers energy to unwanted forms such as heat. Extensive efforts have been spent on designing foils that produce as much lift as possible while creating as little drag as possible. A particular problem to be solved in this optimization process is to keep the flow attached to the surface of the foil at high angles of attack in order to achieve high lift coefficients. While attached, the resulting flow around the foil features streamlines that are mainly parallel to the surface of the foil. Equation 1 shows how the lift force F L  produced by a foil depends on parameters such as the fluid density ρ, the flow velocity ν, the lift coefficient C L  of the foil, and the active area A of the foil. Foil design is generally concerned with optimizing the lift coefficient C L , which may be a function of the flow velocity ν. The flow velocity ν can be described using a non-dimensional parameter known as Reynolds number N R  as shown in Equation 2, where L C  is the foil cord length and μ is the kinematic viscosity of the fluid.  
             Equation   ⁢           ⁢   1   ⁢     :                               F             ⁢   L       =         ρ   ⁢           ⁢     v             ⁢   2                   ⁢   2       ·     C             ⁢   L       ·   A                             Equation   ⁢           ⁢   2   ⁢     :                               N   R     =         L   C     ⁢   v     μ                           
 
         [0002]     The lift coefficient C L  in Equation 1 is generally proportional to the attack angle of the foil until the foil begins to stall. The attack angle indicates the angle between the relative direction of the fluid flow and the foil&#39;s baseline (e.g., the line from the leading edge to the trailing edge of a simple foil.) Stalling results from the tendency of fluid flow to separate from the upper or back side of the foil causing the lift coefficient C L  to drop when the attack angle becomes too large. Accordingly, the stall angle, which is the attack angle corresponding to stall, is the angle of attack where the lift coefficient C L  is largest. Stall limits the performance of known propellers since beyond stall the foil will have greatly increased drag, as well as decreased lift. Further, the stall angle generally decreases with decreasing fluid velocity, so that the maximum lift that a foil can produce generally drops with the fluid velocity. These effects pose a problem for applications where power is to be efficiently transferred to or from a fluid at low flow speeds.  
         [0003]     Varying the attack angle can temporarily produce dynamic lift coefficients that are larger than the maximum lift coefficient C L  that can be achieved when a foil is held statically at a given angle of attack. This effect is known as dynamic lift since it involves dynamically changing the angle of attack of the foil.  FIG. 1  shows a plot  110  of the lift coefficient C L  of a foil held stationary at different attack angles in a range between the positive and negative stall angles. In contrast, plot  120  shows the lift coefficient C L  of the same foil as the attack angle of the foil oscillates between positive and negative attack angles that are greater than the stall angles.  FIG. 1  demonstrates that dynamic lift coefficients can be more than an order of magnitude larger than the static maximum static lift coefficient.  
         [0004]     Dynamic stall vortices are believed to cause of the larger lift coefficients C L  associated with dynamic lift. In particular, the separating fluid flow near the leading edge of a foil  210  as shown in  FIG. 2  can form a vortex  220  in the fluid when foil  210  rotates in a direction  230 . Vortex  220  provides a region of low pressure, which increases the lift coefficient of foil  210 . However, if foil  210  is kept at a stationary relative to the fluid flow ν, the fluid flow ν will move vortex  220  along the surface of foil  210 , and the increase in lift will disappear once vortex  220  moves past the trailing edge of foil  210 . The dynamic increase in lift can thus only be achieved temporarily during a pitching cycle when foil  210  is being rotated. Once foil rotation stops, the lift coefficient falls to the lower static lift coefficient shown by curve  110  in  FIG. 1 .  
