Systems and methods for actively controlling surface drag using selective wave generation

Abstract

A system includes a surface (308), an actuator (304), and a controller (320). The surface (308) has a fluid flowing on the surface. The actuator (304) is coupled to the surface (308) to move the surface relative to the fluid. The controller (320) causes the actuator (304) to cause the surface to generate a surface wave that modifies a drag force in the fluid. The actuator (304) is capable of causing the surface to generate a Love wave.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure claims the benefit of priority to U.S. Provisional Application No. 62 / 773,805, filed on November 30, 2018, entitled “SYSTEMS AND METHODS OF ACTIVE CONTROL OF SURFACE DRAG USING SELECTIVE WAVE GENERATION,” the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates generally to the field of fluid flow on surfaces. More specifically, the present disclosure relates to systems and methods for actively controlling surface drag using selective wave generation. Background Art

[0004] Surface drag is the force that acts against the relative motion of an object moving through a fluid. Air, water, and ground transportation platforms experience some degree of surface drag, which results in slower speeds and lower fuel efficiency. Summary of the invention

[0005] One embodiment relates to a system. The system includes a surface, an actuator, and a controller. The surface has a fluid flowing on the surface. The actuator is coupled to the surface to move the surface relative to the fluid. The controller causes the actuator to cause the surface to generate a surface wave that modifies a drag in the fluid. The actuator causes the surface to generate a Love wave.

[0006] Another embodiment is directed to a method. The method includes coupling an actuator to a surface having a fluid flowing on the surface. The method includes causing the actuator, by a controller, to move the surface relative to the fluid to generate a surface wave that modifies a drag force in the fluid.

[0007] The present disclosure is illustrative only and is not intended to be limiting in any way. Other aspects, inventive features and advantages of the apparatus and / or processes described herein, which are limited only by the claims, will become apparent in the detailed description set forth herein in conjunction with the accompanying drawings, in which like reference numerals refer to like elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a schematic diagram of a transverse surface wave according to an embodiment of the present disclosure.

[0009] Figure 2A-2B is a schematic diagram showing the Love wave phenomenon according to an embodiment of the present disclosure.

[0010] Figure 3 is a schematic diagram of a fluid control system for selectively generating surface waves according to an embodiment of the present disclosure.

[0011] Figure 4A-4B can be used to implement Figure 3 Schematic diagram of the surface acoustic wave device for the fluid control system.

[0012] Figure 5 is a schematic diagram of a pipe flow system for realizing surface wave generation according to an embodiment of the present disclosure.

[0013] Figure 6 is a flow chart of a method of selectively generating surface waves according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0014] Before turning to the drawings that illustrate exemplary embodiments in detail, it should be understood that the present disclosure is not limited to the details or methods set forth in the description or shown in the drawings. It should also be understood that the terminology used herein is for descriptive purposes only and should not be considered limiting.

[0015] Referring generally to the accompanying drawings, the systems and methods described herein can be used to actively control drag on a surface. Drag is a force that acts in opposition to the relative motion of any object moving relative to a surrounding fluid. Drag manipulation techniques have a wide range of applications in improving the performance of systems involving fluid flow over a surface. For example, efficiency and speed can be improved in all modes of air transportation, including commercial aircraft, military aircraft, rockets, drones, and general aviation. Reducing drag can also improve the efficiency of ground transportation, such as in trains, cars, and tractor trailers. Reducing drag can also benefit water transportation, including shipping, personal boating, and undersea transportation. Reducing drag can also improve the efficiency of transporting fluids through pipelines, such as in natural gas or oil pipelines. Manipulating drag can be valuable over a wide range of Reynolds numbers, including relatively high Reynolds numbers experienced by shipping through water and relatively low Reynolds numbers experienced by unmanned aerial vehicles. Selectively increasing drag can also be useful for braking, steering, and other controls of a body moving through a fluid, such as a movable platform (e.g., a vehicle). Being able to actively control the surface drag of an object not only allows surface drag to be minimized when efficiency or speed is most needed, but also allows for better steering and braking capabilities by selectively increasing surface drag.

