NL2034681A - Multi-scale flow control friction drag reduction device in turbulent boundary layer and manfacturing method - Google Patents
Multi-scale flow control friction drag reduction device in turbulent boundary layer and manfacturing method Download PDFInfo
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- NL2034681A NL2034681A NL2034681A NL2034681A NL2034681A NL 2034681 A NL2034681 A NL 2034681A NL 2034681 A NL2034681 A NL 2034681A NL 2034681 A NL2034681 A NL 2034681A NL 2034681 A NL2034681 A NL 2034681A
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64C—AEROPLANES; HELICOPTERS
- B64C21/00—Influencing air flow over aircraft surfaces by affecting boundary layer flow
- B64C21/10—Influencing air flow over aircraft surfaces by affecting boundary layer flow using other surface properties, e.g. roughness
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
- F15D—FLUID DYNAMICS, i.e. METHODS OR MEANS FOR INFLUENCING THE FLOW OF GASES OR LIQUIDS
- F15D1/00—Influencing flow of fluids
- F15D1/10—Influencing flow of fluids around bodies of solid material
- F15D1/12—Influencing flow of fluids around bodies of solid material by influencing the boundary layer
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64C—AEROPLANES; HELICOPTERS
- B64C1/00—Fuselages; Constructional features common to fuselages, wings, stabilising surfaces or the like
- B64C1/0009—Aerodynamic aspects
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64C—AEROPLANES; HELICOPTERS
- B64C23/00—Influencing air flow over aircraft surfaces, not otherwise provided for
- B64C23/005—Influencing air flow over aircraft surfaces, not otherwise provided for by other means not covered by groups B64C23/02 - B64C23/08, e.g. by electric charges, magnetic panels, piezoelectric elements, static charges or ultrasounds
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64C—AEROPLANES; HELICOPTERS
- B64C23/00—Influencing air flow over aircraft surfaces, not otherwise provided for
- B64C23/06—Influencing air flow over aircraft surfaces, not otherwise provided for by generating vortices
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64F—GROUND OR AIRCRAFT-CARRIER-DECK INSTALLATIONS SPECIALLY ADAPTED FOR USE IN CONNECTION WITH AIRCRAFT; DESIGNING, MANUFACTURING, ASSEMBLING, CLEANING, MAINTAINING OR REPAIRING AIRCRAFT, NOT OTHERWISE PROVIDED FOR; HANDLING, TRANSPORTING, TESTING OR INSPECTING AIRCRAFT COMPONENTS, NOT OTHERWISE PROVIDED FOR
- B64F5/00—Designing, manufacturing, assembling, cleaning, maintaining or repairing aircraft, not otherwise provided for; Handling, transporting, testing or inspecting aircraft components, not otherwise provided for
- B64F5/10—Manufacturing or assembling aircraft, e.g. jigs therefor
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
- F15D—FLUID DYNAMICS, i.e. METHODS OR MEANS FOR INFLUENCING THE FLOW OF GASES OR LIQUIDS
- F15D1/00—Influencing flow of fluids
- F15D1/002—Influencing flow of fluids by influencing the boundary layer
- F15D1/0025—Influencing flow of fluids by influencing the boundary layer using passive means, i.e. without external energy supply
- F15D1/003—Influencing flow of fluids by influencing the boundary layer using passive means, i.e. without external energy supply comprising surface features, e.g. indentations or protrusions
- F15D1/0035—Influencing flow of fluids by influencing the boundary layer using passive means, i.e. without external energy supply comprising surface features, e.g. indentations or protrusions in the form of riblets
- F15D1/004—Influencing flow of fluids by influencing the boundary layer using passive means, i.e. without external energy supply comprising surface features, e.g. indentations or protrusions in the form of riblets oriented essentially parallel to the direction of flow
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
- F15D—FLUID DYNAMICS, i.e. METHODS OR MEANS FOR INFLUENCING THE FLOW OF GASES OR LIQUIDS
- F15D1/00—Influencing flow of fluids
- F15D1/002—Influencing flow of fluids by influencing the boundary layer
- F15D1/0065—Influencing flow of fluids by influencing the boundary layer using active means, e.g. supplying external energy or injecting fluid
- F15D1/0075—Influencing flow of fluids by influencing the boundary layer using active means, e.g. supplying external energy or injecting fluid comprising electromagnetic or electrostatic means for influencing the state of the fluid, e.g. for ionising the fluid or for generating a plasma
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
- F15D—FLUID DYNAMICS, i.e. METHODS OR MEANS FOR INFLUENCING THE FLOW OF GASES OR LIQUIDS
- F15D1/00—Influencing flow of fluids
- F15D1/002—Influencing flow of fluids by influencing the boundary layer