         [0005]     U.S. Pat. No. 1,835,018 issued in 1937 to G. J. M. Darrieus discloses a propeller with cyclical thrust generation.  FIG. 3  shows one such propeller  300  that mechanically varies the pitch angles of blades  310  as described in U.S. Pat. No. 1,835,018. Propeller  300  has blades  310  with pivot axes  320  running along the perimeter of a cylinder having a central shaft  330 . A mechanism  340  driven by shaft  330  and attached to blades  310  cyclically changes the pitch of the blades, i.e., the angle between each blade  310  and a line extending from shaft  330  to the pivot  320  of the blade  310 . In propeller  300 , the variation of the attack angle of blades  310  gives propeller  300  a direction of thrust when blades  310  are all submerged in the fluid. However, mechanism  340 , which controls the orientation of blades  310 , can only be optimized for a very limited set of operating parameters, e.g., fluid velocity ν and fluid density ρ. Further, propeller  300  and other current variable pitch propellers have fluid flow that remain attached to the foils at all times, independent of the type of blade pitch control employed. Accordingly, such propellers are unable to achieve the high lift coefficients associated with dynamic lift. This limits the use and efficiency of such propellers at low flow speeds.  
       SUMMARY  
       [0006]     In accordance with an aspect of the invention, a propeller that can extract kinetic energy from a moving fluid or transfer kinetic energy to a fluid to produce thrust or create fluid motion, employs pitch changes of one or more blades in order to produce free vortices in the fluid. The propeller can thus achieve the high lift coefficients associated with dynamic lift. The pitch changes may further be adaptable so that the process of changing the attack angle can adapt according to current operating parameters such as propeller rotational velocity and free flow fluid velocity to optimize energy transfer efficiency. Embodiments of the propeller are particularly suited for low flow speeds, where the effects of low Reynolds number induced flow separation on the blades make other propellers inefficient.  
         [0007]     In the case of energy extraction from the fluid, the fluid flow preferably rotates the propeller, while the pitch changes during propeller rotation may exceed the static stall angle and are of sufficient magnitude to shed vortices and create dynamic lift. In one specific embodiment, the shedding of vortices creates a flow pattern known as von Karman vortex street, and the resulting time averaged flow field distant from the propeller is that of a wake flow. The induced rotation may drive any shaft driven device such as a pump or an electrical generator.  
         [0008]     In the case of momentum transfer to the fluid, the dynamic pitch changes during driven rotation may exceed the static stall angle and are of sufficient magnitude to shed vortices. In one specific embodiment, the shedding of vortices creates a flow pattern that is the inverse to the von Karman vortex street. The resulting time averaged flow field distant from the propeller in this configuration is that of a jet that efficiently produces thrust.  
         [0009]     In another aspect of the invention, a propeller device employs unsteady aerodynamic effects, in order to transfer power efficiently to or from a fluid. While most advantageous at low Reynolds numbers, the propeller device can be employed at all flow speeds. Potential applications include but are not limited to propulsion solutions for mini and micro air vehicles, ocean tidal flow and wave power extraction, river and stream current power extraction, and efficient low wind speed wind power generation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows plots of the lift coefficient of a single propeller blade as a function of angle of attack for static attack angles and for an angle of attack that varies sinusoidally in combination with a sinusoidal heaving motion.  
         [0011]      FIG. 2  illustrates the formation of dynamic stall vortices when the attack angle of a foil increases beyond the stall angle.  
         [0012]      FIG. 3  shows a known propeller including a mechanism that cyclically changes the attack angles of blades mounted transverse to a fluid flow.  
         [0013]      FIG. 4  is a block diagram illustrating a propeller system in accordance with an embodiment of the invention.  
         [0014]      FIG. 5  illustrates parameters of motion of a propeller that in accordance with an embodiment of the invention can be independently controlled and adapted to transfer energy using dynamic lift.  
         [0015]      FIG. 6  illustrates the path and attack angle of a single blade during operation of a propeller in accordance with and embodiment of the current invention.  
         [0016]      FIG. 7  illustrates the shedding of vortices during operation of a propeller in accordance with an embodiment of the invention when the propeller is employed to extract energy from a moving fluid.  
         [0017]      FIG. 8  illustrates the shedding of vortices when a propeller in accordance with an embodiment of the invention is driven to produce thrust in a fluid.  
         [0018]      FIG. 9  shows a system in accordance with an embodiment of the invention including multiple vortex shedding propellers in a cascade configuration to fluid flow or pressure. 
     
    
       [0019]     Use of the same reference symbols in different figures indicates similar or identical items.  
