A virtual variable pit plasma turbulence drag reduction device and a preparation method thereof

Abstract

A virtual variable pit plasma turbulent friction drag reduction device includes a pit flat plate (1) and a virtual pit plasma actuator (2). The pits on the pit flat plate (1) are arranged in a staggered pattern or a matrix pattern. The virtual pit plasma actuator (2) is arranged on the pit flat plate (1) and is a dielectric barrier discharge plasma actuator, including a high-voltage electrode (21), a dielectric layer (22), and a low-voltage electrode (23). There is also provided a virtual variable pit plasma turbulent friction drag reduction method, as well as a rapid preparation method of a flexible plasma actuator based on mask spraying. The present invention combines the plasma actuator active flow control technology with the pit passive flow control technology, and uses the virtual pit surface formed by the plasma actuator to improve the perturbation intensity of the pit on the flow field and enhance the drag reduction effect. In addition, the intensity of the variable pit plasma aerodynamic actuator can be adjusted according to the oncoming flow conditions. The method of the present invention is easy to be combined with sensors to form a closed-loop adaptive turbulent friction drag reduction device.

Description

Technical Field

[0001] The present invention relates to the technical field of flow control, and in particular to a variable virtual pit turbulent friction drag reduction device and a preparation method thereof. Background Art

[0002] With the continuous increase in the flight speed of unmanned aerial vehicles (UAVs), the flight resistance increases rapidly. During the cruise flight stage of high aspect ratio UAVs, frictional resistance accounts for the main part of the flight resistance, and the flow on the flight surface is mostly in a turbulent state during high-speed flight. Therefore, reducing frictional resistance, especially turbulent frictional resistance, plays a crucial role in improving the flight range and endurance, reducing energy consumption and costs, and enhancing the flight performance of UAVs. Existing research has developed a variety of control means, including passive control technologies represented by grooves, pits, and vortex generators, and active flow control technologies represented by wall blowing / suction, wall oscillation, zero-mass jets, and plasma actuators. In order to achieve a drag reduction effect at different Reynolds numbers, the spacing of the grooves needs to match the size of the incoming flow strip structure. Therefore, at high speeds, it is necessary to provide a groove spacing of a small size (μm), which is difficult to process. In contrast, the pit structure is simpler, easier to process, clean, and has better anti-pollution properties. After arranging pits on the surface of a sphere, it is possible to increase the incoming flow turbulence intensity, reduce the flow separation phenomenon behind the sphere, and achieve a stable pressure difference drag reduction effect. However, the turbulent friction drag reduction effect of pits on a flat plate is not very ideal and has certain limitations. When the geometric configuration (such as depth and array spacing) is fixed, the pits can only show an ideal drag reduction effect at the designed wind speed and Reynolds number. Once the flow conditions deviate slightly from the designed state, the drag reduction effect will weaken or disappear, and even an additional drag effect may occur, which is one of the reasons why it has not been applied in practice for a long time. How to expand the application range of the passive drag reduction method of pit drag reduction and achieve stable drag reduction at a wide range of incoming flow velocities and Reynolds numbers remains a key technical problem that has not been solved so far. The present invention proposes to combine active flow control means with passive flow control means, and use plasma virtual aerodynamic excitation to improve the adaptability of the pit turbulent drag reduction technology and broaden the Reynolds number range of effective drag reduction. Summary of the Invention

[0003] Aiming at the problems existing in the prior art, the present invention provides a virtual variable pit plasma turbulent friction drag reduction device, which includes a pit flat plate 1 and a virtual pit plasma actuator 2;

[0004] A space rectangular coordinate system is established, assuming that the x-axis direction is the flow direction, the y-axis direction is the normal direction, and the z-axis direction is the spanwise direction;

[0005] The pit flat plate 1 is made of an insulating and heat-resistant material, and the pits on the pit flat plate 1 are arranged in a staggered pattern or a matrix pattern;

[0006] (1) In the case of staggered arrangement, along the flow direction, the flow direction spacing between the centers of each column of pits and the centers of the pits in the next column is Lx; along the span direction, the span direction spacing between the centers of each row of pits and the centers of the pits in the next row is Lz; the staggered arrangement means that assuming there are 2n + 1 rows and 2m + 1 columns of pits, where n and m are non-zero positive integers, then in adjacent odd or even rows and adjacent odd or even columns, a total of 4 pits are included, and the centers of these 4 pits form a rectangle. Here, the rows and columns refer to the rows and columns of the overall array; between the adjacent odd or even rows, the even or odd rows, and between the adjacent odd or even columns, the even or odd columns, only one pit is included, and the center of this pit is exactly located at the center of the 4 pits.

[0007] (2) In the case of matrix arrangement, it is a standard matrix arrangement, in the form of a P*Q matrix, with P rows and Q columns, where P and Q are positive integers greater than or equal to 2.

[0008] The virtual pit plasma actuator 2 is a dielectric barrier discharge plasma actuator, which includes a high-voltage electrode 21, a dielectric layer 22, and a low-voltage electrode 23 from top to bottom; the virtual pit plasma actuator 2 is arranged on the pit flat plate 1, and its overall size is less than or equal to the size of the pit flat plate 1.

[0009] The low-voltage electrode 23 is integrally in the shape of a rectangular thin sheet, and the projection of its geometric center on the xoz plane coincides with the projection of the geometric center of the pit flat plate 1 on the xoz plane. There is a certain spacing between the four sides of the low-voltage electrode 23 and the four sides of the pit flat plate 1; round holes corresponding to the pits on the pit flat plate 1 are provided in the low-voltage electrode 23, and the size of the round holes is the same as the size of the pits; the low-voltage electrode 23 is provided with a terminal.

[0010] The dielectric layer 22 is a rectangular thin sheet in the horizontal plane projection, on which dielectric layer pits are distributed. The number, position, and shape of the dielectric layer pits are the same as those of the pits on the pit flat plate 1. If the thickness of the low-voltage electrode 23 is ignored, when the dielectric layer 22 is placed on the pit flat plate 1, the lower surface of the dielectric layer 22 is completely attached to the upper surface of the pit flat plate 1; the overall size of the dielectric layer 22 is less than or equal to the size of the pit flat plate 1.