[0016] Air acts as a viscous fluid at subsonic speeds. An object moving through the air can collect a group of air particles that tend to be pulled along as the object moves. Due to molecular adhesion, air particles near the surface of an object tend to move at approximately the same speed as the object. When a flat plate, airfoil, or other object moves through a free stream at a given relative speed, molecular adhesion causes a boundary layer of air with a relative velocity lower than the relative free stream velocity to form adjacent to the surface of the object. A boundary layer can be a layer of air surrounding an object, wherein the relative velocity of the molecular layer closest to the object is zero or close to zero, and wherein the relative velocity at successively farther points from the object increases until it approaches the relative velocity of the free stream, at which point the outer limit of the boundary layer is reached. For example, for an airfoil, a boundary layer can include an interface between the airfoil and the mass of air surrounding the airfoil. The boundary layer can be relatively small compared to the size dimensions of the object. The difference between the relative velocity in the boundary layer region and the free stream velocity contributes to drag. Therefore, increasing the relative velocity in the boundary layer region can reduce drag, and conversely, reducing the relative velocity in the boundary layer region can increase drag. Drag can be caused by disturbances in the flow, which hinder the flow near the surface. Such disturbances are not damped as the Reynolds number increases, which can lead to interactions that increase the disturbance, such as turbulent eddies. The disturbances can flow from the surface to the outer boundary layer, which can cause continuous disturbances. The present solution can control surface drag by controlling the difference between the speed of the free stream and the relative speed of the boundary layer, such as by interrupting the generation of disturbances near the surface.

[0017] Turbulence in the boundary layer can contribute to drag. A turbulent boundary layer can have a large number of eddies that transfer momentum from the faster moving outer portions of the boundary layer to the relatively slower portions closer to the surface. As such, a turbulent boundary layer can have greater surface friction than a laminar boundary layer, which has a smoother flow and is generally free of these eddies. Making the boundary layer less turbulent can be used to reduce drag, whereas making the boundary layer more turbulent can be used to increase drag.

[0018] Overall reference Figure 1-6 , fluid control systems and methods according to the present disclosure can be used to generate transverse surface waves, including using Love waves, to actively control surface drag corresponding to a fluid flowing over a surface. In some embodiments, the fluid control system includes a surface, an actuator, and a controller. The surface has a fluid flowing on the surface. The actuator is coupled to the surface to move the surface relative to the fluid. The controller causes the actuator to cause the surface to generate a surface wave that modifies the drag in the fluid. The actuator causes the surface to generate the Love wave.

[0019] Figure 1A diagram 100 of a transverse surface wave is depicted, which can be used to modify surface drag. A transverse surface wave can have characteristics such as a wavelength λ and frequency ω propagating in a propagation direction (e.g., the direction of mean flow) and an amplitude A according to which the wave moves transversely (e.g., perpendicular to) the propagation direction. However, the effectiveness of transverse surface waves has been demonstrated primarily through numerical or computational methods, and existing implementations may lack feasibility. For example, a transverse surface wave can be approximated by a series of discrete plates. However, the movement of one or more plates relative to each other can require considerable power and generate undesirable flow fields at the interfaces of the plates, thereby reducing the desired effect on surface drag. The large number of interfaces can present significant wear, reliability, and deployment issues relative to a continuous surface. Kagome lattices can alleviate some of these issues in limited cases (e.g., at low frequencies and flow rates), but involve bulk motion of the membrane, which results in undesirable out-of-plane motion. Implementations based on electroactive polymers have been attempted, but suffer from similar problems and are limited in service life and adjustability. Existing actuation methods generally do not scale well in power or size as flow speeds and frequencies increase.