       DETAILED DESCRIPTION  
       [0020]     In accordance with an aspect of the invention, a propeller used to transfer energy to or from a fluid employs pitching blades in order to use the large dynamic lift coefficients that result from shedding of vortices during dynamic pitching. The pitching can provide efficient energy transfers even at low fluid flow rates and can be adapted to changing conditions.  
         [0021]      FIG. 4  illustrates a propeller system  400  in accordance with an exemplary embodiment of the invention. Propeller system  400  uses blades  410  having a size and number that may be selected according to the desired power transfer by propeller system  400 . In operation, one or more of blades  410  will be at least partially submerged or surrounded by a fluid such as water or air to or from which energy is to be transferred, and the average direction of fluid flow is preferably perpendicular to blade shafts  412 . Each blade  410  has a cross-section that is selected to provide a foil having the characteristics required for the target fluid. In general, any type of foil can be employed for blades  410 , but the type of foil may influence the particular pitch variation process employed in system  400  as described further below. At low flow speeds, the particular foil shape used becomes less important, and as described further below, with the proper pitching cycle, even flat plates can perform well as blades  410 .  
         [0022]     System  400  uses an offset mounting of blades  410  so that blades  410  are mounted at one or both ends on a disk or other base  415  at respective radial offsets from a main shaft  420  of propeller system  400 . Each blade  410  has a pivot mounting that permits controlled rotation of the blade  410  for example, by a corresponding servo motor system  430 . Servo motor system  430  may be constructed using a variety of systems including but not limited to an AC or DC servo motor or a hydraulic or pneumatic motor. Each servo system  430  allows a corresponding blade  410  to be rotated with respect to base  415 . In the embodiment of  FIG. 4 , each servo system  430  uses an associated mechanism  435  such a transmission, gear system, a belt and pulley system, or the like to rotate a shaft  412  of the corresponding blade  410 , and an angular position sensor  452  providing a signal indicating the angle of the blade  410  relative to base  415  can be connected to the shaft  412  of the blade  410 . Alternatively, a direct drive system is also possible, where the shaft of the servo motor  430  is directly attached to shaft  412  without a transmission or other mechanism  435 .  
         [0023]     Base  415  is attached to a main shaft  420  and provides a linkage to blades  410 , so that base  415  and main shaft  420  conduct the energy transfer between blades  410  and a device  440 , which may be, for example, a generator or motor. In the embodiment of  FIG. 4 , an optional transmission such as a single stage gear system or similar mechanical drive system  445  is between main shaft  420  and device  440 , but alternatively, device  440  may be directly coupled to main shaft  420 . Device  440  is generally selected according the direction of energy transfer and the task that system  400  performs. For the example, when system  400  extracts energy from a moving fluid, device  440  may be a generator, pump, or other device receiving drive power resulting from the action of the moving fluid on blades  410  during a pitching process. In this case, the lift from blades  410  during the pitching process creates a toque that turns base  415 , main shaft  420 , and mechanism  445  to drive the device  440 , e.g., for electrical power generation or other useful work. Alternatively, device  440  can be a motor that drives main shaft  420  to turn base  415 , so that blades  410  act on the fluid, for example, during a pitching process to create thrust for propulsion or to create a flow in the fluid.  
         [0024]     Both main shaft  420  and blade shafts  412  feature respective angular position sensors  454  and  452  that determine the respective orientations of shafts  420  and  412 . Additional sensors  456  can be used to sense properties of the fluid such as the average speed and direction of free fluid flow, so that at any point in time, a servo control system  450  that controls servo motors  430  can determine the desired pitching schedule of each blade  410  relative to the flow field. Sensors  452  and  454  can be implemented using standard system such as resolvers, tachometers, or encoders of any kind. Sensors  456  can measure any desired characteristic of the fluid including but not limited to measuring the fluid flow direction and magnitude. The flow field for example would preferably be oriented in any direction normal to main shaft  420  and can be measured using an anemometer of any kind and/or a weather vane type device. In limited applications, e.g., when extracting energy from a steady stream of known direction and magnitude, sensors  456  may not be needed and may be eliminated from system  400 .  
