SPECIFICATION CROSS-REFERENCE TO RELATED APPLICATIONS

This PCT application claims the benefit under 35 U.S.C. §120 of U.S.

Provisional Patent Application Serial No. 61/554,687 filed November 2, 2011 entitled "SYSTEMS AND METHODS FOR PERFORMANCE IMPROVEMENT OF PROPELLERS AND TURBINE BLADES" and whose entire disclosure is incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

"Not Applicable"

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK

"Not Applicable"

FIELD OF THE INVENTION

This invention relates generally to active and passive power systems involving foil-shaped blades, e.g., propellers, turbine blades, etc., for use in fluid flows and more particularly to systems and methods for improving the performance of such foil structures in fluid flows.

BACKGROUND OF THE INVENTION

Foil-structures are used to produce a force which is normal to the direction of a fluid flow. The normal force can be used to propel air and water vehicles in an active role, or to be moved by a fluid flow when in a passive role, such as wind turbines and water turbines. Hence, the normal force on the foil can be due to active and passive motions fluid motions. An active motion is where the foil-structure is mechanically forced to move, thereby creating a relative flow adjacent to the foil.

Propellers are included in this category. Passive motions are those which are forced by passing fluid flows. Wind turbines and water (e.g., tidal water) turbines are included in this category.

The profiles (sections) of foil-structures are classified as either symmetric or asymmetric. See the sketches of each in Fig. 1. The symmetry is with respect to the line connecting the leading and trailing edge of the foil. This line is called the chord l (c), and is sketched in Figures IB, 1C and ID. Lifting foils, such as wings and hydrofoils, are normally asymmetric, such as shown in Fig. 1A(2) to generate a lift force. Symmetric foils, such as shown in Fig. 1 A(l), are normally used to control the motions of the host body, such as a ship rudder or a vertical stabilizer of an aircraft. In heavy fluids, such as water, low-pressure boiling (cavitation) can occur. Supercavitating foils, such as shown in Fig. 1A(3), are designed for this normally high-speed condition. The lift (L in Fig. ID) force on the foil is normal to the relative flow direction; whereas, the drag force (D in Fig. ID) is in-line with the flow direction (the direction of the relative velocity V).

The three-dimensionality of a foil geometry includes the taper (where the chord decreases from a maximum value at its root - the point of attachment to another body-part), the dihedral and anhedral (where the tip of the foil is respectively higher or lower than the root) and the sweep (where the chord position on the tip is either forward or aft of the similar chord position on the root). The propeller blades, as in Fig. 2, can have three of the three-dimensional properties: taper, anhedral and sweep. This is also true of turbine blades. The rudder in the figure has taper only.

In Fig. 3 three flow separation conditions on a symmetric foil are shown and are presented to introduce the relevant terminology. In this case, the foils are assumed to be fixed in a steady, uniform flow. In Fig. 3A, five possible regions of viscous effects are shown, namely, the laminar boundary-layer, the transitional boundary-layer, the turbulent boundary-layer, the laminar sublayer and the wake. As can be seen the region of high shear adjacent to the symmetric foil is called the boundary-layer. The flow in this region is always laminar close to the leading edge. When the ratio of the external inertial force and the shear force exceeds a certain value, the flow in the boundary-layer becomes oscillatory. This ratio is usually represented by the dimensionless number called the Reynolds number. For foils, this parameter is based on the chord, and mathematically defined by R _e = V _c /v (1)

Here, V is the design relative speed, c is the chord and v is the kinematic viscosity of the ambient fluid. The oscillations occurring in the boundary-layer are commonly referred to as Tollmien-Schlicting (T-S) waves. These waves can be stable or unstable, depending on the conditions. When the T-S waves become unstable, turbulence occurs downstream from the origin of the T-S waves.

In a relatively slow flow, where there is a small angle of attack, a, a separation "bubble" might occur, which is a localized laminar wake flow, as illustrated in Fig. 3B. As can be seen in that figure a separation bubble occurs close to the leading edge on the suction side of the foil. This is formed when the separation streamline reattaches to the foil. As the attack angle (a) increases, the lift force (L) on the foil decreases; while, the drag force (D) increases. At an extreme angle, such as shown in Fig. 3C, the wake nearly covers the suction side of the foil. In particular, as can be seen the angle of attack, a, is so large that separation occurs just downstream of the leading edge and at the trailing edge of the suction side. In this condition, only the drag (£>) is largely dominant on the foil, since there is little or no lift (the lift (L) is nearly zero), and the drag (D) is large. For an aircraft or water craft, this is condition is one of near-complete stall.