[0020] Wave motions that can be generated in solid matter, such as Love waves, are characterized by side-to-side undulations perpendicular to the direction of propagation, similar to snake-like motion, but with amplitude decreasing with depth. Love waves are a form of surface wave. Rayleigh waves are waves that move up and down perpendicular to the surface like water waves. In contrast, Love waves move laterally with respect to their direction of motion while remaining in the plane of the surface. For example, Figure 2A depicts a representation 200 of Love wave propagation as a function of time, and Figure 2B A representation of the deformation caused by Love waves at the surface is depicted 250. In order for Love waves to exist, two materials with different properties must be stacked together. The shear wave velocity in the top layer must be less than the shear wave velocity in the bottom material. The shear wave velocity is defined by the modulus of rigidity (G) divided by the square root of the material density (ρ), as shown in Equation 1 below:

[0021]

[0022] The present solution may enable more efficient generation of transverse surface waves, including by generating Love waves to cause the resulting surface motion. The present solution may enable continuous local deformation in a desired direction to achieve power efficient and mechanically robust drag control. For example, the present solution may generate Love waves to cause the resulting surface motion using a multilayer stack (e.g., a multilayer stack) comprising at least two layers, the at least two layers comprising a first layer (e.g., the top layer described above) and one or more second layers (the shear wave velocity of the top layer is less than the shear wave velocity of the one or more second layers, such as the bottom layer described above).

[0023] Reference now Figure 3 , a fluid control system 300 is shown. The fluid control system 300 can combine features of various fluid control systems described herein. The fluid control system 300 includes an actuator 304, which is coupled to a surface 308 on which a fluid 312 flows. The actuator 304 moves the surface 308 relative to the fluid 312. For example, the actuator 304 can move the surface 308 to generate a transverse surface wave, such as by propagating Love waves. The actuator 304 can be disposed in a body 316 that defines the surface 308, so that the actuator 304 propagates Love waves in the body 316 to generate a transverse surface wave at the surface 312. In some embodiments, the actuator 304 is coupled to a bulk material (e.g., the body 316) and can be actuated in at least one of an electrostatic or electromagnetic manner to propagate Love waves. Actuator 304 may avoid causing out-of-plane motion of surface 308 by using Love waves, which may be undesirable for effective drag modification (e.g., actuator 304 may not cause out-of-plane motion perpendicular to surface 308; actuator 304 may cause at most a minimal amount of out-of-plane motion perpendicular to surface 308, where surface 308 moves out of the plane of surface 308 less than a threshold distance, e.g., the threshold distance has a size scale of five viscosity length scale units or less relative to the viscosity length scale of fluid 312). Actuator 304 may have a relatively small size compared to surface 308; e.g., actuator 304 may be attached to less than a threshold portion of surface 308 (e.g., less than 75% of surface 308; less than 50% of surface 308; less than 25% of surface 308; less than 10% of surface 308; less than 5% of surface 308; less than 1% of surface 308).

[0024] In some embodiments, surface 308 may include a material that promotes the generation of transverse surface waves. For example, surface 308 may include a metamaterial designed to produce a desired shear wave velocity difference in different layers. Surface 308 may be doped or etched to achieve a desired response to actuation of actuator 304.

[0025] The fluid control system 300 includes a controller 320. The controller 320 may include a processor 324 and a memory 328. The processor 324 may be a general or special purpose processor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGA), a set of processing components or other suitable processing components. The processor 324 is configured to execute computer codes or instructions stored in the memory 328 or received from other computer readable media (e.g., CDROM, network storage, remote server, etc.). The memory 328 may include one or more devices (e.g., memory units, memory devices, storage devices, etc.) for storing data and / or computer codes to complete and / or facilitate the various processes described in the present disclosure. The memory 328 may include a random access memory (RAM), a read-only memory (ROM), a hard drive storage device, a temporary storage device, a non-volatile memory, a flash memory, an optical memory, or any other suitable memory for storing software objects and / or computing instructions. The memory 328 may include a database component, an object code component, a script component, or any other type of information structure for supporting the various activities and information structures described in the present disclosure. The memory 328 may be communicatively connected to the processor 324 via the controller 320 and may include computer code for executing (e.g., by the processor 324) one or more processes described herein. When the processor 324 executes instructions stored in the memory 328, the processor 324 generally configures the controller 320 to perform such activities.

