Method and device for reducing fluid resistance of structure

Abstract

To provide a method and a device for reducing fluid resistance of a structure capable of effectively reducing fluid resistance of the structure by radiating a sonic flow to exert a direct influence thereof on a flow field in a boundary layer without disturbing a flow.SOLUTION: A piezoelectric material 10 which generates elastic surface waves when voltage is applied thereto is pasted on a surface of a structure. A circuit 20, with intervals of a plurality of electrodes determining a wavelength of the elastic waves set so as to be larger than a thickness of the piezoelectric material 10, is formed on the piezoelectric material 10. A radiation direction of a sonic flow, which is determined by a sonic velocity of fluid, a sonic velocity of material surface waves of the piezoelectric material 10 and frequency of the voltage, is controlled so as to be on a surface side. Then, flow resistance of the structure due to the fluid is reduced by changing velocity distribution in a boundary layer of the fluid formed on the surface of the structure with the sonic flow applied to the boundary layer and / or controlling velocity fluctuation in the boundary layer.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for reducing fluid resistance of a structure, such as a ship or a pipe, and an apparatus for reducing fluid resistance of a structure. [Background technology]

[0002] The International Maritime Organization (IMO) has established international treaties that regulate the reduction of total greenhouse gas emissions from ships constructed in the future to 30% by 2025. Furthermore, a strategic goal is to reduce total greenhouse gas emissions in the maritime sector to 50% of the 2008 level by 2050. Since turbulent frictional resistance accounts for 70-80% of a ship's total resistance, the development of a method to reduce turbulent frictional resistance is essential to achieving the target of reducing greenhouse gas emissions from ships. Meanwhile, even if some means can be used to laminarize the turbulent boundary layer, reducing frictional resistance on ships requires that this state be maintained over a wide area, which means that it is difficult to achieve this goal in 10 9 The difficulty lies in obtaining a frictional resistance reduction effect in a developed turbulent boundary layer with a very high Reynolds number on the order of 1. Therefore, the only practical means currently available for reducing fluid resistance, such as turbulent friction resistance, on ships is the air lubrication method, which the present inventor also contributed to achieving. Against this background, the inventors of the present invention came up with the idea of ​​using a piezoelectric material that generates surface acoustic waves, which has been a hot topic recently, to cause the radiated acoustic streaming to act on the turbulent boundary layer, thereby reducing frictional resistance. Patent Document 1 discloses a submerged propulsion device that includes a main body and a high-frequency vibration actuator attached to the main body so that it is in at least partial contact with the liquid when the main body is placed on the surface of or in the liquid, and that generates ultrasonic waves in the liquid using the high-frequency vibration actuator, thereby obtaining propulsive force from the acoustic radiation pressure generated at the interface between the surface of the high-frequency vibration actuator and the liquid. Patent Document 2 also discloses a method for promoting the sliding of one or more droplets on a substrate, in which ultrasonic surface waves of sufficient amplitude to deform the droplets in an inertial capillary natural vibration mode are generated on the substrate, thereby reducing the adhesive force of the droplets to the substrate and promoting the movement of the droplets under the action of an external force, and the amplitude of the ultrasonic surface waves is insufficient to cause a symmetric deformation of the droplets to cause the droplets to move in the absence of an external force in the direction of propagation of the ultrasonic surface waves. Also, in paragraph 0087, it is stated that the acoustic Weber number is We ac It is described as generating waves of less than 0.5. Patent Document 3 discloses a transversal surface acoustic wave device that uses a quartz substrate with Euler angles, and has an input IDT and an output IDT that excite a SAW in the upper mode of the stop band, and grooves recessed into the substrate located between the electrode fingers that constitute the input IDT and output IDT, in which the wavelength of the SAW and the depth of the groove satisfy a specified relationship, and the depth of the groove and the line occupancy rate of the IDT satisfy a specified relationship. Furthermore, Patent Document 4 discloses a droplet spraying device that is composed of a piezoelectric substrate having piezoelectric properties and a comb-tooth electrode fabricated on the substrate, and generates surface acoustic waves to atomize and spray droplets on the substrate, in which the comb-tooth electrodes are arranged facing each other across the droplets, and a discontinuous surface is disposed in the middle that inhibits the propagation of surface acoustic waves. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-32956 A [Patent Document 2] Special Publication No. 2019-508301 [Patent Document 3] JP 2015-84535 A [Patent Document 4] JP 2013-99734 A Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Documents 1 to 4 do not attempt to reduce fluid resistance in structures such as ships by any method other than the air lubrication method. Therefore, an object of the present invention is to provide a method for reducing fluid resistance in a structure, which can directly affect the flow field in the boundary layer by radiating acoustic streaming without disturbing the flow, thereby effectively reducing fluid resistance, and a device for reducing fluid resistance in a structure. [Means for solving the problem]

[0005] A method for reducing fluid resistance in a structure corresponding to claim 1 is a method for reducing fluid resistance in a structure over whose surface a fluid flows, comprising the steps of: attaching a piezoelectric material that generates a surface acoustic wave when a voltage is applied to the surface of the structure; forming a circuit on the piezoelectric material in which the spacing between multiple electrodes that determine the wavelength of the surface acoustic wave is set to be greater than the thickness of the piezoelectric material; controlling the radiation direction of acoustic streaming, which is determined by the fluid sound speed of the fluid, the material surface wave sound speed of the piezoelectric material, and the frequency of the voltage, to be toward the surface; and acting on the acoustic streaming to a boundary layer formed on the surface of the structure for the fluid to change the velocity distribution of the boundary layer and / or suppress velocity fluctuations in the boundary layer, thereby reducing the fluid resistance of the structure due to the fluid. According to the present invention as recited in claim 1, by radiating acoustic streaming toward the surface side, the velocity component perpendicular to the surface of the structure is reduced and the velocity component parallel to the surface is increased, thereby directly affecting the flow field in the boundary layer without disturbing the flow, and effectively reducing fluid resistance.