As will be appreciated by those skilled in the art, the control of aircraft foil- structures to avoid the effects of stalling is of considerable importance for a safe flight. For example, on landing the pilot adjusts the angle of attack of the airfoil to cause the airfoil to stall just at the point that the aircraft's landing gear meets the runway. If the lift and drag of that airfoil could be controlled during landing without necessitating entering a stall condition, landing could be achieved more smoothly and safely. So too, the control of the steering of a ship and other water-born vessel operating at a slow speed is also dependent upon the angle of attack of the rudder (foil). In particular, if the angle of attack is such that the rudder is stalled, the rudder effectively becomes a drag and thus loss of steering occurs.

With respect to a propeller and a turbine, both have design (operational) speeds. That is, there is a rotational speed for each device which is optimum at a design condition. In the off-design conditions, propellers and turbines becomes rather inefficient in their performances partially due to flow separation. Thus, if the propeller is operated at a low speed, then propulsion is compromised. Similarly, if a turbine operates at a low speed, then the energy-conversion efficiency is compromised.

Thus, there presently exists a need for a system which addresses the foregoing problems of the prior art. The subject invention addresses that need by providing a means and method for enabling the fluid dynamic forces on propellers blades and turbine blades operating at slow speeds to be readily controlled without the stalling.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is disclosed a system for introducing acoustic waves into fluid flow (e.g., air or water), comprising Tollmien-Schlichting (T-S) waves, about an aerodynamic surface or a hydrodynamic surface (e.g., an aircraft wing, a blade structure such as a helicopter rotor blade, or a propeller, turbine, hydrofoil, etc.) to improve the lift-drag ratio and thereby enhance the performance of the aerodynamic or hydrodynamic surface in the fluid flow. The system comprises: at least one acoustic transducer (e.g., a speaker, woofer, etc.) positioned on or adjacent the aerodynamic or hydrodynamic surface (e.g., an aircraft wing, a blade structure such as a helicopter rotor blade, propeller, turbine, hydrofoil, etc.) to direct the acoustic waves along the aerodynamic or hydrodynamic surface; and a controller that activates the at least one acoustic transducer to emit the acoustic waves along the aerodynamic or hydrodynamic surface to cause the Tollmien- Schlichting (T-S) waves to become unstable and to cause a point of flow separation to move towards the trailing edge of the aerodynamic or hydrodynamic surface, thereby reducing the drag force on said aerodynamic or hydrodynamic surface and increasing the lift force on the aerodynamic or hydrodynamic surface.

In accordance with another aspect of this invention there is disclosed a method for improving the lift-drag ratio of an aerodynamic or hydrodynamic surface (e.g., an aircraft wing, a blade structure such as a helicopter rotor blade, or a propeller, turbine, hydrofoil, etc.), wherein the aerodynamic or hydrodynamic surface has a trailing edge in a fluid flow in which Tollmien-Schlichting waves occur adjacent the aerodynamic or hydrodynamic surface. The method comprises: positioning at least one acoustic transducer (e.g., a speaker, woofer, etc.) on or adjacent the aerodynamic or hydrodynamic surface; activating the at least one acoustic transducer to emit acoustic waves along the aerodynamic or hydrodynamic surface to cause the Tollmien-Schlicting (T-S) waves to become unstable and to cause a point of flow separation to move towards the trailing edge of the aerodynamic or hydrodynamic surface, thereby reducing the drag force on said aerodynamic or hydrodynamic surface and increasing the lift force on the aerodynamic or hydrodynamic surface. DESCRIPTION OF THE DRAWING

Fig. 1A(1) is an illustration of a symmetric foil section for use in a fluid flow for control;

Fig. 1A(2) is an illustration of an asymmetric foil section for use in a fluid flow for lift;

Fig. 1A(3) is an illustration of a supercavitating foil section;

Fig. IB is an illustration of the geometry of a foil;

Fig. 1C is an illustration of a foil showing various features and notation relating thereto;