[0011] The high-voltage electrode 21 includes a plurality of spherical electrodes with the same number as the pits, corresponding positions one by one, and shapes exactly fitting inside the dielectric layer pits; these spherical electrodes are connected by connection lines extending along the first diagonal direction of the virtual pit plasma actuator 2 and all directions parallel to the first diagonal direction. These connection lines are only used to connect adjacent spherical electrodes; at the same time, the spherical electrodes in the second diagonal direction are connected; the high-voltage electrode 21 is provided with a terminal; the terminals of the low-voltage electrode 23 and the high-voltage electrode 21 are not in the same place and maintain a certain distance.

[0012] The projections of the centers of the circular holes of the low-voltage electrode 23 and the centers of the corresponding spherical electrodes of the high-voltage electrode 21 on the xoz plane coincide. The edge of the spherical electrode is exactly aligned with the edge of the circular hole, and the sizes of both are the same as the diameter of the circular cross-section of the pit on the pit flat plate 1.

[0013] In a specific embodiment of the present invention, along the flow direction, the flow direction spacing between the centers of each column of pits and the centers of the pits in the next column is Lx = 10 mm; along the span direction, the span direction spacing between the centers of each row of pits and the centers of the pits in the next row is Lz = 10 mm.

[0014] In another specific embodiment of the present invention, the pit flat plate 1 has a length a = 120 mm, a width b = 120 mm, and a height h = 3 mm; the diameter D of the circular cross-section of a single pit is 10 mm, the depth d is 0.5 mm, the edge curvature radius r is 0.25 mm, the arc curvature radius R of the pit is 25 mm, and there are a total of 41 pit regions on the flat plate.

[0015] In an embodiment of the present invention, the material of the pit flat plate 1 is organic glass, ceramic, polyether ether ketone, polytetrafluoroethylene, or ABS photosensitive resin.

[0016] In yet another specific embodiment of the present invention, a spacing of 13 mm is reserved between the four sides of the low-voltage electrode 23 and the four sides of the pit flat plate 1; the rectangular edges of the low-voltage electrode 23 are rounded.

[0017] In another embodiment of the present invention, the thickness range of the electrode layer is 50 - 150 μm; the dielectric layer material is selected from quartz glass, ceramic, polyimide, polytetrafluoroethylene, etc.; the thickness of the dielectric layer 22 is 100 - 200 μm.

[0018] There is also provided a virtual variable pit plasma turbulent friction reduction system, which is based on the above virtual variable pit plasma turbulent friction reduction device. The positive electrode of the plasma power supply 3 is connected to the high-voltage electrode 21, and the negative electrode is grounded together with the low-voltage electrode 23 to form a complete discharge circuit.

[0019] In addition, there is also provided a virtual variable pit plasma turbulent friction reduction method, which is based on the above virtual variable pit plasma turbulent friction reduction system, specifically as follows:

[0020] When the virtual pit plasma actuator 2 is not working, the pit flat plate plays a role in introducing a non-smooth surface into the flow field. Within a certain range of Reynolds numbers, the fluid generates spanwise velocity under the action of the pit surface, increasing the dissipation of turbulent energy to make the flow more stable and producing a drag reduction effect. However, when the Reynolds number changes significantly, the perturbation weakens, the drag reduction effect disappears, and the pit simultaneously brings an increase in pressure difference drag, even resulting in a drag increase effect. The plasma actuator can induce a wall jet starting from the high-voltage electrode 21 on the surface of the dielectric layer 22 under the discharge condition. Since low-voltage electrodes 23 are arranged around the high-voltage electrode 21, the generated wall jets collide with each other to form an induced jet 4. Also, because the plasma excitation induces a jet with zero mass flow rate and there is a starting vortex, it inevitably induces a downward vertical flow of the air flow 5 around the structure of the induced jet 4, forming a virtual surface similar to a pit shape. By adjusting the plasma excitation intensity, the virtual equivalent height of the pit is changed, thereby changing the perturbation effect on the flow field to adapt to the change in the Reynolds number.

[0021] In addition, a rapid preparation method of a flexible plasma actuator based on mask spraying is also provided. Based on the above virtual variable pit plasma turbulent friction drag reduction device, the specific steps are as follows:

[0022] Step S1: First, use CAD software to design the shapes and layouts of the high-voltage electrode and low-voltage electrode of the actuator to generate a vector line drawing of the electrode outer contour; the file format of this vector line drawing can be recognized by the laser cutting machine drive software.

[0023] Step S2: Import the vector line drawing into the laser cutting machine drive software to generate the movement path of the laser head; then place two acrylic flat plates with a certain thickness on the workbench of the laser cutting machine respectively, optimize the settings of the coordinate origin, movement speed, laser energy, and focusing distance of the laser head, and cut out the contours of the high-voltage electrode 21 and low-voltage electrode 23 respectively according to the laser path; further, take out the cut-off areas to obtain the mask templates of the high-voltage electrode 21 and low-voltage electrode 23 of the actuator; the length and width of the acrylic flat plate should be greater than the length and width of the prepared actuator respectively.

[0024] Step S3: First, press the low-voltage electrode mask tightly onto the surface of the flexible insulating medium to ensure good adhesion between the two; then, spray the conductive metal paint evenly on the surface of the low-voltage electrode mask in a mist form through a high-pressure sprayer. The metal paint leaves the required configuration of the low-voltage electrode 23 on the insulating medium surface through the hollow part on the surface of the low-voltage electrode mask; after spraying, dry and cure the insulating medium and the mask in place; during the drying process, the organic matter in the conductive metal paint on the insulating medium surface will volatilize, leaving a thin metal electrode; perform multiple "spraying + drying" processes; when the low-voltage electrode 23 is prepared, cover it with another layer of flexible insulating medium, and use the same process on its surface to prepare the high-voltage electrode 21. Since the surface to be sprayed is curved, the metal paint can adhere evenly to its surface during the spraying process, forming a spherical high-voltage electrode 21; further, prepare a flexible insulating medium with the required surface curvature;

[0025] Step S4: Remove the mask on the surface to obtain the prepared flexible plasma actuator.