[0006] The present invention as recited in claim 2 is characterized in that the radiation direction of the acoustic streaming is controlled to be 34 degrees or less with respect to the surface. According to the present invention as recited in claim 2, by emitting acoustic streaming in a direction along the surface of a structure, the perpendicular velocity component of the flow is reduced and the parallel velocity component is increased, thereby increasing the density of the acoustic streaming and improving the energy efficiency of acoustic streaming control.

[0007] The present invention as recited in claim 3 is characterized in that the acoustic streaming is emitted only in one direction, the direction of the fluid flow, due to the circuit configuration. According to the present invention as defined in claim 3, it is possible to increase the efficiency of acoustic streaming control.

[0008] The present invention as set forth in claim 4 is characterized in that the material surface wave speed is more than 1 time and not more than 1.2 times the fluid sound speed. According to the fourth aspect of the present invention, the acoustic streaming can be emitted in a substantially tangential direction by making the acoustic streaming radiation angle nearly parallel to the surface of the structure.

[0009] The present invention as set forth in claim 5 is characterized in that a base material is attached between the piezoelectric material and the structure, and the material surface wave speed is adjusted by combining the surface wave speed characteristics of the piezoelectric material and the base material. According to the present invention as defined in claim 5, by combining surface wave acoustic velocity characteristics, it becomes easier to bring the radiation direction of acoustic streaming closer to an angle close to parallel to the surface of the structure, compared to a case where no base material is used.

[0010] The present invention as defined in claim 6 is characterized in that a coating is applied to the side surface of the electrode or the top surface of the circuit, and the spacing between the electrodes is set to be greater than the total thickness obtained by adding the thickness of the coating to the thickness of the piezoelectric material. According to the present invention as set forth in claim 6, when a coating is applied to the side surface of the electrode, it is possible to eliminate unevenness in the circuit and reduce fluid resistance. Also, when a coating is applied to the top surface of the circuit, by setting the distance between the electrodes to be greater than the total thickness, the total thickness becomes smaller than the wavelength of the surface acoustic wave, making it possible to control the acoustic velocity of the surface wave of the piezoelectric material.

[0011] The present invention as set forth in claim 7 is characterized in that the level of voltage applied to a piezoelectric material is changed to reduce the temporal fluctuation of the flow velocity of a fluid in a turbulent boundary layer. According to the seventh aspect of the present invention, by suppressing the temporal fluctuation, it is possible to deal with the temporal fluctuation of the flow velocity of the fluid caused by, for example, a vortex, and efficiently reduce the fluid resistance.

[0012] The present invention as recited in claim 8 is characterized in that the voltage level is changed over time in response to the temporal variation of the flow velocity. According to the present invention as defined in claim 8, the fluid resistance can be reduced more efficiently.

[0013] The present invention as set forth in claim 9 is characterized in that the flow velocity is detected and the voltage level is changed based on the detected flow velocity. According to the present invention as set forth in claim 9, the flow resistance can be effectively reduced based on the detected flow velocity.

[0014] The fluid resistance reduction device for a structure corresponding to claim 10 is a fluid resistance reduction device for a structure over whose surface a fluid flows, comprising: a piezoelectric material that is attached to the surface of the structure and generates surface acoustic waves upon application of a voltage; a circuit in which the spacing between a plurality of electrodes that determine the wavelength of the surface acoustic wave formed on the piezoelectric material is set to be greater than the thickness of the piezoelectric material; and a control means that controls the radiation direction of an acoustic stream, which is determined by the fluid sound speed of the fluid, the material surface wave sound speed of the piezoelectric material, and the frequency of the voltage, to be toward the surface side, and the acoustic stream is caused to act on a boundary layer formed on the surface of the structure for the fluid to change the velocity distribution of the boundary layer and / or suppress velocity fluctuations in the boundary layer, thereby reducing the fluid resistance of the structure caused by the fluid. According to the present invention as set forth in claim 10, by radiating acoustic streaming toward the surface side, the velocity component perpendicular to the surface of the structure is reduced and the velocity component parallel to the surface is increased, thereby directly affecting the flow field in the boundary layer without disturbing the flow, and effectively reducing fluid resistance.

[0015] The present invention according to claim 11 is characterized in that the control means controls the radiation direction of the acoustic streaming to be 34 degrees or less with respect to the surface. According to the present invention as set forth in claim 11, by emitting acoustic streaming in a direction along the surface of a structure, the perpendicular velocity component of the flow is reduced and the parallel velocity component is increased, thereby increasing the density of the acoustic streaming and improving the energy efficiency of acoustic streaming control.

[0016] The present invention as set forth in claim 12 is characterized in that the circuit has a circuit configuration in which the acoustic streaming is emitted in only one direction, which is the direction of the fluid flow. According to the twelfth aspect of the present invention, the efficiency of acoustic streaming control can be improved.

[0017] The present invention as described in claim 13 is characterized in that the circuit configuration is a comb-tooth circuit in which comb-tooth electrodes connected to the main circuit are combined facing each other and have an open floating electrode and a short-circuit floating electrode between the electrodes. According to the present invention as set forth in claim 13, by combining comb-shaped electrodes facing each other and having an open floating electrode and a short-circuited floating electrode between the electrodes, it becomes possible to radiate acoustic streaming in only one direction as a unidirectional type, thereby improving the efficiency of acoustic streaming control.

[0018] The present invention as set forth in claim 14 is characterized in that the material surface wave speed is set to be more than 1 time and not more than 1.2 times the fluid sound speed. According to the fourteenth aspect of the present invention, the acoustic streaming can be emitted in a substantially tangential direction by making the acoustic streaming radiation angle nearly parallel to the surface of the structure.

[0019] The present invention as set forth in claim 15 is characterized in that a base material is attached between the piezoelectric material and the structure, and the surface wave speed of the material is adjusted by combining the surface wave speed characteristics of the piezoelectric material and the base material. According to the present invention as defined in claim 15, by combining surface wave sound velocity characteristics, it becomes easier to bring the radiation direction of the acoustic streaming closer to an angle close to parallel to the surface of the structure, compared to a case where no base material is used.