Fig. ID is an illustration of the fluid dynamic forces on a foil;

Fig. 2 is a partial view of the propeller - rudder orientation of a ship;

Fig. 3 A is an illustration of flow separation conditions, e.g., five possible regions of viscous effects, on a symmetric foil;

Fig. 3B is an illustration of another flow separation condition, e.g., a separation bubble, on a symmetric foil;

Fig. 3C is an illustration of another flow separation condition, e.g., a symmetric foil in a stalled state;

Fig. 4 is an exemplary acoustic system for implementing an acoustic tripping of the Tollmien-Schlicting (T-S) waves on a variety of apparatus;

Fig. 5 depicts one exemplary system of the subject invention for introducing sound waves into a laminar boundary-layer at the propeller of a ship to produce turbulent boundary-layer flow by acoustic tripping;

Fig. 6A depicts another exemplary system of the subject invention for introducing sound waves into a laminar boundary- layer along the wing of an aircraft to produce turbulent boundary-layer flow by acoustic tripping;

Fig. 6B depicts another exemplary system of the subject invention for introducing sound waves into a laminar boundary-layer along the rotors of a helicopter to produce turbulent boundary-layer flow by acoustic tripping;

Fig. 7 depicts another exemplary system of the subject invention for introducing sound waves into a laminar boundary-layer along the rotors of a wind turbine to produce turbulent boundary-layer flow by acoustic tripping;

Fig. 8A depicts another exemplary system of the subject invention for introducing sound waves into a laminar boundary-layer along a hydrofoil of a boat to produce turbulent boundary-layer flow by acoustic tripping; Fig. 8B is an enlargement of the area indicated in Fig. 8A showing the positioning of the speakers of the acoustic system on the hydrofoil; and

Fig. 9 depicts another exemplary system of the subject invention for introducing sound waves into a laminar boundary-layer along the rotors of a water turbine (e.g., a tidal water turbine) to produce turbulent boundary-layer flow by acoustic tripping.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

"Acoustic tripping" refers to the introduction of sound waves into a laminar boundary-layer on a body in order to prematurely trip the laminar boundary-layer flow to produce turbulent boundary-layer flow. By doing so, the flow separation producing either a wake or a separation "bubble" (enclosing wake-type flows) is moved aft. This, in turn, increases the lifting force on the body and reduces the drag. As discussed by McCormick, Knese and Korman (2010), the first recorded recognition of acoustic tripping was due to an observation by a German aerodynamicist, F. W. Schmitz inl942. Mr. Schmitz noticed that blowing a whistle near a wind tunnel in which a model airfoil was mounted caused a reduction in the critical Reynolds number value. In the following decade, the phenomenon of acoustic tripping was studied in aerodynamic experiments at the University of Notre Dame and elsewhere. One result was a video of a smoke tunnel test that demonstrated how audible sound could cause the disappearance of the laminar wakes downstream of airfoils and spheres. The video was produced at Notre Dame by Brown in 1958. The other smoke- and wind-tunnel studies of acoustic tripping include those reported by Mueller and Batill (1982), M. E. Goldstein (1984), W. S. Saric and E. Reshotoko (1998) and A. M. Gurun (2006). Mueller and Batill (1982) report that the flow fields about bodies could be changed by a number of frequencies at the same Reynolds numbers. Goldstein (1984) speculates that the acoustical effects are to hasten the instability of the wave systems in the laminar boundary layer, the Tollmien-Schlichting (T-S) waves. Zaman and McKinzie (1989) performed a series of tests on two-dimensional airfoils to study the control of laminar separation using acoustic excitation. This cause-and-effect discussed by Goldstein (1984) was still speculative as of the beginning of the present millennium, according to Grundy, Keefe and Lowson (2001).

McCormick, Knese and Korman (2010) focused their study effects of acoustic tripping on the fluid dynamic forces on airfoils and hydrofoils. The force reduction in the airfoil study exceeded 50% at a Reynolds number (based on the chord) ^' of approximately 31,500 and a sound frequency between 100Hz and 1kHz. A lower force reduction value was obtained on a hydrofoil at a Reynolds number of about 63,100 and a similar frequency range. The hydrofoil study produced a drag reduction slightly over 5% for a frequency in the audible range of 100Hz - 1kHz. The McCormick-Knese-Korman study was the first in which acoustic tripping was experimentally obtained in water.