[0026] Furthermore, the present invention provides a virtual variable pit plasma turbulent drag reduction device, which includes a pit plate 1, a high-voltage electrode 21, and a low-voltage electrode 23 from top to bottom; specifically as follows:

[0027] Establish a spatial rectangular coordinate system, assuming that the x-axis direction is the flow direction, the y-axis direction is the normal direction, and the z-axis direction is the span direction;

[0028] The pit plate 1 is made of insulating and heat-resistant materials, and the pits on the pit plate 1 are arranged in a staggered pattern or a matrix pattern;

[0029] (1) In the case of staggered arrangement, along the flow direction, the flow direction spacing between the centers of each column of pits and the centers of the pits in the next column is Lx; along the span direction, the span direction spacing between the centers of each row of pits and the centers of the pits in the next row is Lz; the staggered arrangement means that assuming there are 2n + 1 rows and 2m + 1 columns of pits, where n and m are non-zero positive integers, then the adjacent odd or even rows and adjacent odd or even columns include a total of 4 pits, and the centers of these 4 pits form a rectangle. Here, the rows and columns refer to the rows and columns of the overall array; the even or odd rows between the adjacent odd or even rows and the even or odd columns between the adjacent odd or even columns contain only one pit, and the center of this pit is exactly located at the center of the 4 pits;

[0030] (2) In the case of matrix arrangement, it is a standard matrix arrangement, in the form of a P*Q matrix, with P rows and Q columns, where P and Q are positive integers greater than or equal to 2;

[0031] The low-voltage electrode 23 is arranged under the pit flat plate 1, presenting a rectangular thin sheet as a whole. The projection of its geometric center on the xoz plane coincides with the projection of the geometric center of the pit flat plate 1 on the xoz plane. A certain distance is reserved between the four sides of the low-voltage electrode 23 and the four sides of the pit flat plate 1. Round holes corresponding to the pits on the pit flat plate 1 are provided in the low-voltage electrode 23, and the sizes of the round holes are the same as those of the pits. The low-voltage electrode 23 is provided with a terminal;

[0032] The high-voltage electrode 21 is arranged on the pit flat plate 1, and its overall size is less than or equal to that of the pit flat plate 1. The high-voltage electrode 21 includes a plurality of spherical electrodes with the same number as the pits, corresponding positions one by one, and shapes exactly fitting the inside of the pits of the dielectric layer. These spherical electrodes are connected by connection lines extending in the first diagonal direction of the pit flat plate 1 and all directions parallel to the first diagonal direction. These connection lines are only used to connect adjacent spherical electrodes. At the same time, the spherical electrodes in the second diagonal direction are connected. The high-voltage electrode 21 is provided with a terminal. The terminals of the low-voltage electrode 23 and the high-voltage electrode 21 are not in the same place and maintain a certain distance;

[0033] The projection of the center of the round hole of the low-voltage electrode 23 and the center position of the corresponding spherical electrode of the high-voltage electrode 21 on the xoz plane coincides. The edge of the spherical electrode is exactly aligned with the edge of the round hole, and their sizes are both the same as the diameter of the circular cross-section of the pit on the pit flat plate 1.

[0034] The present invention proposes a new type of turbulent drag reduction control method combining active and passive means. Aiming at the problem in the passive drag reduction method of pits that the pit size is fixed and the perturbation intensity of the flow field is limited, resulting in difficulty in effectively reducing the drag of the high-speed boundary layer under off-design conditions, it is proposed to combine the plasma excitation active flow control technology with the pit passive flow control technology. By using the virtual pit-shaped surface formed by plasma excitation, the perturbation intensity of the pit on the flow field is increased, and the drag reduction effect is enhanced. At the same time, since the intensity of the variable pit plasma aerodynamic excitation device invented can be adjusted according to the oncoming flow conditions, the method proposed by the present invention can be easily combined with a sensing device to form a closed-loop adaptive turbulent friction drag reduction device. Description of the Drawings

[0035] Figure 1 The device diagram of the present invention is shown, where Figure 1 (a) shows an exploded laminated view, Figure 1 (b) shows the pit flat plate, Figure 1 (c) shows the plasma exciter;

[0036] Figure 2 The partial cross-sectional view of the device of the present invention is shown;

[0037] Figure 3 The electrode plan view of the virtual pit plasma exciter is shown, whereFigure 3 (a) shows the plan view of the low-voltage electrode, Figure 3 (b) shows the plan view of the high-voltage electrode;

[0038] Figure 4 shows the relationship diagram of the drag reduction device;

[0039] Figure 5 shows the rapid preparation steps of the virtual pit actuator;

[0040] Figure 6 shows the implementation case of the rapid preparation method: virtual pit plasma actuator.

[0041] Explanation of the attached figure labels:

[0042] 1 Pit plate 2 Virtual pit plasma actuator 21 High-voltage electrode 22 Dielectric layer 23 Low-voltage electrode 3 Plasma power supply 4 Induced jet 5 Airflow 6 High-voltage electrode contour 7 Mask plate 8 Spray gun 9 Prepared virtual pit plasma actuator 91 Prepared high-voltage electrode 92 Prepared dielectric layer 93 Prepared low-voltage electrode Detailed implementation manners

[0043] A virtual variable pit plasma turbulent friction drag reduction device mainly consists of a pit plate 1 and a virtual pit plasma actuator 2. As Figure 1 shown, the x-axis direction is the flow direction, the y-axis direction is the normal direction, and the z-axis direction is the span direction.

[0044] According to different application scenarios and requirements, the specific geometric dimensions of the pit plate 1 and the virtual pit plasma actuator 2 can be adaptively designed. In the Figure 1-3 shown case, the pits on the pit plate 1 can be arranged in a staggered pattern or a matrix pattern. Here, the staggered pattern is taken as an example. As Figure 1-3 shown, along the flow direction, the flow direction spacing between the centers of each column of pits and the centers of the pits in the next column below is Lx = 10 mm; along the span direction, the span direction spacing between the centers of each row of pits and the centers of the pits in the next row below is Lz = 10 mm. The so-called staggered arrangement means that assuming there are 2n + 1 rows and 2m + 1 columns of pits, where n and m are non-zero positive integers, then the adjacent odd (even) rows and adjacent odd (even) columns include a total of 4 pits, and the centers of these 4 pits form a rectangle. Here, the rows and columns refer to the rows and columns of the overall array; the even (odd) rows between the adjacent odd (even) rows and the even (odd) columns between the adjacent odd (even) columns only contain one pit, and the center of this pit is exactly located at the center of the 4 pits.