[0020] The present invention as described in claim 16 is characterized in that a coating is provided on the side surface of the electrode or the top surface of the circuit, and the spacing between the electrodes is set to be greater than the total thickness obtained by adding the thickness of the coating to the thickness of the piezoelectric material. According to the present invention as set forth in claim 16, when a coating is applied to the side surface of the electrode, it is possible to eliminate unevenness in the circuit and reduce fluid resistance. Also, when a coating is applied to the top surface of the circuit, by setting the distance between the electrodes to be greater than the total thickness, the total thickness becomes smaller than the wavelength of the surface acoustic wave, making it possible to control the acoustic velocity of the surface wave of the piezoelectric material.

[0021] The present invention as set forth in claim 17 is characterized in that the control means changes the level of the voltage applied to the piezoelectric material to reduce the temporal fluctuation of the flow velocity of the fluid in the turbulent boundary layer. According to the seventeenth aspect of the present invention, by suppressing the temporal fluctuation, it is possible to deal with the temporal fluctuation of the flow velocity of the fluid caused by, for example, a vortex, and efficiently reduce the fluid resistance.

[0022] The present invention as set forth in claim 18 is characterized in that the control means changes the voltage level over time in response to the temporal variation of the flow velocity. According to the eighteenth aspect of the present invention, the fluid resistance can be reduced more efficiently.

[0023] The present invention as set forth in claim 19 is characterized in that the control means changes the voltage level based on the flow velocity detected by the flow velocity detection means for detecting the flow velocity. According to the nineteenth aspect of the present invention, the flow resistance can be effectively reduced based on the detected flow velocity. Effect of the Invention

[0024] According to the method for reducing fluid resistance of a structure of the present invention, the acoustic streaming is radiated toward the surface side, thereby reducing the velocity component perpendicular to the surface of the structure and increasing the velocity component parallel to the surface, thereby directly affecting the flow field within the boundary layer without disturbing the flow, and effectively reducing fluid resistance.

[0025] Furthermore, when the radiation direction of acoustic streaming is controlled so that it is 34 degrees or less with respect to the surface, the acoustic streaming can be radiated in a direction along the surface of the structure, thereby reducing the perpendicular velocity component of the flow and increasing the parallel velocity component, thereby increasing the density of the acoustic streaming and improving the energy efficiency of acoustic streaming control.

[0026] Furthermore, when the acoustic streaming is emitted in only one direction, the direction of the fluid flow, due to the circuit configuration, the efficiency of acoustic streaming control can be improved.

[0027] Furthermore, when the material surface wave speed is more than 1 time and not more than 1.2 times the fluid speed of sound, the acoustic streaming can be emitted in a nearly tangential direction by making the radiation angle of the acoustic streaming nearly parallel to the surface of the structure.

[0028] In addition, when a base material is attached between the piezoelectric material and the structure and the material surface wave speed is adjusted by combining the surface wave speed characteristics of the piezoelectric material and base material, the combination of surface wave speed characteristics makes it easier to bring the radiation direction of the acoustic flow closer to an angle that is closer to parallel to the surface of the structure compared to when a base material is not used.

[0029] In addition, when a coating is applied to the side surface of the electrode or the top surface of the circuit, and the electrode spacing is set to be greater than the total thickness obtained by adding the thickness of the coating to the thickness of the piezoelectric material, applying a coating to the side surface of the electrode can eliminate unevenness in the circuit and reduce fluid resistance. In addition, when a coating is applied to the top surface of the circuit, by setting the electrode spacing to be greater than the total thickness, the total thickness becomes smaller than the wavelength of the surface acoustic wave, making it possible to control the surface wave speed of the piezoelectric material.

[0030] In addition, when the level of voltage applied to the piezoelectric material is changed to reduce the temporal variation in the flow velocity of the fluid in the turbulent boundary layer, the temporal variation can be suppressed to accommodate, for example, the temporal variation in the flow velocity of the fluid due to vortices, thereby efficiently reducing fluid resistance.

[0031] Furthermore, if the voltage level is changed over time in accordance with the temporal fluctuation of the flow velocity, the flow resistance can be reduced more efficiently.

[0032] Furthermore, when the flow velocity is detected and the voltage level is changed based on the detected flow velocity, the flow resistance can be effectively reduced by being based on the detected flow velocity.

[0033] According to the device for reducing fluid resistance for a structure of the present invention, the acoustic streaming is radiated toward the surface side, thereby reducing the velocity component perpendicular to the surface of the structure and increasing the velocity component parallel to the surface, thereby directly affecting the flow field within the boundary layer without disturbing the flow, and effectively reducing fluid resistance.

[0034] Furthermore, when the control means controls the radiation direction of the acoustic streaming to be 34 degrees or less with respect to the surface, the acoustic streaming is radiated in a direction along the surface of the structure, thereby reducing the perpendicular velocity component of the flow and increasing the parallel velocity component, thereby increasing the density of the acoustic streaming and improving the energy efficiency of acoustic streaming control.

[0035] Furthermore, if the circuit has a circuit configuration in which the acoustic streaming is emitted in only one direction, that is, the direction of the fluid flow, the efficiency of acoustic streaming control can be improved.

[0036] Furthermore, when the circuit configuration is a comb-tooth circuit in which comb-tooth electrodes connected to the main electric circuit are combined opposite each other and have an open floating electrode and a short-circuit floating electrode between the electrodes, by having the comb-tooth electrodes combined opposite each other and the open floating electrode and the short-circuit floating electrode between the electrodes, it becomes possible to radiate acoustic streaming in only one direction as a unidirectional type, thereby improving the efficiency of acoustic streaming control.

[0037] Furthermore, when the material surface wave speed is set to be more than 1 time and not more than 1.2 times the fluid sound speed, the acoustic streaming radiation angle can be made nearly parallel to the surface of the structure, and acoustic streaming can be radiated in a nearly tangential direction.

