Abstract:
A system for producing variable amplitude pneumatic pulses, such as may be used to prevent flow separation on airfoil and diffuser surfaces to control stalling, utilizes a source of pressurized air which is regulated and fed to parallel outlet lines. Each outlet line has a control valve therein attached to a controller for activation of the valves. The first parallel line leads to an actuator which supplies the pulsed air to a desired surface. The second parallel line leads to the atmosphere. By cycling the valves at about ninety degrees out of phase, pneumatic pulses of large amplitude are obtained with a simple device capable of being transported on the aircraft.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to fluid flow control. The present invention relates specifically to a practical pneumatic oscillator for generating large amplitude pressure and velocity oscillations suitable to prevent stalling on airfoil and diffuser surfaces. 
     2. Discussion of the Related Art 
     Active air flow control is an area of intense research in the aerospace industry. In particular, air flow control using pulsed air injection through slots and holes on various surfaces of the aircraft and aircraft engines is known to improve performance when operating near stalled conditions. Air flow control with pulsed air injection has been demonstrated under laboratory conditions to control forebody flow vortices on aircraft and missiles at high angles of attack, delay the stall of wings on aircraft, enhance the lift characteristics of helicopter blades, suppress the stall in engine compressor inlets, and enhance the performance of vanes in axial flow compressors. 
     The aerospace industry is attempting to transition laboratory results of flow control experiments into prototype applications. The principal difficulty in most cases has been to produce large amplitude velocity oscillations at the air-to-surface interface with equipment that is practical to be carried in the aircraft. In the laboratory one typically uses large loudspeakers and acoustic drivers with massive power amplifiers, or a large rotating valve with a drive mechanism to produce the pressure oscillations. One problem with the speaker approach is that the equipment needed is too large. Secondly, the speaker techniques are limited by the push-pull nature of the loud speaker diaphragms to low frequency bandwidths which produce only small amplitude oscillations superposed on a steady jet of air. Similarly, known techniques utilizing a single valve in a single pressurized line produce only small amplitude oscillations superposed on a steady jet of air. When the frequency of the single valve opening goes up, the amplitude of the pressure changes goes down. What is needed is a device that is sufficiently compact and simple enough to be practical for implementation on aircraft while producing pressure oscillations near the maximum possible amplitude. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above problems by providing both a device and a process for using the device. In broad outline, the device comprises a pair of activatable valves connected in first and second parallel lines downstream from a pressurized air supply. “Parallel” will be understood to mean that the lines are interconnected and could share a parallel flow, and not that the lines must be physically parallel. The downstream side of the first valve is connected via a suitable air conduit, or actuator line, to an actuator outlet which provides pulsed air, to the controlled surface. The controlled surface may be, e.g., an airfoil surface or a diffuser surface such as a vane or compressor as may be found on turbine engines. This conduit between the first valve and the actuator may be referred to as the actuator line. The actuator line typically leads to the controlled surface through a thin slot, sometimes herein called the actuator outlet, of smaller dimensions than the actuator line. Thus, by way of explanation and not limitation, it is believed that as pressure rises in the actuator line, air velocity will increase through the actuator outlet at the controlled surface. The downstream side of the second valve can be either open to the atmosphere or connected to a vacuum line or other source, or area, of lower pressure. A controller is used to set the activation and frequency of oscillation of the two valves and the phase difference between the two valves. In some embodiments the duty cycles of the valves may also be controlled. 
     Pressurized air is suitably provided by an onboard source such as may be commonly found in the compressor stages of jet engines or otherwise provided, such as by a dedicated onboard compressor. The supply line for the pressurized air is then preferably fed to a pressure regulator to supply an output stream of known pressure. The regulator may provide a fixed value pressure or may be adjustable, such as through control signal input from the controller function. The proper operation of the device then produces the needed amplitude of pneumatic oscillations at the actuator. 
     The process, or operational, component of the invention uses offset timing of the second valve, from activation of the first valve, to create an air-ejector effect through the second line thereby decreasing pressure in the actuator line. Typically the opening and closing of the second valve will lag the opening and closing of the first valve by about a quarter cycle, or about ninety degrees. The large amplitude oscillations occur because the second valve acts like an air ejector when it is open, which lowers the pressures in each parallel line, and concurrently increases velocity through the second line by the Bernoulli principle, and particularly for purposes of the present invention, lowers the pressure in the first parallel line. The combined high and low pressure phases of the two line system, due to the offset opening and closing of the valves, produce pressure oscillations in the first, or actuator line, of an amplitude and frequency sufficient for creating air velocity pulses at the actuator outlet to control air flow at the airfoil or diffuser surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a device according to one embodiment of the present invention. 
     FIG. 2 is a graph of a pressure wave of the present invention superposed over the operating cycles of the first valve and the second valve. 
     FIG. 3 is a graph comparing pulse performance of an embodiment of the present invention and a system utilizing a single valve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As seen in FIG. 1, a pneumatic oscillator  11  according to the present invention comprises an air supply line  13  which is connected to a source of pressurized air  14  such as an engine compressor typically found in a jet aircraft engine or a dedicated compressor. The air supply line  13  leads to a pressure regulator  15  for control of the air flow to primary and secondary air lines  17 ,  19 , respectively, which are parallel and downstream from the pressure regulator  15 . The pressure regulator  15  may be variably controlled for selectable pressure as indicated by the dashed signal line  21  leading from an electronic controller  23 . Alternatively, the pressure regulator may be set to a fixed regulation point and not connected to the controller  23 . 