[0026] The controller 320 may generate a control signal to control the operation of the actuator 304, such as to cause the actuator 304 to move the surface 308 relative to the fluid 312. The controller 320 may select parameters (such as frequency and amplitude) of the control signal based on desired transverse surface wave characteristics. For example, the controller 320 may select parameters of the control signal that correspond to an expected change in the drag of the fluid 312 relative to the surface 308. The controller 320 may execute a wave function that maps the parameter(s) of the control signal to parameters of the resulting transverse surface waves generated using the operation of the actuator 304, and a drag function that maps the parameters of the resulting transverse surface waves to a corresponding change in the drag of the fluid 312 as it flows over the surface 308, and may execute the wave function and the drag function to select the parameter(s) of the control signal. As further described in Figure 4A , 4B As discussed in connection with FIG. 5 , the controller 320 may generate the control signal to include a plurality of control signals having different parameters (including different phases).

[0027] The controller 320 may control the operation of the actuator 304 to control one or more parameters of the surface wave, such as at least one of the amplitude, wavelength, or frequency of the surface wave (e.g., amplitude A, wavelength λ, ω, e.g., Figure 1 312). For example, the controller 320 may control the amplitude, frequency, or intermittency of the movement of the actuator 304 to control one or more parameters of the surface. The controller 320 may control one or more parameters of the surface wave based on the parameters of the fluid 312. For example, the controller 320 may control the operation of the actuator 304 to achieve a target value for one or more parameters of the surface wave based on the viscosity length scale η of the fluid 312, where η is defined according to the following equation 3, where ν is the kinematic viscosity of the fluid 312, τ w is the wall shear stress of fluid 312 relative to surface 308, and ρ is the density of the fluid:

[0028]

[0029] Reference now Figure 4A-4B , depicting a flow control device 400. The flow control device 400 may be used to implement reference Figure 3 The described fluid control system 300, such as to receive a control signal from a controller 316 and respond to the control signal to implement the functions of the actuator 304, including generating a transverse surface wave, which results in a desired drag change. The flow control device 400 can be implemented as a surface acoustic wave device. The flow control device 400 can be relatively thin and continuous, thereby alleviating the shortcomings of existing systems such as multi-plate systems. The flow control device 400 can be used to generate various surface waves, including transverse surface waves (e.g., generated via Love waves), standing waves, and traveling waves. The flow control device 400 can also be used to generate Rayleigh waves and Stoneley waves. However, such waves may be less effective in controlling drag than Love waves. Due to the relatively small size and current requirements of the operation of the flow control device 400, the flow control device 400 can be used to achieve the desired transverse surface wave generation while reducing power requirements and wear compared to existing systems.

[0030] The flow control device 400 includes a first layer 404 and a second layer 408. The first layer 404 can be a guide layer. The first layer 404 can be made of silicon dioxide. The first layer 404 can include a sensor area 408, which can include one or more sensors that detect characteristics of a fluid flowing on the first layer 404 and / or characteristics of the motion of the first layer 404. The first layer 404 can have a lower shear wave velocity or phase velocity than the second layer 412, which can enable the generation of Love waves.

[0031] The second layer 412 may be used to generate waves, such as generating Love waves. The second layer 412 may be a piezoelectric layer. The second layer 412 may include a cut crystal, such as AT-cut quartz. The second layer 412 may include lithium niobate. The second layer 412 may include PVDF (e.g., polyvinylidene fluoride; polyvinylidene fluoride).

[0032] In some embodiments, the second layer 412 includes a plurality of electrodes 416 that can be excited to generate acoustic waves that cause the desired wave motion. The plurality of electrodes 416 can be deposited on the second layer 412 between the first layer 404 and the second layer 412. In some embodiments, the plurality of electrodes 416 includes at least one pair of electrode patterns 416a, 416b. The electrode patterns 416a, 416b can be interdigital transducers. For example, as Figure 4B , electrode pattern 416a may include multiple spaced apart fingers of electrode 420a, and electrode pattern 416b may include multiple spaced apart fingers of electrode 420b. The positioning and spacing of electrode patterns 416a, 416b may be selected to control the direction of waves generated using second layer 412. The dimensions of electrode patterns 416a, 416b may be selected to control the amplitude and other parameters of the generated waves, thereby controlling the change in drag.