[0045] The shape of the pits described in the present invention is known to those skilled in the art. For example, the main profile shape of the pits is an arc with a certain curvature at the edge, which can be regarded as a curve composed of three tangent arcs, and will not be elaborated here (Qin Liguo, Gong Chaoyong, Sun Hongjiang, et al. Research progress on drag reduction of non-smooth surfaces [J]. Surface Technology, 2022, 51(8): 107-122.).

[0046] Essentially, the staggering means that the arrangement of adjacent two rows (columns) along the flow direction is offset. For example, the arrangements of the 1st row and the 3rd row are the same (i.e., the arrangement methods of odd rows are the same), the 2nd row is offset from the 1st and 3rd rows by a certain distance, and the arrangement method of the columns is similar. The two spacings, the flow direction spacing Lx and the spanwise spacing Lz mentioned above, do not necessarily have to be the same. It is precisely this spacing that ensures that the center distance between the pit in the first row and the first column and the pit in the second row and the second column is (Lx^2 + Lz^2)^0.5, which can be referred to Figure 3 (b) for understanding.

[0047] The matrix arrangement is a standard matrix arrangement, in the form of a P*Q matrix, with P rows and Q columns, and both P and Q are positive integers greater than or equal to 2.

[0048] In a specific embodiment of the present invention, the pit plate 1 has a length a = 120 mm, a width b = 120 mm, and a height h = 3 mm; the diameter D of the circular cross-section of a single pit is 10 mm, the depth d is 0.5 mm, the edge curvature radius r is 0.25 mm, and the arc curvature radius R of the pit is 25 mm. There are a total of 41 pit regions on the plate. When the ratio of the pit depth to the diameter d / D is less than 10%, the flow structure development stage is less, and when it is less than 6%, there is no separated flow. In order to better obtain the drag reduction effect, the preferred depth-to-diameter ratio d / D = 5%. The pit plate 1 can be manufactured by 3D printing, machining, high-energy beam machining, etching, sheet metal stamping forming, etc. The material can be plexiglass, ceramic, polyether ether ketone, polytetrafluoroethylene, ABS photosensitive resin, etc. Considering the need for insulation and good heat resistance when combined with the plasma actuator, polytetrafluoroethylene is preferably used as its raw material here. Note that the overall shape of the device is not limited to a flat plate shape and can be adaptively bent according to the application scenario and the shape of the component.

[0049] The virtual pit plasma actuator 2 is essentially a dielectric barrier discharge plasma actuator, which includes a high-voltage electrode 21, a dielectric layer 22, and a low-voltage electrode 23 from top to bottom, and the stacking order is as Figure 2 shown. The virtual pit plasma actuator 2 is arranged on the pit plate 1, and its overall size is the same as that of the pit plate 1, or the overall size is smaller than that of the pit plate 1 (that is, the projection size of the virtual pit plasma actuator 2 on the horizontal plane is smaller than the projection size of the pit plate 1, but the centers of the two coincide, and their four sides are parallel to each other). As Figure 3As shown, the low-voltage electrode 23 is in the shape of a rectangular thin sheet as a whole. The projection of its geometric center on the xoz plane coincides with the projection of the geometric center of the concave-pit flat plate 1 on the xoz plane. A certain distance is reserved between the four sides of the low-voltage electrode 23 and the four sides of the concave-pit flat plate 1. In a specific embodiment of the present invention, the distance is 13 mm. To avoid charge accumulation at the sharp corners of the electrode, the rectangular edges are rounded. A round hole corresponding to the concave pit on the concave-pit flat plate 1 is provided in the low-voltage electrode 23, and the size of the round hole is the same as that of the concave pit. In a specific embodiment of the present invention, a connection terminal extends from the lower left corner of the low-voltage electrode 23, and the end of the connection terminal is, for example, Figure 3 the circular electrode shown in (a). The dielectric layer 22 is a rectangular thin sheet in the horizontal plane projection, and dielectric layer concave pits are distributed thereon. The number, position, and shape of the dielectric layer concave pits are the same as those of the concave pits on the concave-pit flat plate 1. If the thickness of the low-voltage electrode 23 is ignored, when the dielectric layer 22 is placed on the concave-pit flat plate 1, the lower surface of the dielectric layer 22 is completely attached to the upper surface of the concave-pit flat plate 1. In an embodiment of the present invention, a round hole with a diameter of 5 mm is provided in the lower left corner to facilitate the connection of the connection part of the low-voltage electrode 23. In a specific embodiment of the present invention, the size of the dielectric layer 22 is the same as that of the concave-pit flat plate, which is 120*120 mm 2 . The high-voltage electrode 21 includes a plurality of spherical electrodes (the spherical electrodes are small hemispherical surfaces, and the size is smaller than the hemispherical surface. Here, the spherical electrodes mainly reflect their curved surface shapes, and their shapes fit the concave pit shapes, rather than complete spherical surfaces) with the same number as the concave pits, corresponding positions one by one, and shapes exactly fitting the inside of the dielectric layer concave pits. These spherical electrodes are connected by connection lines extending along the first diagonal direction (for example, the upper right to lower left diagonal) of the virtual concave-pit plasma exciter 2 and all directions parallel to the first diagonal direction. These connection lines are only used to connect adjacent spherical electrodes; at the same time, the spherical electrodes in the second diagonal direction (for example, the upper left to lower right diagonal) are connected. In a specific embodiment of the present invention, a connection terminal extends from the lower right corner of the high-voltage electrode 21, and the end of the connection terminal is, for example, Figure 3The circular electrode shown in (b). The projections of the centers of the circular holes of the low-voltage electrode 23 and the centers of the corresponding spherical electrodes of the high-voltage electrode 21 on the xoz plane coincide. The edge of the spherical electrode is exactly aligned with the edge of the circular hole, and their sizes are both the same as the diameter of the circular cross-section of the pit on the pit flat plate 1, for example, 10 mm. In a specific embodiment of the present invention, to reduce the influence of the non-circular electrode area on the circuit, the preferred connection line width is 1 mm; the electrode coverage area, that is, the plasma generation area, is 94*94 mm2. The high-voltage electrode 21 and the low-voltage electrode 23 can be made of copper or other conductive metal coatings (such as silver, tungsten, etc.). In an embodiment of the present invention, the thickness range of the electrode layer is 50-150 μm, and considering the spraying quality and ensuring the reliability of the circuit, 35 μm is preferred. The dielectric layer material can be selected from quartz glass, ceramics, polyimide, polytetrafluoroethylene, etc. Considering the need for flexible arrangement of the plasma actuator, polyimide flexible film is preferred. The thickness of the dielectric layer 22 needs to be determined according to the discharge voltage, and the typical range is 100-200 μm, and 190 μm is preferred. The positive electrode of the plasma power supply 3 is connected to the high-voltage electrode 21, and the negative electrode is grounded together with the low-voltage electrode 23 to form a complete discharge circuit. The plasma power supply 3 can be divided into sinusoidal alternating current, nanosecond pulse, and pulsed direct current dielectric barrier discharge plasma excitation according to different driving voltage waveforms. The present invention does not specifically limit the frequency and voltage range of the driving waveform, and can be adaptively adjusted according to the application scenario requirements.