[0038] In addition, when a base material is attached between the piezoelectric material and the structure and the material surface wave speed is adjusted by combining the surface wave speed characteristics of the piezoelectric material and base material, the combination of surface wave speed characteristics makes it easier to bring the radiation direction of the acoustic flow closer to an angle that is closer to parallel to the surface of the structure compared to when a base material is not used.

[0039] In addition, when the side surface of the electrode or the top surface of the circuit has a coating and the electrode spacing is set to be greater than the total thickness of the piezoelectric material plus the coating thickness, applying a coating to the side surface of the electrode can eliminate unevenness in the circuit and reduce fluid resistance. In addition, when a coating is applied to the top surface of the circuit, by setting the electrode spacing to be greater than the total thickness, the total thickness becomes smaller than the wavelength of the surface acoustic wave, making it possible to control the surface wave speed of the piezoelectric material.

[0040] Furthermore, when the control means changes the level of voltage applied to the piezoelectric material to reduce the temporal variation in the flow velocity of the fluid in the turbulent boundary layer, the temporal variation can be suppressed, thereby responding to the temporal variation in the flow velocity of the fluid due to, for example, vortices, and efficiently reducing fluid resistance.

[0041] Furthermore, if the control means changes the voltage level over time in response to the temporal variation of the flow rate, the flow resistance can be reduced more efficiently.

[0042] Furthermore, when the control means changes the voltage level based on the flow velocity detected by the flow velocity detection means, the flow resistance can be effectively reduced by being based on the detected flow velocity. [Brief description of the drawings]

[0043] [Figure 1] FIG. 1 is a diagram showing a configuration of a fluid resistance reduction device for a structure according to an embodiment of the present invention; [Diagram 2] Conceptual diagram of boundary layer flow field control by acoustic streaming emission using the fluid drag reduction device [Diagram 3] A diagram showing the flow velocity distribution when Reynolds stress occurs due to the same turbulent flow. [Figure 4] A diagram showing the radiation direction of acoustic streaming using the fluid drag reduction device. [Diagram 5] FIG. 13 is a diagram showing an example of the relationship between the radiation angle of the acoustic wave and the acoustic Weber number. [Figure 6] A diagram showing the circuit with a coating applied to its top surface. [Figure 7] Figure showing the estimated surface wave sound velocity of the same composite piezoelectric material [Figure 8] Photo of the appearance of a composite piezoelectric material made of polyvinylidene fluoride and aluminum as the base material. [Figure 9] Photo of the appearance of a composite piezoelectric material made of lithium niobate and polystyrene resin as the base material. [Figure 10] FIG. 14 shows the power input waveform when controlling flow velocity fluctuations due to turbulence. [Figure 11] FIG. 1 shows an example of the parallel circuit. [Figure 12] FIG. 13 shows an example of a unidirectional circuit using the comb-shaped electrodes. [Figure 13] FIG. 1 is a diagram showing a configuration of a fluid resistance reduction device for a structure according to another embodiment of the present invention; [Figure 14] FIG. 1 shows an example of a circuit combined with the flow velocity detection means. [Figure 15] Diagram showing the Rayleigh angle (angle of radiation of acoustic streaming) [Figure 16] A diagram showing an example of a bidirectional circuit using interdigital electrodes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] A method for reducing fluid resistance in a structure and an apparatus for reducing fluid resistance in a structure according to an embodiment of the present invention will be described. FIG. 1 is a diagram showing the configuration of a device for reducing fluid resistance in a structure according to this embodiment. The fluid resistance reduction device 1 is used to reduce the fluid resistance of structures (including fluid machinery) over whose surfaces a fluid flows, such as ships, aircraft, trains, automobiles, or piping. As described above, for example, in a ship, turbulent friction resistance accounts for 70 to 80% of the total resistance. In order to reduce the friction resistance in a developed turbulent boundary layer (also called a boundary layer), it is necessary to continuously and actively suppress the turbulent vortices associated with the velocity distribution and velocity fluctuations of the turbulent boundary layer, and in order to do this effectively, it is effective to control it from the surface side of the structure, which is the source of the turbulence. The fluid resistance reduction device 1 includes a piezoelectric material 10 that is attached to the surface of a structure and generates a surface acoustic wave when a voltage is applied thereto, a circuit 20 having a plurality of electrodes, a control means 30 that controls the radiation direction of an acoustic stream, a power supply means 40, and a frequency control means 50 that controls the frequency of the power input from the power supply means 40 to the circuit 20, and generates a surface acoustic wave on the surface of the structure, and causes the acoustic stream radiated from the surface of the structure to act on a boundary layer formed in the vicinity of the surface of the structure of the fluid to change the velocity distribution of the boundary layer and / or suppress the velocity fluctuation of the boundary layer, thereby reducing the fluid resistance of the structure caused by the fluid. Note that the state in which the piezoelectric material 10 is attached to the surface of the structure includes a state in which it is buried facing the surface, buried in the vicinity of the surface, attached to the surface and covered, attached including related materials, etc.