- F15D1/0085—Methods of making characteristic surfaces for influencing the boundary layer
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64C—AEROPLANES; HELICOPTERS
- B64C2230/00—Boundary layer controls
- B64C2230/12—Boundary layer controls by using electromagnetic tiles, fluid ionizers, static charges or plasma
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64C—AEROPLANES; HELICOPTERS
- B64C2230/00—Boundary layer controls
- B64C2230/26—Boundary layer controls by using rib lets or hydrophobic surfaces
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T50/00—Aeronautics or air transport
- Y02T50/10—Drag reduction
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- Fluid Mechanics (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Aviation & Aerospace Engineering (AREA)
- Electromagnetism (AREA)
- Plasma & Fusion (AREA)
- Manufacturing & Machinery (AREA)
- Transportation (AREA)
- Moulds For Moulding Plastics Or The Like (AREA)
- Plasma Technology (AREA)
Abstract
The present invention relates to a friction drag reduction device for multi-scale flow control and a manufacturing method. Strip-shaped DBD-VG and riblets surfaces are sequentially and alternately arranged in a streamwise direction, and the strip-shaped DBD-VG form a streamwise vortex array arranged in a spanwise direction so as to limit a spanwise irregular movement of large-scale flow structures in turbulent boundary layer and reduce friction drag components caused by the large-scale flow structures under a medium-high Reynolds number. The riblets surfaces reduce riblets surface friction drag caused by flow structures of near-wall region in the turbulent boundary layer, and control small-scale flow structures in the near-wall region. A region for controlling the multi- scale flow structures is formed in a combined action range of the streamwise vorteX array generated by the DBD-VG and the riblets surfaces.
Description
MULTI-SCALE FLOW CONTROL FRICTION DRAG REDUCTION DEVICE
IN TURBULENT BOUNDARY LAYER AND MANFACTURING METHOD
[DI] The present invention relates to the technical field of flow control, and in particular to a multi-scale flow control friction drag reduction device in turbulent boundary layer and a manufacturing method.
[02] Aerodynamic drag is the necessary cost paid out when an aircraft moves at a relative high speed, where frictional drag caused by a relative movement of a surface of the aircraft and a fluid accounts for about 50% of the total aerodynamic drag of the aircraft. For a large aircraft, a large amount of friction drag is generated by turbulent boundary layer attached to a surface of the large aircraft, and therefore, performing friction drag reduction research on the turbulent boundary layer helps significantly reduce the total drag of the aircraft.
[03] Under a given incoming flow condition and an aerodynamic configuration of the aircraft, the friction drag is often reduced by controlling a multi-scale flow structure in the turbulent boundary layer. Owing to having the advantages of being free of energy injection, easy in engineering deployment, low in additional mass, free of damage to a machine body structure, etc., the riblets surface technology has become a passive friction drag reduction flow control technology with a wide application prospect on an aircraft.
However, riblets drag reduction has not been currently applied in a large scale. In addition to riblets machining, and difficulty in laying mounting and maintenance, the following problems also limit a riblets drag reduction effect:
[04] (1) Requirements of the riblets drag reduction on incoming flow conditions are harsh, in practical application, the Reynolds number span is large, and it is often difficult to maintain the optimal drag reduction condition of riblets in a wide space-time range, such that the phenomenon of increased friction drag occurs when the designed Reynolds number is deviated.
[05] (2) Flowing in a real working condition is complex and changeable, and a local incoming flow tends not to coincide with a main direction of the riblets, such that an actual drag reduction effect is significantly reduced or even has a risk of drag increase.
[06] (3) A flow structure with rich scales is present in the turbulent boundary layer.
Large-scale flow structures (a size is greater than a nominal thickness of the boundary layer) of an outer area of the turbulent boundary layer under a medium-high Reynolds number is increasingly obvious to the friction drag of near-wall region, and in this case, an effect of reducing the friction drag only by controlling small-scale flow structures (a size is smaller than the nominal thickness of the boundary layer) of near riblets surfaces is limited.