[0050] The present invention also provides a virtual variable pit plasma turbulent friction reduction method, and the specific implementation is as follows. When the virtual pit plasma actuator 2 is not working, the pit flat plate plays a role in introducing a non-smooth surface in the flow field. Within a certain Reynolds number range, the fluid generates spanwise velocity under the action of the pit surface, increasing the dissipation of turbulent energy to make the flow more stable and producing a drag reduction effect. However, when the Reynolds number changes greatly, the perturbation weakens, the drag reduction effect disappears, and the pit also brings an increase in pressure difference resistance, and even an increase in resistance effect occurs. As Figure 2 shown, the plasma actuator can induce a wall jet starting from the high-voltage electrode 21 on the surface of the dielectric layer 22 under the discharge condition. Since the low-voltage electrodes 23 are arranged around the high-voltage electrode 21, the generated wall jets collide with each other to form an induced jet 4. Also, because the plasma excitation induces a zero-mass flow jet and there is a starting vortex, it will inevitably induce a downward vertical flow of the air flow 5 around the structure of the induced jet 4, forming a virtual surface similar to a pit shape. By adjusting the plasma excitation intensity, the virtual equivalent height of the pit is changed, thereby changing the perturbation effect on the flow field and adapting to the change of the Reynolds number.

[0051] The following further elaborates on a closed-loop control system for a drag reduction method proposed based on the above device. Figure 4As shown. According to the specific usage scenario, the virtual variable pit plasma turbulence drag reduction device of the present invention is applied to the surface of the component to be tested, and corresponding sensors such as pitot tubes, hot films, microbalances, etc. are arranged in the flow field to collect flow field information. Specifically as follows:

[0052] Step 1, flow field information collection. Under the action of the oncoming flow, the flow field data is collected by using the above-mentioned sensors, and is converted into digital signals through an analog transmitter and transmitted to the analysis and control platform.

[0053] Step 2, calculate the error signal. The analysis and control platform analyzes the boundary layer state in combination with relevant software, determines whether the pit plate produces a drag reduction effect or reaches the expected value under the current flow field, and obtains the error signal.

[0054] Step 3, generate and send control instructions. According to the error signal, the control algorithm makes a decision on the excitation intensity of the pit actuator according to the actual working needs, and the control power supply sends a control signal to the pit actuator.

[0055] Step 4, the pit actuator adjusts its working state. After receiving the control signal, the pit actuator discharges to induce jets with different intensities to disturb the flow field and change the working conditions of the device to adapt to the current oncoming flow conditions.

[0056] Step 5, repeat the above steps until the error signal converges within the preset range to achieve the purpose of closed-loop control.

[0057] Thus, a process of a closed-loop control system is formed, continuously iterating until the best drag reduction is achieved, and ensuring that the device can respond to complex oncoming flow conditions in real time.

[0058] The following introduces the rapid preparation method of the virtual variable pit plasma turbulence drag reduction device of the present invention:

[0059] Traditional active flow control experiments generally adopt the method of first preparing a flexible plasma actuator and then manually pasting it on the model surface, with poor accuracy (in the order of 1 mm), long time consumption (in the order of hours), extremely low efficiency, and the magnetron sputtering preparation process is complex and costly. The pits on the pit plate 1 of the device of the present invention are curved surfaces, and the virtual pit plasma actuators are also curved surfaces. Aiming at the problem of how to quickly and low-cost prepare curved surface plasma electrodes, the present invention proposes a rapid preparation method of flexible plasma actuators based on mask spraying. Compared with the manual pasting scheme under laboratory conditions, this method can be used to prepare various complex electrode shapes, with high dimensional accuracy (0.1 mm), short time consumption (<20 min), and is convenient for rapid replacement.

[0060] Figure 5The process of the rapid preparation method of the flexible plasma actuator of the present invention is given as follows:

[0061] Step S1: First, use CAD software (such as SolidWorks, UG, Catia, AutoCAD, etc.) to design the shapes and layouts of the high-voltage electrode and the low-voltage electrode of the actuator, and generate a vector line drawing of the outer contour of the electrode. The file format of this vector line drawing is not limited (such as dxf, dwg, or PS, etc.), as long as it can be recognized by the laser cutting machine driving software.

[0062] Step S2: Import this vector line drawing into the laser cutting machine driving software to generate the movement path of the laser head. Then, place two acrylic plates with a thickness of 3 - 5 mm (preferably 3 mm) on the workbench of the laser cutting machine respectively, optimize the settings of the coordinate origin, movement speed, laser energy, focusing distance, etc. of the laser head, and cut out the contours of the high-voltage electrode 21 and the low-voltage electrode 23 respectively according to the laser path. Further, take out the cut-off area to obtain the mask plates of the high-voltage electrode 21 and the low-voltage electrode 23 of the actuator. It should be noted that: the length and width of the acrylic plate should be greater than the length and width of the prepared actuator respectively.