[0045] FIG. 2 is a conceptual diagram of the flow field control in the boundary layer by acoustic streaming radiation using a fluid drag reduction device, in which the velocity distribution in the boundary layer without flow field control is shown on the left side, and the velocity distribution in the boundary layer with flow field control is shown on the right side. Note that a base material 60 is provided between the structure and the piezoelectric material 10. Here, the structure is not shown, but is a ship, an aircraft, a train, an automobile, or a pipe located on the lower surface of the base material 60 in FIG. 2, and the piezoelectric material 10 and the base material 60 are attached to the surface of the structure. In addition, the surface of the structure can be applied to any surface, whether flat or curved, as long as the piezoelectric material 10 and the base material 60 can be attached to it. In addition, when the base material 60 is not required, the piezoelectric material 10 is directly attached to the surface of the structure. When AC power is input from power supply means 40 to piezoelectric material 10, surface acoustic waves are generated on the surface of the structure, exciting acoustic streaming, which is then radiated from the surface. Fluid resistance reduction device 1 reduces fluid resistance by controlling the flow field in the boundary layer using the radiated acoustic streaming. The closer the fluid is to the surface of a structure, the slower the fluid speed becomes, so a velocity gradient occurs according to the distance from the surface of the structure. The fluid resistance reduction device 1 emits acoustic streaming in the tangential direction of the surface, accelerating the fluid close to the surface as shown in the circled area on the right side of Figure 2, and changing the velocity distribution in the boundary layer. This reduces the difference in speed between the fluid close to the surface and the fluid far from the surface, making the velocity gradient smaller and reducing fluid resistance. When the velocity in the normal direction of the acoustic streaming increases, the acoustic streaming pushes the fluid mass upward in FIG. 2, bringing it closer to the mainstream, which results in an increase in the so-called Reynolds stress and an increase in frictional resistance. This Reynolds stress will be explained with reference to Fig. 3. Fig. 3 is a diagram showing the flow velocity distribution when Reynolds stress is generated by turbulent flow. When there is a flow along a solid wall, the force τ acting on a surface parallel to the wall is expressed by the following equation (1).

number

[0046] The technology of a SAW (Surface Acoustic Wave) device is used as a means for generating surface acoustic waves for emitting acoustic streaming. A SAW device is a device that excites surface acoustic waves on the surface by forming comb-shaped electrodes on a piezoelectric material 10 and passing a current through it. The piezoelectric material 10 is attached to a surface (wall) of a structure that comes into contact with a fluid, such as the outer plate of a ship's hull, the inner surface of a pipe, etc. The circuit 20 is formed on the piezoelectric material 10 and determines the wavelength of the surface acoustic wave. Here, FIG. 15 is a diagram showing the Rayleigh angle (the radiation angle of acoustic streaming). The Rayleigh angle, or the radiation direction (radiation angle) that acoustic streaming makes with the surface of the structure 100, is determined by the propagation speed (sound speed) of longitudinal waves on the surface of the structure 100 and the propagation speed (sound speed) of longitudinal waves in the fluid, as shown in the following equation (2). Therefore, simply generating surface acoustic waves will result in the radiation angle of the acoustic streaming being large with respect to the surface of the structure 100, and will actually disrupt the flow field.

number

[0047] Therefore, the control means 30 controls the radiation direction of the acoustic streaming to face the surface side. The radiation direction of the acoustic streaming is determined by the sound speed of the fluid (fluid sound speed), the material surface wave sound speed of the piezoelectric material 10, and the frequency of the voltage. By reducing the velocity component perpendicular to the surface in the velocity distribution within the boundary layer, which disturbs the flow, and increasing the velocity component parallel to the surface, the acoustic streaming has a direct effect on the flow field within the boundary layer without disturbing the flow, thereby enhancing the resistance reduction effect achieved by controlling the velocity distribution within the boundary layer, and effectively reducing fluid resistance.

[0048] FIG. 4 is a diagram showing the radiation direction of acoustic streaming using a fluid resistance reduction device. The material surface wave acoustic velocity of the piezoelectric material 10 can be controlled by making the thickness of the piezoelectric material 10 smaller (thinner) than the wavelength of the surface acoustic wave. Since the wavelength of the generated surface acoustic wave is determined by the distance between the electrodes of the circuit 20, the thickness t of the piezoelectric material 10 can be made smaller than the wavelength λ of the surface acoustic wave by setting the distance between the electrodes in the circuit 20 to be larger than the thickness t of the piezoelectric material 10. To direct the acoustic streaming toward the surface, the angle should be as close to parallel as possible to the surface of the structure, ie, 45 degrees or less (Rayleigh angle 45 degrees or more). By controlling the acoustic streaming direction to 34 degrees or less (Rayleigh angle 56 degrees or more) with respect to the surface by the control means 30, an effective reduction in turbulent friction resistance can be achieved, but it is preferable to control the acoustic streaming direction to 30 degrees or less (Rayleigh angle 60 degrees or more), and more preferably to 25 degrees or less. By radiating the acoustic streaming in a direction along the surface of the structure, the perpendicular velocity component of the flow is reduced and the parallel velocity component is increased, thereby enhancing the reduction in fluid resistance, and the density of the acoustic streaming can be increased, thereby improving the energy efficiency of acoustic streaming control.

[0049] The fluid resistance reduction device 1 can arbitrarily control the speed of the radiated acoustic stream by controlling the power input from the power supply means 40 using the control means 30. By controlling the material surface wave speed to match the fluid sound speed and emitting acoustic streaming tangential to the surface of the structure, it is possible to directly affect the flow field within the boundary layer without disturbing the flow, thereby reducing turbulent friction resistance and making flow field control more efficient. The control means 30 controls the material surface wave speed so that it is slightly faster than the fluid sound speed. Here, "slightly faster" means that the material surface wave speed is in the range of more than 1 time and not more than 1.2 times the fluid sound speed. The material surface wave speed is preferably set to more than 1 time and not more than 1.16 times the fluid sound speed, and more preferably to more than 1 time and not more than 1.1 times. When the material surface wave speed is set to 1.16 times the fluid sound speed, the radiation direction of the acoustic stream is about 30 degrees (Rayleigh angle is about 60 degrees) with respect to the surface. When the material surface wave speed is set to 1.2 times the fluid sound speed, the radiation direction of the acoustic stream corresponds to about 34 degrees with respect to the surface, and when it is set to 1.1 times, it corresponds to about 25 degrees. By setting the material surface wave speed to be slightly faster than the fluid sound speed in this way, the acoustic streaming radiation angle can be made nearly parallel to the surface of the structure, and acoustic streaming can be radiated in a nearly tangential direction.