[0033] In some embodiments, at the flow control device 400 (e.g., from the reference Figure 3 The control signal received by the controller 316 described in the embodiment includes a first control signal and a second control signal. The first control signal can be used to excite the electrode pattern 416a, and the second control signal can be used to excite the electrode pattern 416b. In some embodiments, the first control signal and the second control signal have different phases, and they can be selected to generate a desired surface wave. In some embodiments, the first control signal and the second control signal have different frequencies, so that the electrode patterns 416a, 416b can generate a beat frequency (for example, based on an intermediate frequency) based on the first operating frequency of the electrode pattern 416a and the second operating frequency of the electrode pattern 416b.

[0034] like Figure 4A As depicted in FIG. 4 , the flow control device 400 generates Love waves that propagate in a direction 428 (e.g., occur at different locations along the direction 428 as a function of time). The propagation of the Love waves includes displacement of particles of the first layer 404 in a direction 432 that is perpendicular to the direction 428, similar to Figure 2A-2B As depicted in .

[0035] Reference now Figure 5, depicting a flow system 500. The flow system 500 includes a body through which a fluid 502 flows, depicted as a pipe 504, having an inner layer 512 disposed on an inner surface 508 of the pipe 504. The inner layer 512 can be implemented using the fluid control system 300 and / or the flow control device 400. For example, the inner layer 512 can include a piezoelectric material that can be actuated to generate transverse surface waves that change the drag in the fluid 502 flowing through the pipe 504.

[0036] Reference now Figure 6 , depicting a method 600 for actively controlling surface drag using selective wave generation. The method 600 may be performed using various systems and devices described herein, including the fluid control system 300 and the fluid control device 400.

[0037] At 605, an actuator is coupled to a surface having a fluid flowing on the surface. The surface may be defined by a body over which the fluid flows, such as a movable platform (e.g., a vehicle). The surface may be defined by a body through which the fluid flows, such as a pipe. The fluid may include at least one of air, water, or oil. In some embodiments, the surface includes a semiconductor material doped to achieve a desired response to actuator actuation. In some embodiments, the actuator includes a plurality of electrodes. For example, the actuator may include a surface acoustic wave device including an interdigitated electrode pattern.

[0038] At 610, the controller causes the actuator to move the surface relative to the fluid, thereby generating surface waves that modify the drag in the fluid, such as to increase or decrease the drag of the fluid relative to the surface. In some embodiments, transverse surface waves are generated. In some embodiments, Love waves are generated.

[0039] In some embodiments, shear waves are generated. In some embodiments, standing waves are generated. In some embodiments, the actuator includes a plurality of electrodes, and the controller causes the plurality of electrodes to generate a beat frequency based on a first operating frequency of a first subset of the plurality of electrodes and a second operating frequency of a second subset of the plurality of electrodes.

[0040] As used herein, the terms "substantially," "approximately," "substantially," and similar terms are intended to have a broad meaning consistent with the ordinary and recognized usage by those of ordinary skill in the art to which the subject matter of the present disclosure relates. It should be understood by those skilled in the art who review the present disclosure that these terms are intended to allow description of certain features described and claimed without limiting the scope of those features to the precise numerical ranges provided. Thus, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the present disclosure as set forth in the appended claims.

[0041] As used herein, the term "coupled" refers to two components being engaged with each other directly or indirectly. Such engagement may be static (e.g., permanent or fixed) or movable (e.g., removable or releasable). Such engagement may be achieved by coupling two components directly to each other, by coupling two components to each other using a separate intermediate component and any additional intermediate components that are coupled to each other, or by coupling two components to each other using an intermediate component that is integrally formed as a single entity with one of the two components. Such components may be coupled mechanically, electrically, and / or fluidically.