Because acoustic tripping is considered to be a low-Reynolds number phenomenon, the phenomenon has drawn only scientific interest, with the exception of the McCormick-Knese-Korman study. The subject invention makes use of acoustic tripping, i.e., the introduction of audible sound into the flows about foil- structures (propellers, rudders, hydrofoils, airfoils, turbine blades, etc.), in order to improve the lift-drag ratio, and thereby the performance of these appendages.

The performance of a foil in its designed task (lift, motion control, etc.) is diminished as the point of separation approaches the leading edge of the foil. This might occur on a rudder of a ship traveling at a low speed with a large rudder angle of attack. It also occurs on aircraft wings when in a landing scenario. In fact, as discussed above, prior to set-down, aircraft are normally stalled intentionally. Another example concerns propellers. As discussed above in off-design conditions, a propeller becomes rather inefficient in its performance partially due to flow separation.

The subject invention uses acoustic tripping by introducing sound waves into flows about foils to cause the Tollmien-Schlichting (T-S) waves to become unstable. This causes the point of flow separation (see Fig. 3a) to move towards the trailing edge of the foil, thereby reducing the drag force (D) and increasing the lift force (L) on the foil. Thus, for aircraft one or more audio transducers (e.g., speakers, including woofers) can be mounted on or adjacent the wing of the aircraft and operated during landing to cause the Tollmien-Schlichting (T-S) waves to become unstable and move toward the trailing edge of the wing. By so doing one can land the aircraft with true control of the lift simply by the speed of the aircraft and not the angle of attack of the wing. The positioning of the separation point toward the trailing edge of the foil will be a function of the amplitude and frequency of the acoustic waves (audible sound) produced by the speakers. To that end, any suitable means (not shown) is provided coupled to the transducers (e.g., woofers) to adjust the frequency and amplitude of the signals provided to them so that they produce the audible sound.

In Fig. 4 one exemplary embodiment of a system 20 constructed in accordance with this invention is shown. The system 20 comprises a speaker system that comprises at least one acoustic transducer, e.g., a speaker 24 (e.g., a woofer) that operates under the control of a controller 22 (e.g., microcontroller or microprocessor) 22 which also controls a frequency synthesizer 22A which generates the particular acoustic tripping frequency. The frequency synthesizer 22A permits the adjustment of the frequency and amplitude of the acoustic wave. The output of the frequency synthesizer 22 A is coupled to the speaker 24 via a bus or other conductor 25 via respective digital-to-analog converters and amplifiers, the combination of which is indicated by 26. The processor 22 activates the speaker 24 with a particular frequency, as discussed previously, and such activation may be timed or it can be initiated based on sensor inputs from sensor(s) 28 that are passed through an analog-to-digital converter (A/D) 23. The location of the speaker 24 is on the aerodynamic or hydrodynamic element itself (e.g., a wing of an aircraft, a rotor blade of a helicopter or wind turbine, a hydrofoil etc.,) or the immediately adjacent vicinity (e.g., along a host body such as a fuselage of an aircraft, or a hub of a turbine, or the hull of a boat, etc.) all of which is indicated by 21. Thus, the term "aerodynamic surface" and "hydrodynamic surface" as used throughout this Specification means the aerodynamic element or its immediately adjacent vicinity and the hydrodynamic element or its immediately adjacent vicinity. The preferred embodiment 20 utilizes a plurality of speakers 24 that can be distributed on the aerodynamic surface or the hydrodynamic surface 21.

The following are examples of the system 20 being implemented on various devices, by way of example only. It should be noted that the phrase "upper surface" indicates the side of the aerodynamic airfoil or hydrodynamic hydrofoil that experiences the decreased air or water pressure, respectively, during movement of the aerodynamic or hydrodynamic element and is also referred to as the "suction side."

In Fig. 5, one exemplary embodiment of the system 20 constructed in accordance with this invention is shown in place on a large ship 10. In particular, as can be seen a plurality of audio transducers 24 (e.g., speakers, including woofers) are mounted on the ship's hull 12 forward and above the propeller 14. The effective propulsion of that ship 10 at very slow speeds can be achieved with the propeller 14 rotating slowly by applying the sound waves to the propeller to thereby reduce the blade-wake. This results in a high lift-to-drag ratio, so that the propeller maintains its ability to be an effective propulsive appendage of host vehicle. Moreover, the acoustic waves also make the propeller 14 more efficient notwithstanding the fact that it is rotating at much lower speed than its optimal rotational speed. The same can be said of a turbine blade operating in a low flow. Although not shown, the sensor(s) 28 that can be used here is a nautical speed sensor.