[0050] However, in the case of lithium niobate (LiNbO3, sound speed 3992m / s), which is commonly used as a piezoelectric material in devices that generate surface acoustic waves, the Rayleigh angle θ of the acoustic streaming radiated to water (sound speed 1483m / s) is 22.5 degrees (the radiation direction is 67.5 degrees to the surface), and when used alone, the velocity component perpendicular to the surface of the structure dominates the velocity component in the tangential direction. Also, when polyvinylidene fluoride (PVDF, sound speed 1300m / s) is used alone as a piezoelectric material, acoustic streaming is not radiated because the sound speed is slower than that of water. For this reason, it is difficult to make the acoustic streaming radiation direction nearly parallel to the surface of a structure using a typical piezoelectric material alone. On the other hand, by attaching a substrate made of a different material to the back surface of the piezoelectric material 10, which is thinner than the wavelength of the generated surface acoustic wave, the sound speed characteristics of the surface acoustic wave can be changed, thereby changing the sound speed. Therefore, in this embodiment, a composite piezoelectric material is formed by attaching a base material 60 between the piezoelectric material 10 and the structure. In this case, the thickness of the piezoelectric material 10 is made smaller than the wavelength of the surface acoustic wave, and the piezoelectric material 10 is attached onto the base material 60, which in combination allows the material surface wave speed to be changed and adjusted. Compared to when the base material 60 is not used, the surface wave speed characteristics are combined to make it easier to bring the radiation direction of the acoustic stream closer to an angle close to parallel to the surface of the structure (an angle close to the tangent direction of the surface). The base material 60 is metal, resin, glass, etc. By adjusting the surface wave acoustic velocity characteristics of the piezoelectric material 10 and the base material 60 when they are bonded together, it is possible to design the desired surface wave acoustic velocity characteristics. Furthermore, if the required surface wave acoustic velocity characteristics are obtained with only the piezoelectric material 10, the base material 60 may not be used.

[0051] The acoustic Weber number We is an index that represents the force with which an acoustic wave accelerates the surface of a medium (liquid). In the case of surface waves, the acoustic Weber number We is expressed by the following equation (3). We = ρ(A ω) 2 R / (σ (cosθ) 2 ) ···(3) ρ: density of the medium (liquid), A: longitudinal amplitude of the acoustic wave, ω: angular frequency of longitudinal vibration, R: (3V / 2π) 1 / 3 Equivalent radius of the medium (liquid), θ; Rayleigh angle, σ; surface tension between the object surface and the medium (liquid)

[0052] FIG. 5 is a diagram showing an example of the relationship between the radiation angle of an acoustic wave and the acoustic Weber number. The longitudinal wave amplitude A of the acoustic wave is calculated using the following formula (4), which is an approximation obtained by correcting the electromechanical coupling coefficient based on the measured data of LiNbO3128°Y. A = 1.67E-10 * VIN (m) (4) VIN; Applied voltage (V 0-p ) As shown in Figure 5, when the Rayleigh angle θ is 22 degrees, the acoustic Weber number We is 1.0, when the Rayleigh angle θ is 68 degrees, the acoustic Weber number We is 6.14, and when the Rayleigh angle θ is 85 degrees, the acoustic Weber number We is 114. When the Rayleigh angle θ is 85 degrees, the acoustic Weber number We is approximately 100 times that of when the Rayleigh angle θ is 22 degrees. By making the Rayleigh angle θ as close to 90 degrees as possible, the inertial force of the fluid can be increased, and the flow field can be efficiently affected.

[0053] FIG. 6 shows the state in which a coating has been applied to the upper surface of the circuit. It is preferable to apply a coating 70 to prevent corrosion on the upper surface of the circuit 20 formed on the piezoelectric material 10. The material of the coating 70 is preferably one having a surface wave acoustic velocity close to that of the piezoelectric material 10, and may be the same material as the piezoelectric material 10. When coating 70 is provided on the upper surface of circuit 20, the distance between the electrodes is set to be greater than the total thickness t1+t2 obtained by adding the thickness t2 of coating 70 to the thickness t1 of piezoelectric material 10. This makes the combined thickness of piezoelectric material 10 and coating 70 (total thickness t1+t2) smaller than the wavelength λ of the surface acoustic wave, making it possible to control the acoustic velocity of the material surface wave of piezoelectric material 10. In addition, a coating 70 can be applied to the side surface of the electrode. This can eliminate unevenness in the circuit 20 and reduce fluid resistance.

[0054] The following formula (5) is an estimation formula for the surface wave sound velocity and the radiation angle of acoustic streaming of the composite piezoelectric material based on the physical properties and thickness of the piezoelectric material 10, the thickness of the base material 60, and the wavelength set in the circuit 20.

number

[0055] Using the estimation formula (5), various combinations of fluid resistance reduction devices 1 were designed by changing the type and thickness of the piezoelectric material 10, the material of the base material 60, and the wavelength of the circuit 20. Since the constant α has a range depending on the combination of the piezoelectric material 10 and the base material 60, the composite piezoelectric material is selected taking this range into consideration. FIG. 8 is a photograph of the appearance of a composite piezoelectric material in which the piezoelectric material is polyvinylidene fluoride (PVDF resin sheet) and the base material 60 is aluminum, and FIG. 9 is a photograph of the appearance of a composite piezoelectric material in which the piezoelectric material is lithium niobate and the base material 60 is polystyrene resin. The composite piezoelectric material shown in Fig. 8 has a circuit 20 formed on a PVDF resin sheet, which is a piezoelectric material 10. The PVDF resin sheet has piezoelectricity at a sound speed relatively close to that of water. The material surface wave sound speed of the PVDF resin sheet is 1300 m / s, which is slower than the sound speed of water, 1483 m / s, so acoustic streaming is not emitted into water as is. Therefore, a composite piezoelectric material was made by combining it with aluminum, a metal material with a high surface wave sound speed, as the base material 60. 9, a circuit 20 is formed on a lithium niobate thin film, which is a piezoelectric material 10. The surface acoustic velocity of the lithium niobate thin film is 3992 m / s, which is faster than the acoustic velocity of water. Therefore, a composite piezoelectric material was made by combining it with a polystyrene resin, which has a slow surface acoustic velocity, as a base material 60. Although not shown in the figure, a composite piezoelectric material was also produced in which the piezoelectric material 10 was made of polyvinylidene fluoride and the base material 60 was made of stainless steel. The resonance frequency of the composite piezoelectric materials produced was measured, and the change in the surface wave sound velocity was confirmed. For example, the sound velocity of polyvinylidene fluoride alone was 1300 m / s, but when polyvinylidene fluoride was attached to the base material 60, the sound velocity changed to 1776 m / s, and the accompanying change in frequency characteristics (resonance frequency 5.48 MHz) was confirmed. In addition, the surface wave acoustic velocity of the fabricated composite piezoelectric material was calculated by searching for the tuning frequency of the circuit 20, and it was confirmed that the surface wave acoustic velocity could be controlled. Furthermore, when using single materials such as BSO (bismuth silicon oxide Bi12SiO20) and BGO (bismuth germanium oxide Bi12GeO20), whose longitudinal sound velocity of surface waves (approximately 1620m / s to 1720m / s) is slightly faster than the sound velocity of water (1483m / s), it is expected that acoustic streaming can be emitted at a radiation angle of about 25° to the wall surface even with the material alone. Furthermore, by making these materials into a composite material, it is possible to achieve a longitudinal sound velocity of surface waves closer to the sound velocity of water, making it possible to radiate acoustic streaming that is nearly horizontal, and enabling more ideal flow field control.