[0042] As used herein, the term "or" is used in its inclusive sense (and not in its exclusive sense), such that when used to connect a list of elements, the term "or" refers to one, some, or all of the elements in the list. Unless expressly stated otherwise, connective language such as the phrase "at least one of X, Y, and Z" should be understood to convey that an element can be any one of X, Y, Z; X and Y; X and Z; Y and Z; or X, Y, and Z (i.e., any combination of X, Y, and Z). Therefore, unless otherwise indicated, such connective language is generally not intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.

[0043] References to the positions of elements (e.g., "top," "bottom," "above," "below," etc.) herein are only used to describe the orientation of the various elements in the drawings. It should be noted that the orientation of the various elements may be different according to other exemplary embodiments, and such variations are intended to be covered by the present disclosure.

[0044] The hardware and data processing components used to implement various processes, operations, illustrative logic, logic blocks, modules and circuits described in conjunction with the embodiments disclosed herein can be implemented or executed with a general-purpose single-chip or multi-chip processor designed to perform the functions described herein, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components or any combination thereof. A general-purpose processor can be a microprocessor, or any conventional processor, controller, microcontroller or state machine. The processor can also be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors combined with a DSP core, or any other such configuration. In some embodiments, specific processes and methods can be performed by circuit systems specific to a given function. A memory (e.g., a memory, a memory unit, a storage device, etc.) may include one or more devices for storing data and / or computer code (e.g., RAM, ROM, flash memory, hard disk storage, etc.) to complete or facilitate the various processes, layers and modules described in this disclosure. The memory may be or include volatile memory or non-volatile memory and may include a database component, an object code component, a script component, or any other type of information structure for supporting the various activities and information structures described in the present disclosure. According to an exemplary embodiment, the memory is communicatively connected to the processor via the processing circuit and includes computer code for executing (e.g., by the processing circuit and / or the processor) one or more processes described herein.

[0045] The present disclosure contemplates methods, systems and program products for implementing various operations on any machine-readable medium. The embodiments of the present disclosure may be implemented using an existing computer processor, or by a dedicated computer processor for an appropriate system incorporated for this purpose or other purposes, or by a hard-wired system. Embodiments within the scope of the present disclosure include program products, which include machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media may be any available medium that can be accessed by a general or special-purpose computer or other machine with a processor. For example, such machine-readable media may include RAM, ROM, EPROM, EEPROM or other optical disk storage devices, magnetic disk storage devices or other magnetic storage devices, or may be used to carry or store desired program codes in the form of machine-executable instructions or data structures and any other medium that can be accessed by a general or special-purpose computer or other machine with a processor. The above combination is also included in the scope of machine-readable media. Machine-executable instructions include instructions and data that, for example, enable a general-purpose computer, a special-purpose computer or a special-purpose processing machine to perform a specific function or group of functions.

[0046] Although the accompanying drawings and description can illustrate the specific order of method steps, unless otherwise specified above, the order of these steps may be different from the order depicted and described. Moreover, two or more steps may be performed concurrently or partially concurrently, unless otherwise specified above. Such variations may, for example, depend on the selected software and hardware systems and depend on the designer's choice. All of these variations are within the scope of the present disclosure. Similarly, software implementations of the described methods may be accomplished with standard programming techniques having rule-based logic and other logic to accomplish various connection steps, processing steps, comparison steps, and decision steps.

[0047] It is important to note that the structure and arrangement of the fluid control system and the fluid control method shown in the various exemplary embodiments are merely illustrative. In addition, any element disclosed in one embodiment may be combined or utilized with any other embodiment disclosed herein. Although only one example of an element from one embodiment that may be combined or utilized in another embodiment has been described above, it should be appreciated that other elements of the various embodiments may be combined or utilized with any other embodiment disclosed herein.

Claims

1. A system for controlling surface drag, comprising: a surface having a fluid flowing over the surface; an actuator coupled to the surface to move the surface relative to the fluid; as well as a controller that causes the actuator to generate a surface wave in the plane of the surface that modifies the drag in the fluid, Wherein the actuator comprises a plurality of electrodes arranged to generate a beat frequency based on a first operating frequency of a first subset of the plurality of electrodes and a second operating frequency of a second subset of the plurality of electrodes.