In Fig. 6A, another exemplary embodiment of the system 20 constructed in accordance with this invention is shown installed as part of a wing 16 (or other aerodynamic surfaces such as a rudder or elevator/stabilator) of an aircraft and integrated with the flight control system (FCS). The plurality of speakers 24 are dispersed along the upper surface of the wing 16. Although not shown, the sensor(s) 28 that may be used here is a radar altimeter or barometric altimeter, along with an airspeed sensor. Thus, when the aircraft is entering a landing mode as it flies in the direction of 16A, the system 20 is automatically activated to effect acoustic tripping of the T-S waves. Similarly, as shown in Fig. 6B, the system 20 can be installed in a helicopter 11 (e.g., the CH-47 Chinook Helicopter) along the upper surface of the rotor blades 13. Again, although not shown, the sensor(s) 28 that may be used here is a radar altimeter or barometric altimeter, along with an airspeed sensor. Thus, when the helicopter is entering a landing hovering mode, with the front rotor rotating in the direction indicated by the arrows 13 A and the rear rotor rotating in the direction indicated by the arrows 13B, the system 20 is automatically activated to effect acoustic tripping of the T-S waves.

In Fig. 7, another exemplary embodiment of the system 20 constructed in accordance with this invention is shown installed as part of a wind turbine 17 whose rotor blades 19 rotate in the direction 19A. The plurality of speakers 24 are dispersed along the upper surface of the turbine blades 19. Although not shown, the sensor(s) 28 that may be used here is an angle of attack (viz., blade pitch) and/or rotor speed sensor. Alternatively, the plurality of speakers 24 (shown in phantom) may be located on the hub 23 of the turbine and directed toward the upper surface of a respective turbine blade 19.

In Figs. 8A-8B, a further exemplary embodiment of the system 20 constructed in accordance with this invention is shown installed as part of a hydrofoil 32 on a boat 30. As shown most clearly in Fig. 8B, the plurality of speakers 24 are dispersed along the upper surface of the hydrofoil 32. The speakers 24 would also be dispersed on the inward surface of the hydrofoil side segments 34A and 34B in a similar manner, as shown in Fig. 8A. Although not shown, the sensor(s) 28 that can be used here is a nautical speed sensor.

In Fig. 9, another exemplary embodiment of the system 20 constructed in accordance with this invention is shown which includes a water turbine 36 submerged in water 2. The water turbine 36, like the wind turbine 17, may comprise a plurality of rotor blades 38 that rotate to generate electricity due to tidal motion when the turbine 36 is submerged in water as shown in Fig. 9; the rotor blades 38 rotate in the direction 38 A. A plurality of speakers 24 is dispersed along the upper surface of each rotor blade 38. Although not shown, the sensor(s) 28 that may be used here is an angle of attack (viz., blade pitch) and/or rotor speed sensor. Alternatively, the plurality of speakers 24 (shown in phantom) may be located on the hub 40 of the turbine and directed toward the upper surface of a respective turbine blade 38.

It should be pointed out at this juncture that the exemplary embodiments shown and described above constitute only a few examples of a large multitude of systems that can be constructed in accordance with this invention. Moreover, the frequency range may be larger than that disclosed above, e.g., the lower frequency may be 50Hz or below.

Although not shown, it is within the broadest aspect of the present invention to include a continuous acoustic transducer that runs the span from rotor or blade root to tip (meaning from where the rotor/blade is secured to its free end), rather than a plurality of discrete acoustic transducers 24 (e.g., speakers). For example, the continuous acoustic transducer could form a single acoustic transducing element that emits the acoustic signal continuously along the surface 21. Similarly, the acoustic transducers shown with regard to the ship hull (e.g., Fig. 5) could be replaced with a continuous transducing element rather than discrete ones.

Without further elaboration the foregoing will so fully illustrate my invention that others may, by applying current or future knowledge, adopt the same for use under various conditions of service.