[0056] FIG. 10 is a diagram showing the power input waveform during flow velocity fluctuation control due to turbulence. The power input from the power supply means 40 to the electrodes is input as an alternating current, and its frequency is set to 1 MHz to 10 MHz. If the frequency is lower than the frequency at which surface acoustic waves are generated, the waveform can be superimposed to periodically change the radiation velocity of acoustic streaming. Since the drive frequency of the fluid resistance reduction device 1 is set to 1 MHz to 10 MHz, while the frequency of flow velocity fluctuations due to turbulence is on the order of 10 KHz, if the frequency of the flow field fluctuations is superimposed on the drive frequency, acoustic streaming can be emitted so as to suppress the flow field fluctuations, and the turbulent friction resistance can be suppressed. In a turbulent boundary layer, the temporal fluctuation of the flow velocity is one of the causes of turbulent friction resistance, so the temporal fluctuation of the flow velocity of the fluid in the turbulent boundary layer is reduced by changing the level of the voltage applied to the piezoelectric material 10. By suppressing the temporal fluctuation, it is possible to deal with the temporal fluctuation of the flow velocity of the fluid caused by, for example, turbulent eddies, and efficiently reduce fluid resistance such as turbulent friction resistance. It is preferable that the voltage level is changed over time in response to the temporal variation of the flow velocity, thereby making it possible to reduce the flow resistance more efficiently.

[0057] In addition, in a turbulent boundary layer, spatial fluctuations in flow velocity are one of the causes of turbulent friction resistance. Therefore, by suppressing spatial fluctuations, turbulent friction resistance can be reduced efficiently. Although the spatial scale of a turbulent vortex is on the order of 10 to 100 μm, by miniaturizing the circuit 20, it is possible to refine the spatial resolution to the scale of the wavelength of a surface acoustic wave (for example, 324 μm at 5 MHz).

[0058] Fig. 11 is a diagram showing an example of a parallel circuit, and Fig. 12 is a diagram showing an example of a unidirectional circuit using comb-shaped electrodes, where Fig. 12(a) is an overall view and Fig. 12(b) is a partially enlarged view. Here, Fig. 16 is a diagram showing an example of a bidirectional circuit using comb-tooth electrodes. A typical comb-tooth electrode is structured so that comb-tooth electrodes connected to a main electric circuit are combined facing each other, and when AC power is input, surface acoustic waves are excited in both directions of the comb teeth, and acoustic streaming is also emitted in both directions. In contrast, circuit 20 of this embodiment achieves unidirectional acoustic streaming radiation by combining comb-like electrodes connected to the main electric circuit facing each other and forming a comb-like circuit having open floating electrodes 21 and short-circuit floating electrodes 22 between the electrodes. In this way, by combining comb-like electrodes facing each other and having open floating electrodes 21 and short-circuit floating electrodes 22 between the electrodes, it becomes possible to radiate acoustic streaming in only one direction as a unidirectional type, thereby improving the efficiency of acoustic streaming control.

[0059] Next, a method for reducing fluid resistance of a structure and an apparatus for reducing fluid resistance of a structure according to another embodiment of the present invention will be described. Note that the same reference numerals are used to designate the same functional members as those in the above-described embodiment, and the description thereof will be omitted. Fig. 13 is a diagram showing the configuration of a fluid resistance reduction device for a structure according to this embodiment, and Fig. 14 is a diagram showing an example of a circuit combined with a flow velocity detection means. Note that the numbers 1 to 4 in Fig. 14 indicate each position.

[0060] The velocity distribution in the boundary layer near the surface of the structure changes with the change in the velocity of the structure, so the control means 30 adjusts the voltage input from the power supply means 40 to the piezoelectric material 10 in accordance with the velocity of the structure, thereby changing the velocity of the radiated acoustic streaming. The fluid resistance reduction device 1 of this embodiment is provided with a flow velocity detection means 80 on the upstream side of the circuit 20 for detecting the flow velocity of the fluid. The flow velocity detection means 80 has an electrode type shear force meter and an electrode type thermometer. In addition, a memory means (not shown) connected to the control means 30 stores the relationship between the flow velocity change and the shear force change, and the relationship between the flow velocity change and the temperature change. The shear force meter and the thermometer output the measured values ​​to the control means 30. The control means 30 detects the flow velocity of the fluid using the instantaneous value of the shear stress measured by the shear force meter, and changes the voltage level based on the detected flow velocity (detected flow velocity) to control the acoustic streaming emission. By using the detected flow velocity as the basis, it is possible to effectively reduce the fluid resistance. Furthermore, the control means 30 detects the flow velocity of the fluid using the temperature measured by the thermometer, and by using the detected flow velocity as well, it is possible to perform more precise control. [Industrial Applicability]