         [0025]     Servo control system  450  can be implemented using application specific hardware or a general purpose processing system programmed to select and implement a pitching schedule for varying the attack angles of blades  410 . Servo control system  450  can be attached to base  415  or be separate from base  415  and communicate with systems  430  and  452  on rotating base  415  via wired or wireless connections. In particular, servo control system  450  can use the information transmitted from sensors  452 ,  454 , and  456  to determine a pitching schedule, direct servo motor systems  430  to individually vary the pitches of respective blades  410 , and monitor angular sensors  452  and  454  to determine whether blades  310  are pitching as required to generate a desired vortex shedding pattern in the fluid. As described further below, the desired vortex shedding pattern generally depends on whether energy is being extracted from or applied to the fluid.  
         [0026]     While the example system of  FIG. 4  shows an electronic control system  450  that adjusts the pitch schedules, control systems may employ mechanical linkages or other means of actuation, like hydraulic or pneumatic actuators, to achieve a pitching cycle that varies the attack angle of blades  410  sufficiently to shed vortices and achieve high dynamic lift coefficients.  
         [0027]     Propeller system  400  as described above has at least one blade that is mounted eccentrically to a main shaft  420 . A propeller having a single blade  410  is easier to analytically analyze since the wake field of one blade in a multi-blade propeller can affect the flow at other blades.  FIG. 5  shows four positions of a single blade  410  on base  415  with a view along the direction of main shaft  420 . Blade  410  can be rotated as described above with respect to base  415  and shaft  420 . In  FIG. 5 , a blade angle α defines the relative angle of a blade  410  to the tangent of the circle that blade shaft  412  follows as base  415  and main shaft  420  rotate. A rotation angle θ defines the position of blade  410  as base  415  rotates. For example,  FIG. 5  shows blade  410  at positions where rotation angle θ is 0, 90°, 180°, and 270°.  
         [0028]     The attack angle of blade  410  generally depends on blade angle α, rotation angle θ, an angular velocity ω of base  420 , and the direction and velocity of free stream fluid flow ν. More specifically, the orientation of the base line of a foil of blade  410  depends on angles α and θ. The fluid velocity at the foil, which is a vector sum of the blade velocity and the free stream fluid flow ν, depends on free stream fluid flow ν, rotation angle θ, and an angular velocity ω of base  415 . However, if the free stream fluid velocity ν is small when compared to the rotational velocity of a blade  410 , the attack angle of a blade  410  is approximately equal to angle α, and the pitching schedule for extracting energy from a constant free stream fluid flow or for momentum transfer in a specific can be a function rotation angle θ. Accordingly, for a low fluid velocity, mechanical linkages, gears or an active servo system with a fixed pitching schedule can vary blade angle α as a function of rotation angle θ, which has a defined relation to the background fluid flow vector ν.  
         [0029]     One pitching schedule for low fluid velocities sinusoidally varies blade angle α with a frequency equal to the rotational frequency of base  415 . This causes blade  410  to perform a combined pitching and plunging motion with respect to the flow, leading to an oscillatory blade path as shown in  FIG. 6 . To achieve dynamic lift and shed vortices, the amplitude of the pitching schedule is such that the angle of attack of blade  410  exceeds the stall angle twice during each rotation of base  415 . Accordingly, the pitch control system must be able to vary the blade angle by amounts sufficient to create dynamic stall vortices. In contrast, a linkage of the type shown in  FIG. 3  only allows for small angular angle of attack changes of the blade with respect to the tangent direction. Such small changes are sufficient to perform energy transfers when the Reynolds number is large and the desired flow pattern is that of attached flow, but do not produce dynamic stall vortices and the high efficiency provided by dynamic lift.  
         [0030]     The single blade configuration illustrated in  FIG. 6  provides lift forces F and moments that are periodic with twice the rotational frequency for a propeller with a single blade. During the oscillations, torque varies between zero at the shaft when the propeller blade is at its upper or lower position, e.g., at θ=90° or 270° and α=0, to the maximum torque when blade  410  reaches the greatest blade angle and shedding of vortices reaches maximum strength, e.g., at θ=0° or 180° and the magnitude of α is greater than the stall angle. A steadier torque may be provided by adding more blades, but careful consideration needs to be paid to scheduling the vortex shedding so that individual vortices from different blades do not cancel each other as they travel downstream.  