[0063] Step S3: First, press and cover the low-voltage electrode mask plate tightly on the surface of the flexible insulating medium to ensure good adhesion between the two. Then, spray the conductive metal paint evenly on the surface of the low-voltage electrode mask in a mist form through a high-pressure spray gun. The metal paint leaves the required configuration of the low-voltage electrode 23 on the insulating medium surface through the hollow part on the surface of the low-voltage electrode mask. After spraying, heat air is passed through the insulating medium and the mask plate in place for drying and curing treatment. During the drying process, the organic matter in the conductive metal paint on the insulating medium surface will volatilize, leaving a thin metal electrode. In order to spray a thicker electrode on the insulating medium surface, multiple "spraying + drying" treatments can be carried out. When the low-voltage electrode 23 is prepared, cover another layer of flexible insulating medium on its surface, and use the same process to prepare the high-voltage electrode 21 on its surface. Since the surface to be sprayed is curved, the metal paint can be evenly attached to its surface during the spraying process, forming a spherical high-voltage electrode 21. In this step, the material of the flexible insulating medium is preferably polyimide, and it can also be other materials such as PET film. Further, the flexible insulating medium can also be prepared by spraying according to the curvature requirement of the processing surface, and the processing process is similar to that of spraying the electrode.

[0064] Step S4: Remove the mask plate on the surface to obtain the prepared flexible plasma actuator.

[0065] Figure 6 The virtual pit plasma actuator prepared by using the present invention. Taking the spraying preparation of the high-voltage electrode 21 as an example, as Figure 6As shown in (a), the contour 6 of the high-voltage electrode is in a dendritic shape, extending obliquely from the circular connection at the lower right corner to the upper left, and connecting each spherical electrode in series. The specific dimensions are the same as those described above. The acrylic mask plate 7 cut according to the electrode contour 6 is as shown in Figure 6 (b). The thickness of the mask plate 7 is 3 mm, and the outer dimensions are 120 mm * 120 mm. The brown color on the surface is the kraft paper that has not been torn off, which plays a role in protecting and preventing surface scratches during the laser cutting process. The spray gun 8 used is a common upper-pot high-voltage spray gun on the market, and the metal paint is copper paint. The prepared insulating medium 92 material is polyimide, which is directly sprayed on the surface to be attached and processed, with a thickness of about 190 μm. Then, the mask plate 7 is pressed on the surface of the insulating medium. After single-time copper paint spraying and about 5 minutes of air drying treatment, the prepared high-voltage electrode 91 is as shown in Figure 6 (c). From the visual effect, the surface of the prepared high-voltage electrode 21 is uniform and the contour is clear, without burrs, chipping and other phenomena.

[0066] From the perspective of preparation accuracy, the electrode size error is mainly caused by the cutting error of the laser cutter, and the typical value is 0.1 mm. From the perspective of preparation time, the cutting time of the mask plate is 2 - 3 minutes, the single-time spraying time is 1 minute, and the hot air drying time is 5 - 8 minutes. The total time is only 15 minutes, far lower than the magnetron sputtering and manual pasting methods. From the perspective of applicable scenarios, since the actuator uses flexible polyimide as the insulating medium, it can be easily pasted on the leading edge of the wing and the lip of the intake duct of the aircraft, etc., and at the same time can overcome the problem of difficult preparation of the curved electrode of the traditional ceramic-based plasma actuator. In addition, this preparation method is very easy to industrialize, can be used for mass production of plasma actuators, the mask plate can also be used multiple times, and the manufacturing cost is low.

[0067] Furthermore, the configuration of the virtual variable pit plasma turbulence drag reduction device can be optimized. Since the pit plate 1 is made of insulating and heat-resistant materials such as ceramics, polyether ether ketone, and polytetrafluoroethylene, it can replace the dielectric layer 22 and play the role of insulation and discharge blocking. At this time, the virtual pit plasma actuator 2 can be composed of a laminated combination of a high-voltage electrode 21, a pit plate 1, and a low-voltage electrode 23 whose geometric centers coincide in the projection on the xoz plane. The high-voltage electrode 21 and the low-voltage electrode 23 can also be sprayed and made by the aforementioned rapid preparation method. This configuration directly uses the pit plate 1 as the dielectric layer 22, eliminating the spraying of the dielectric layer 22 (insulating medium 92). Only the high-voltage electrode 21 and the low-voltage electrode 22 need to be sprayed and prepared on the upper and lower surfaces of the pit plate 1 respectively. The processing technology is simpler, the processing efficiency is higher, it is easier to prepare, and the manufacturing cost is further reduced.

[0068] From the above working methods and structural descriptions, it is not difficult to conclude that the advantages and effects of the present invention are mainly as follows:

[0069] 1. Easy to implement and wide application range. Compared with the groove-shaped surface, the pit-shaped surface has a larger gap without sharp points on the surface, so dust and particles cannot adhere for a long time and can self-clean under the oncoming flow, with higher anti-pollution ability. In addition, the pit structure is relatively simple, with various arrangement forms and relatively low processing costs. The virtual pit actuator has a flexible feature that can better fit the curved surface and reduce the impact on the structural surface after arrangement.

[0070] 2. Can be adaptively adjusted and has good robustness. Compared with general flow control means, the present invention combines a pit plate with a virtual pit plasma actuator to achieve the combination of active and passive flow control. Compared with traditional pit drag reduction methods, by adjusting the plasma excitation intensity, the virtual equivalent height of the pit can be changed, effectively improving the problems of single disturbance form and limited disturbance intensity of the pit, broadening the working range of pit drag reduction, and adapting to more complex working conditions.

[0071] 3. Low cost and can be prepared quickly and reliably. Compared with the traditional manual pasting method for preparing plasma actuators, the preparation method of the present invention has high precision, good repeatability, low time consumption, and can prepare three-dimensional curved surface electrodes. Compared with the flexible circuit board processing method, the preparation method has a simple process, and the mask can be reused, with low manufacturing costs.