2. The system as described in claim 1 further includes a multilayer stack, which includes at least a first layer facing the fluid and a second layer on the side of the first layer opposite to the fluid, the first layer includes the surface, the first layer has a shear wave velocity different from that of the second layer, and the first layer is made of a material different from that of the second layer.

3. The system of claim 1, wherein the controller is configured to control at least one of an amplitude, a wavelength, or a frequency of the surface waves based on a parameter of the fluid.

4. The system of claim 1, wherein the controller controls the actuator to cause the surface to generate Love waves on the surface.

5. The system of claim 1, wherein the surface is defined by a membrane covering an interior surface of a body through which the fluid flows.

6. The system of claim 5, wherein the body comprises a pipe.

7. The system of claim 1, wherein the surface is defined by a membrane covering an outer surface of a body through which a fluid moves.

8. The system of claim 1, wherein the actuator comprises a surface acoustic wave device.

9. The system of claim 1, wherein the actuator comprises a piezoelectric material.

10. The system of claim 1, wherein the actuator is attached to less than a threshold portion of the surface.

11. The system of claim 1 , wherein the actuator comprises a plurality of electrodes arranged along one or more parallel axes.

12. The system of claim 11, wherein the controller drives the plurality of electrodes to generate surface waves including standing waves.

13. The system of claim 1, wherein the surface comprises a semiconductor material that is doped to achieve a desired response to actuation by the actuator.

14. The system of claim 1, wherein the actuator does not move the surface out of the plane of the surface.

15. A method of controlling surface drag, comprising: coupling an actuator to a surface having a fluid flowing over the surface; as well as causing an actuator, via a controller, to move the surface relative to the fluid to generate a surface wave in the plane of the surface that modifies drag in the fluid; The method of controlling surface drag further comprises: driving, by the controller, a first subset of the plurality of electrodes of the actuator at a first operating frequency; as well as A second subset of the plurality of electrodes is driven by a controller at a second operating frequency different from the first operating frequency to generate surface waves at a beat frequency corresponding to the first operating frequency and the second operating frequency.

16. A method as claimed in claim 15, wherein the surface is provided as a first layer of a multi-layer stack, the first layer faces the fluid, the multi-layer stack includes a second layer on the side of the first layer opposite to the fluid, the first layer includes the surface, the first layer has a shear wave velocity different from that of the second layer, and the first layer is made of a material different from that of the second layer.

17. The method of claim 15, further comprising: At least one of an amplitude, a wavelength, or a frequency of the surface waves is controlled by a controller based on a parameter of the fluid.

18. The method of claim 15, comprising: The actuator is controlled by a controller to cause the surface to generate Love waves on the surface.

19. The method of claim 15, wherein the surface is defined by a membrane covering an interior surface of a body through which the fluid flows.

20. The method of claim 19, wherein the body comprises a pipe.

21. The method of claim 15, wherein the surface is defined by a membrane covering an outer surface of a body through which a fluid moves.

22. The method of claim 15, comprising: The plurality of electrodes of the actuator are driven by a controller.

23. The method of claim 22, wherein the actuator comprises a surface acoustic wave device.

24. The method of claim 15, wherein the actuator comprises a piezoelectric material.

25. The method of claim 15, wherein the actuator is attached to less than a threshold portion of the surface.

26. The method of claim 15, wherein the actuator comprises a plurality of electrodes arranged along one or more parallel axes.

27. The method of claim 15, comprising: The plurality of electrodes are driven by a controller to generate surface waves including standing waves.

28. The method of claim 15, wherein the surface comprises a semiconductor material, and the semiconductor material is doped to achieve a desired response to actuation by an actuator.

29. The method of claim 15, comprising: The surface is moved by the actuator out of the plane of the surface by less than the thickness of the surface perpendicular to the plane.

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