         [0031]     Sinusoidal pitch variation such as previously described is only one example of a pitching schedule. More generally, a propeller system such as system  400  of  FIG. 4  can measure fluid properties and/or angular velocity of base  415  and adjust the pitching schedule for current conditions, for example, to change the amplitude or time dependence of the variation of blade angle α to adapt to changes in free stream fluid flow direction or speed. In general, an adaptable pitching schedule is easiest to implement using an electronic servo control system such as illustrated in  FIG. 4 .  
         [0032]     The resulting pitching and plunging action of the selected pitching schedule of blades  410  can cause vortex shedding from blades  410  in a pattern precisely defined in space and time. In general, vortex shedding occurs as a result of pitching a foil past its stall angle of attack, at which point separation will occur as described above and illustrated in  FIG. 2 .  FIG. 2  shows that for a positive (clockwise) angle of attack of the foil  210  in a moving fluid, a clockwise rotating vortex  220  forms on the upper side of foil  220 . This vortex  220  will then be swept downstream with the mean fluid flow towards the trailing edge of foil  210 , entering the wake of foil  210 . Similarly, a negative angle of attack will produce a counter clockwise rotating vortex on the bottom side of the foil. Accordingly, a propeller system  400  that causes blades  410  to oscillate between extremes that are greater than the stall angle of attack while extracting energy from a fluid flow will generate a vortex pattern such as shown in  FIG. 7 .  
         [0033]     Propeller  400  can efficiently extract energy from the fluid when the pattern of shed vortices forms a pattern known as the von Karman Vortex Street. In general, the pitching schedule required to produce a von Karman Vortex Street pattern depends on various operating parameters such as the fluid flow speed and rotational speed of the propeller, but the pitching schedule should always be oscillatory. Similarly, the pitch schedule of the foils needs to be controlled precisely to achieve the desired vortex shedding pattern. The left side of  FIG. 7  illustrates uniform flow field incident on propeller system  400 . Propeller system  400  interacting with the flow field creates the von Karman Vortex street wake pattern and causes a net transfer of flow momentum to propeller system  400 , such that the time averaged flow field is that of a wake flow, shown on the right side of  FIG. 7 . The momentum transfer causes the propeller to turn in the indicated direction, making torque and therefore shaft power available at the main shaft. This shaft power can subsequently be used to drive devices like generators or pumps. This operating mode can therefore be that of a windmill or watermill.  
         [0034]     It should be noted that, a mill in accordance with an embodiment of the invention can be self starting provided the proper angles of attack are set by mechanical or other means. In particular, a programmable control system such as servo control system  400  in  FIG. 4  can orient the blades so that the current fluid flow causes torque, at which point the propeller will start rotating and variation of the blade angles can begin. Further, the self starting ability works even for a single blade located in any angular position of rotation.  
         [0035]     The momentum deficit due to energy extraction as illustrated by the flow profiles before and after propeller system  400  in  FIG. 7  imposes a net force in the downstream direction on propeller system  400 . The mounting structure of propeller system must be able to withstand this force.  
         [0036]      FIG. 8  shows the inverse operating mode of propeller system  400 , where thrust is generated. In  FIG. 8 , propeller  400  operates in propeller mode, with a time averaged jet type flow resulting. The flow is preferably the inverse of the von Karman vortex street. Inverse refers to the fact that the rotation direction of the vortices is the opposite of those shown in  FIG. 7 . Propeller  400  in this use will experience a net force in the upstream direction, but a motor or engine of some sort needs to provide shaft power to maintain rotation of propeller  400 . While a typical application of this operating mode is thrust generation in order to propel land, air or watercraft, propeller  400  can also be used as a fan device in order to deliver an fluid current for heating, cooling or pneumatic transport of particles, as well as providing a pressure rise like in any air or gas compressor. For the latter purpose, several of these propellers  400 A,  400 B, and  400 C may be cascaded in order as shown in  FIG. 9  to increase the overall pressure rise.  
         [0037]     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.