Claims

1. A virtual variable pit plasma turbulent friction reduction device, which includes a pit flat plate (1) and a virtual pit plasma actuator (2); characterized in that A space rectangular coordinate system is established. It is assumed that the x-axis direction is the flow direction, the y-axis direction is the normal direction, and the z-axis direction is the spanwise direction; The pit flat plate (1) is made of insulating and heat-resistant materials, and the pits on the pit flat plate (1) are arranged in a staggered pattern or a matrix pattern; (1) In the case of staggered arrangement, along the flow direction, the flow direction spacing between the centers of each column of pits and the centers of the pits in the next column is Lx; along the spanwise direction, the spanwise spacing between the centers of each row of pits and the centers of the pits in the next row is Lz; The staggered arrangement means that assuming there are 2n + 1 rows and 2m + 1 columns of pits in total, where n and m are non-zero positive integers, then the adjacent odd or even rows and adjacent odd or even columns include a total of 4 pits, and the centers of these 4 pits form a rectangle. Here, the rows and columns refer to the rows and columns of the overall array; The even or odd rows between the adjacent odd or even rows and the even or odd columns between the adjacent odd or even columns contain only one pit, and the center of this pit is exactly located at the center of the 4 pits; (2) In the case of matrix arrangement, it is a standard matrix arrangement, in the form of a P*Q matrix, with P rows and Q columns, where P and Q are both positive integers greater than or equal to 2; The virtual pit plasma actuator (2) is a dielectric barrier discharge plasma actuator, which includes a high-voltage electrode (21), a dielectric layer (22), and a low-voltage electrode (23) from top to bottom; The virtual pit plasma actuator (2) is arranged on the pit flat plate (1), and its overall size is less than or equal to the size of the pit flat plate (1); The low-voltage electrode (23) is in the shape of a rectangular thin sheet as a whole, and the projection of its geometric center on the xoz plane coincides with the projection of the geometric center of the pit flat plate (1) on the xoz plane. There is a certain spacing between the four sides of the low-voltage electrode (23) and the four sides of the pit flat plate (1); Round holes corresponding to the pits on the pit flat plate (1) are provided in the low-voltage electrode (23), and the size of the round hole is the same as the size of the pit; The low-voltage electrode (23) is provided with a terminal; The dielectric layer (22) is a rectangular thin sheet in the horizontal plane projection, and dielectric layer pits are distributed on it. The number, position, and shape of the dielectric layer pits are the same as those of the pits on the pit flat plate (1). If the thickness of the low-voltage electrode (23) is ignored, when the dielectric layer (22) is placed on the pit flat plate (1), the lower surface of the dielectric layer (22) is completely attached to the upper surface of the pit flat plate (1); The overall size of the dielectric layer (22) is less than or equal to the size of the pit flat plate (1); The high-voltage electrode (21) includes a plurality of spherical electrodes, the number of which is the same as that of the pits, the positions of which correspond to the pits one by one, and the shapes of which exactly fit the inside of the pits of the dielectric layer; these spherical electrodes are connected by connection lines extending along the first diagonal direction of the virtual pit plasma exciter (2) and all directions parallel to the first diagonal direction, and these connection lines are only used to connect adjacent spherical electrodes; at the same time, the spherical electrodes in the second diagonal direction are connected; the high-voltage electrode (21) is provided with a terminal; the terminals of the low-voltage electrode (23) and the high-voltage electrode (21) are not in the same place and maintain a certain distance; The projection of the center of the circular hole of the low-voltage electrode (23) on the xoz plane coincides with the center position of the corresponding spherical electrode of the high-voltage electrode (21), the edge of the spherical electrode coincides exactly with the edge of the circular hole, and the sizes of both are the same as the diameter of the circular cross-section of the pit on the pit flat plate (1).

2. The virtual variable pit plasma turbulent friction drag reduction device according to claim 1, wherein Along the flow direction, the flow distance between the centers of each column of pits and the centers of the pits in the next column is Lx = 10 mm; along the span direction, the span distance between the centers of each row of pits and the centers of the pits in the next row is Lz = 10 mm.

3. The virtual variable pit plasma turbulent flow drag reduction device according to claim 1, characterized in that, The pit flat plate (1) has a length a = 120 mm, a width b = 120 mm, and a height h = 3 mm; the diameter D of the circular cross-section of a single pit is 10 mm, the depth d is 0.5 mm, the edge curvature radius r is 0.25 mm, the arc curvature radius R of the pit is 25 mm, and there are a total of 41 pit regions on the flat plate.

4. The virtual variable pit plasma turbulent friction drag reduction device according to claim 1, characterized in that, The material of the pit flat plate (1) is plexiglass, ceramic, polyetheretherketone, polytetrafluoroethylene or ABS photosensitive resin.

5. The virtual variable pit plasma turbulent flow drag reduction device according to claim 1, wherein A spacing of 13 mm is reserved between the four sides of the low-voltage electrode (23) and the four sides of the pit flat plate (1); the rectangular edges of the low-voltage electrode (23) are rounded.

6. The virtual variable pit plasma turbulent flow friction reduction device according to claim 1, characterized in that The thickness range of the electrode layer is 50 - 150 μm; the dielectric layer material is selected from quartz glass, ceramic, polyimide or polytetrafluoroethylene, etc.; the thickness of the dielectric layer (22) is 100 - 200 μm.

7. A virtual variable pit plasma turbulent flow friction reduction system, which is based on the virtual variable pit plasma turbulent flow friction reduction device according to any one of claims 1 to 6, and is characterized in that, The positive electrode of the plasma power supply (3) is connected to the high-voltage electrode (21), and the negative electrode is grounded together with the low-voltage electrode (23) to form a complete discharge circuit.