     Connected across the primary air line  17  is a first valve  25  for controlling, e.g., opening and closing, air flow through the actuator line section  27  of the primary air line  17  downstream from the first valve  25 . The first valve  25  may be an electrically activated solenoid although other suitable valves or means of activation may be utilized according to the known skill in the art without deviating from the present invention. The electronic controller  23  is shown as electrically connected by a signal line  26  for activation of the first valve  25 . The construction and arrangement of a suitable controller are considered to be within the skill of the art and are left to the individual designer for implementation. Other suitable sensing and control apparatus for monitoring and adjusting system performance, e.g., to add efficiency, safety, or redundancy to the system, may of course be added to various control points or subsystems of the present invention. 
     The actuator line section  27  of the primary air line  17  supplies an actuator  29  which is the operative end of the pneumatic oscillator  11  and may, for example, have an outlet which is a thin slot  28 , or tube with a series of holes, leading to the surface of an aircraft wing  30  (in phantom) or rotor needing pulsed air to improve its flight characteristics. 
     Connected across the secondary air line  19  is a second valve  31 , preferably of the identical type as the first valve  25 , for controlling, e.g., opening and closing, air flow through the secondary air line  19 . The second valve  31  is shown as connected to the electronic controller  23  through a signal line  32  for activation thereof in conjunction with the first valve  25 , as further explained below. 
     A low pressure section  33  of the secondary air line  19  is located downstream from the second valve  31  and is open to the atmosphere for simplicity of construction in the preferred embodiment. Alternatively, the low pressure section  33  of the secondary line  19  may open to a vacuum line  34  (in phantom) as may be provided on an aircraft. “Vacuum” as used herein will mean any area of negative pressure. Other areas having a pressure lower than that of the actuator line section  27  may also be used within the meaning of the present invention depending on the oscillation performance demanded of the system. 
     Referencing FIGS. 1 and 2, FIG. 2 shows the opening and closing duty cycles  35  of the first and second valves  25 ,  31  superposed for a comparison with a graph of the pressure wave  37  in the actuator line section  27 , with the X axis being a time line of 0.1 seconds, and the Y axis being pressure in pascals at ten to the fourth power. The solid line  39  shows the first valve cycle and the dotted line  41  shows the second valve cycle. The phase difference between the first valve and the second valve may be optimized to give the largest pressure amplitude in the actuator line section  27 . The second valve, which controls the low pressure phase of the cycle, is delayed by approximately 90 degrees relative to the first valve. The valves are oscillating at about 30 hertz and have about a 50 percent duty cycle. 
     At time T 1  on the pressure wave graph both the first valve and the second valve are fully open, representing the lowest pressure on the pressure wave  37 . It will be noted that the pressure drops briefly below zero pascals gauge pressure indicating that, when the second valve opens the secondary line  19  fully to the atmosphere, air is actually being withdrawn from the primary line  17  for a brief interval. 
     At time T 2 , the first valve is closed and the second valve is open, raising the pressure on the pressure wave  37 , i.e., in the actuator line section  27 . At time T 3 , both the first valve and the second valve are closed, representing the highest pressure at the actuator  29 . At time T 4 , the first valve is open and the second valve remains closed, decreasing air pressure at the actuator. At time T 5 , both valves have opened again, bringing the pressure to its lowest point. 
     FIG. 3 compares the performance of an embodiment of the present invention to a single-valve arrangement, as further explained below. A graph  43  of the line pressure in the actuator line  27  downstream of the first valve  25  is shown when the valves are oscillating at 30 Hz. The pressure graph  45  of the single-valve configuration, also oscillating at 30 Hz, is shown as the dotted line. The peak-to-peak pressure amplitude of the present invention, at about −1 to about +21 kilopascals, is between four and five times larger than the single-valve system which ranges from about 14 to about 19 kilopascals. 
     Double Valve Experimental Details 
     Air was supplied from a shop air supply  14  at 100 psig to an Ingersoll Rand pressure regulator  15 . The valves  25 ,  31  were connected between the regulator  15  and an airfoil model  30  with 0.5 inch inner diameter rubber tubing. The airfoil model  30  had an actuator outlet slot of one-eighth inch wide by twelve inches long. The pressure regulator  15  reduced the pressure to the solenoid valves  25 ,  31  and controlled the overall flow rate through the system  11  to 13.6 cfm. Valve  31  opened to the atmosphere. Measurements of the fluctuating pressure were made with a Kulite pressure transducer, model no. xcs-093-5g, from Kulite Semiconductor Products, Inc. of Leonia, N.J. The Kulite transducer was located at the actuator  29 . A Setra model no. 239 pressure transducer, from Setra System, Inc. of Boxborough, Mass., was used to measure the pressure in the tubing downstream of the regulator  15 . The valves  25 ,  31  used were 24 VAC solenoid valves manufactured by WaterMaster from Orbit Irrigation Products, Inc. of Bountiful, Utah. The valves were opened and closed by computer control using LABVIEW™ software from National Instruments of Austin, Tex. The time delays between valve openings and duty cycles were adjusted with the LABVIEW™ software. The data signals from the pressure transducers and the signals to the solenoid valves were also acquired with the LABVIEW™ program. The data shown in FIG. 2 were acquired at a total flow rate of 13.6 cfm. 
     Single Valve Experimental Details 
     The single valve experiments were identical to the double-valve experiment except the valve  31  opening to atmosphere was completely closed. 
     Having thus disclosed a system including apparatus and method of operation for a pneumatic pressure oscillator having two parallel valve-operated lines for imposing large amplitude pressure oscillations on a stream of pressurized air, such as may be suitably employed for active air flow control on airfoil and diffuser surfaces, it will be appreciated that many aspects of the present apparatus and method may be adjusted to alter the performance characteristics of the system of the present invention without deviating from the spirit of the invention. The present invention is intended to be limited only by the appended claims.