[0061] The present invention can be applied to structures such as ships, aircraft, trains, automobiles, or piping, over whose surfaces fluids flow, to reduce fluid resistance occurring on the external or internal surfaces. When applied to ships, the invention may be applied to the entire submerged surface, or only to the parts where the effect is greatest, such as the bow and stern or the parts of the bow and stern with large curvature. When applied to propellers, the invention may be applied to the entire propeller, or only to the parts of the propeller with large curvature, the blade leading edge, etc. [Explanation of symbols]

[0062] 1. Fluid resistance reduction device 10 Piezoelectric materials 20 Circuit 21 Open floating electrode 22 Short circuit floating electrode 30 Control Means 60 Base Materials 70 Coating 80 Flow velocity detection means

Claims

1. A method for reducing fluid resistance of a structure over a surface of which a fluid flows, comprising the steps of: a piezoelectric material that generates a surface acoustic wave upon application of a voltage to the surface of the structure, a circuit formed on the piezoelectric material in which the spacing between multiple electrodes that determine the wavelength of the surface acoustic wave is set to be greater than the thickness of the piezoelectric material, the radiation direction of an acoustic stream determined by the fluid sound speed of the fluid, the material surface wave sound speed of the piezoelectric material, and the frequency of the voltage is controlled to be toward the surface, and the acoustic stream is caused to act on a boundary layer of the fluid formed on the surface of the structure to change the velocity distribution of the boundary layer and / or suppress velocity fluctuations of the boundary layer, thereby reducing the fluid resistance of the structure due to the fluid.

2. The method for reducing fluid resistance in a structure according to claim 1, further comprising controlling the radiation direction of the acoustic streaming to be 34 degrees or less with respect to the surface.

3. 3. The method for reducing fluid resistance in a structure according to claim 1, wherein the acoustic streaming is emitted only in one direction, that is, the direction of flow of the fluid, due to a configuration of the circuit.

4. 4. The method for reducing fluid resistance of a structure according to claim 1, wherein the material surface wave speed is more than 1 time and not more than 1.2 times the fluid speed.

5. A method for reducing fluid resistance in a structure described in any one of claims 1 to 4, characterized in that a base material is attached between the piezoelectric material and the structure, and the material surface wave sound velocity is adjusted by combining the surface wave sound velocity characteristics of the piezoelectric material and the base material.

6. A method for reducing fluid resistance in a structure described in any one of claims 1 to 5, characterized in that a coating is applied to a side surface of the electrode or an upper surface of the circuit, and the spacing between the electrodes is set to be greater than a total thickness obtained by adding the thickness of the coating to the thickness of the piezoelectric material.

7. 7. A method for reducing fluid resistance in a structure according to claim 1, further comprising changing the level of the voltage applied to the piezoelectric material to reduce temporal fluctuations in the flow velocity of the fluid in a turbulent boundary layer.

8. 8. The method for reducing fluid resistance of a structure according to claim 7, further comprising the step of: varying a level of said voltage over time in response to said temporal variation of said flow velocity.

9. 9. The method for reducing fluid resistance in a structure according to claim 7, further comprising the step of detecting the flow velocity and changing a level of the voltage based on the detected flow velocity.

10. A fluid resistance reduction device for a structure over whose surface a fluid flows, comprising: a piezoelectric material that is affixed to the surface of the structure and generates a surface acoustic wave upon application of a voltage; a circuit in which the spacing between a plurality of electrodes formed on the piezoelectric material, which determine the wavelength of the surface acoustic wave, is set to be greater than the thickness of the piezoelectric material; and a control means for controlling the radiation direction of an acoustic stream, which is determined by the fluid sound speed of the fluid, the material surface wave sound speed of the piezoelectric material, and the frequency of the voltage, to be toward the surface, wherein the acoustic stream acts on a boundary layer of the fluid formed on the surface of the structure to change the velocity distribution of the boundary layer and / or suppress velocity fluctuations in the boundary layer, thereby reducing the fluid resistance of the structure due to the fluid.

11. 11. The device for reducing fluid resistance in a structure according to claim 10, wherein the control means controls the radiation direction of the acoustic streaming to be 34 degrees or less with respect to the surface.

12. 12. The device for reducing fluid resistance in a structure according to claim 10 or 11, wherein the circuit has a circuit configuration in which the acoustic stream is radiated in only one direction, that is, a direction of flow of the fluid.

13. The fluid resistance reduction device for a structure as described in claim 12, characterized in that the circuit configuration is a comb-tooth circuit in which the comb-tooth-shaped electrodes connected to a main electrical circuit are combined facing each other and have an open floating electrode and a short-circuit floating electrode between the electrodes.

14. 14. The device for reducing fluid resistance of a structure according to claim 10, wherein the material surface wave speed is set to be more than 1 time but not more than 1.2 times the fluid speed.

15. A fluid resistance reduction device for a structure described in any one of claims 10 to 14, characterized in that a base material is attached between the piezoelectric material and the structure, and the material surface wave sound velocity is adjusted by combining the surface wave sound velocity characteristics of the piezoelectric material and the base material.

16. A device for reducing fluid resistance in a structure described in any one of claims 10 to 15, characterized in that a coating is provided on a side surface of the electrode or an upper surface of the circuit, and the spacing between the electrodes is set to be greater than a total thickness obtained by adding the thickness of the coating to the thickness of the piezoelectric material.

17. The device for reducing fluid resistance of a structure according to any one of claims 10 to 16, characterized in that the control means changes the level of the voltage applied to the piezoelectric material to reduce temporal fluctuations in the flow velocity of the fluid in the turbulent boundary layer.

18. 18. The device for reducing fluid resistance in a structure according to claim 17, wherein the control means changes a level of the voltage over time in response to the temporal variation of the flow velocity.

19. 19. The device for reducing fluid resistance in a structure according to claim 17 or 18, characterized in that the control means changes a level of the voltage based on a flow velocity detected by a flow velocity detection means for detecting the flow velocity.

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