8. A virtual variable pit plasma turbulent friction reduction method, which is based on the virtual variable pit plasma turbulent friction reduction system as described in claim 7, and is characterized in that When the virtual pit plasma actuator (2) is not working, the pit plate plays a role in introducing a non-smooth surface into the flow field. Within a certain range of Reynolds numbers, the fluid generates spanwise velocity under the action of the pit surface, increasing the dissipation of turbulent energy to make the flow more stable and producing a drag reduction effect. However, when the Reynolds number changes significantly, the perturbation weakens, the drag reduction effect disappears, and the pit simultaneously brings an increase in pressure difference drag, even resulting in a drag increase effect. The plasma actuator can induce a wall jet starting from the high-voltage electrode (21) on the surface of the dielectric layer (22) under discharge conditions. Since low-voltage electrodes (23) are arranged around the high-voltage electrode (21), the generated wall jets collide with each other to form an induced jet (4). Also, because the plasma excitation induces a jet with zero mass flow rate and there is a starting vortex, it will inevitably induce a downward vertical flow of the air flow (5) around the structure of the induced jet (4), forming a virtual surface similar to a pit shape. By adjusting the plasma excitation intensity, the virtual equivalent height of the pit is changed, thereby changing the perturbation effect on the flow field to adapt to the change in the Reynolds number.

9. A rapid preparation method of a flexible plasma actuator based on mask spraying, which is based on the virtual variable pit plasma turbulent friction drag reduction device according to any one of claims 1 to 6, and is characterized in that, Specifically as follows: Step S1: First, use CAD software to design the shapes and layouts of the high-voltage electrode and low-voltage electrode of the actuator to generate a vector line diagram of the electrode outer contour; the file format of this vector line diagram can be recognized by the laser cutting machine drive software. Step S2: Import this vector line diagram into the laser cutting machine drive software to generate the movement path of the laser head; then place two acrylic plates with a certain thickness on the workbench of the laser cutting machine respectively, optimize the settings of the coordinate origin, movement speed, laser energy, and focusing distance of the laser head, and cut out the contours of the high-voltage electrode (21) and low-voltage electrode (23) according to the laser path respectively; further, take out the cut-off area to obtain the mask plates of the high-voltage electrode (21) and low-voltage electrode (23) of the actuator; the length and width of the acrylic plate should be greater than the length and width of the prepared actuator respectively. Step S3: First, press and cover the low-voltage electrode mask plate tightly on the surface of the flexible insulating medium to ensure good fitting between the two; then evenly spray the conductive metal paint on the surface of the low-voltage electrode mask in a mist form through a high-pressure spray pot. The metal paint leaves the required configuration of the low-voltage electrode (23) on the insulating medium surface through the hollow part on the surface of the low-voltage electrode mask; after spraying, perform drying and curing treatment on the insulating medium and the mask plate in place; during the drying process, the organic matter in the conductive metal paint on the surface of the insulating medium will volatilize, leaving a thin metal electrode; perform multiple "spraying + drying" treatments; when the low-voltage electrode (23) is prepared, cover another layer of flexible insulating medium on its surface, and use the same process to prepare the high-voltage electrode (21) on its surface. Since the surface to be sprayed is curved, the metal paint can evenly adhere to its surface during the spraying process, forming a spherical high-voltage electrode (21); further, prepare a flexible insulating medium with the required surface curvature. Step S4: Remove the mask plate on the surface to obtain the prepared flexible plasma actuator.

10. A virtual variable pit plasma turbulent friction reduction device, which includes a high-voltage electrode (21), a pit flat plate (1), and a low-voltage electrode (23) from top to bottom; characterized in that A spatial rectangular coordinate system is established. It is assumed that the x-axis direction is the flow direction, the y-axis direction is the normal direction, and the z-axis direction is the spanwise direction; The pit flat plate (1) is made of insulating and heat-resistant materials, and the pits on the pit flat plate (1) are arranged in a staggered pattern or a matrix pattern; (1) In the case of staggered arrangement, along the flow direction, the flow distance between the centers of each column of pits and the centers of the pits in the next column is Lx; along the spanwise direction, the spanwise distance between the centers of each row of pits and the centers of the pits in the next row is Lz; The staggered arrangement means that assuming there are 2n + 1 rows and 2m + 1 columns of pits, where n and m are non-zero positive integers, then the adjacent odd or even rows and adjacent odd or even columns include a total of 4 pits, and the centers of these 4 pits form a rectangle. Here, the rows and columns refer to the rows and columns of the overall array; the even or odd rows between the adjacent odd or even rows and the even or odd columns between the adjacent odd or even columns contain only one pit, and the center of this pit is exactly located at the center of the 4 pits; (2) In the case of matrix arrangement, it is a standard matrix arrangement, in the form of a P*Q matrix, with P rows and Q columns, where P and Q are positive integers greater than or equal to 2; The low-voltage electrode (23) is arranged under the pit flat plate (1), presenting a rectangular thin sheet as a whole. The projection of its geometric center on the xoz plane coincides with the projection of the geometric center of the pit flat plate (1) on the xoz plane. There is a certain distance reserved between the four sides of the low-voltage electrode (23) and the four sides of the pit flat plate (1); round holes corresponding to the pits on the pit flat plate (1) are provided in the low-voltage electrode (23), and the size of the round holes is the same as the size of the pits; the low-voltage electrode (23) is provided with a terminal; The high-voltage electrode (21) is arranged on the pit flat plate (1), and its overall size is less than or equal to the size of the pit flat plate (1); the high-voltage electrode (21) includes a plurality of spherical electrodes with the same number as the pits, corresponding positions one by one, and shapes exactly fitting the inside of the dielectric layer pits; these spherical electrodes are connected by connecting lines extending in the first diagonal direction of the pit flat plate (1) and all directions parallel to the first diagonal direction. These connecting lines are only used to connect adjacent spherical electrodes; at the same time, the spherical electrodes in the second diagonal direction are connected; the high-voltage electrode (21) is provided with a terminal; the terminals of the low-voltage electrode (23) and the high-voltage electrode (21) are not in the same place and maintain a certain distance; The projection of the center of the round hole of the low-voltage electrode (23) and the center of the corresponding spherical electrode of the high-voltage electrode (21) on the xoz plane coincides. The edge of the spherical electrode is exactly aligned with the edge of the round hole, and their sizes are both the same as the diameter of the circular cross-section of the pit on the pit flat plate (1).

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