[07] An objective of the present invention is to provide a friction drag reduction device for multi-scale flow control and a manufacturing method, which jointly apply control over two modes of large-scale flow structures and small-scale flow structures in turbulent boundary layer, thereby improving a drag reduction effect.
[08] In order to achieve the above objective, the present invention provides the solutions as follows:
[09] A friction drag reduction device for multi-scale flow control. The friction drag reduction device includes: strip-shaped Dielectric-Barrier-Discharge Vortex Generators (DBD-VG) and niblets surfaces.
[10] the number of the strip-shaped DBD-VG and the number of the riblets surfaces are the same and at least one, the strip-shaped DBD-VG and the riblets surfaces are sequentially and alternately arranged in a streamwise direction,
[11] the strip-shaped DBD-VG are configured to form a streamwise vortex array arranged in a spanwise direction so as to reduce riblets surface friction drag caused by irregular flow behaviors of large-scale structures in turbulent boundary layer, and
[12] The riblets surfaces are configured to reduce riblets surface friction drag caused by small-scale flow structures of near-wall region in the turbulent boundary layer.
[13] Optionally, in a normal direction, the strip-shaped DBD-VG and the riblets surfaces are arranged at the same solid riblets surface height, in the streamwise direction, the strip-shaped DBD-VG are arranged upstream of airflow, the riblets surfaces are arranged downstream of the airflow, leading edges of the riblets surfaces are tightly attached to trailing edges of the strip-shaped DBD-VG, and in the spanwise direction, a spanwise width of the strip-shaped DBD-VG are consistent with a spanwise width of the riblets surfaces.
[14] Optionally, each of the strip-shaped DBD-VG includes: an insulating dielectric layer, a plurality of upper electrodes and a plurality of lower electrodes.
[15] The plurality of lower electrodes are arranged inside the insulating dielectric layer, the plurality of lower electrodes are sequentially and uniformly arranged in the spanwise direction, and the plurality of upper electrodes are located on the insulating dielectric layer.
[16] Long sides of both the plurality of upper electrodes and the plurality of lower electrodes are parallel to the streamwise direction, the plurality of upper electrodes and the plurality of lower electrodes are sequentially and alternately arranged in the spanwise direction, a distance between the adjacent upper electrodes is equal to a width of the lower electrodes, and the lower electrodes are located under areas between the adjacent upper electrodes.
[17] The plurality of upper electrodes and the plurality of lower electrodes are configured to form an electric field, and under an action of the formed electric field, near-riblets surface gas between the adjacent upper electrodes forms a spanwise opposite-blowing jet, and forms a pair of reverse-arranged streamwise vortexes under an action of an incoming flow, and the plurality of upper electrodes jointly act to form a streamwise vortex array arranged in the spanwise direction.
[18] Optionally, each of the strip-shaped DBD-VG further includes: an upper common electrode, a lower common electrode, an alternating current power supply and a voltage measurement device.
[19] A leading edge of the insulating dielectric layer is provided with the upper common electrode extending in the spanwise direction, and a trailing edge of the insulating dielectric layer is provided with the lower common electrode extending in the spanwise direction.
[20] The plurality of upper electrodes are all connected to the upper common electrode, and the plurality of lower electrodes are all connected to the lower common electrode. The upper common electrode and the lower common electrode are connected to two ends of the alternating current power supply respectively, and two ends of the voltage measurement device are connected to the upper common electrode and the lower common electrode respectively.
[21] Optionally, the riblets surface is of a riblets surface structure formed by a plurality of riblets extending in the streamwise direction and sequentially arranged in the spanwise direction.
[22] A manufacturing method for the friction drag reduction device mentioned above. The manufacturing method includes:
[23] determining geometric parameters of the strip-shaped DBD-VG and the riblets surfaces respectively;
[24] manufacturing the strip-shaped DBD-VG according to the geometric parameters of the strip-shaped DBD-VG;
[25] manufacturing the riblets surfaces by using a polydimethylsiloxane polymer mold overturning process according to the geometric parameters of the riblets surfaces; and
[26] sequentially and alternately arranging the machined strip-shaped DBD-VG and the riblets surfaces in a streamwise direction to form the friction drag reduction device.
[27] Optionally, the determining geometric parameters of the strip-shaped DBD-
VG and the riblets surfaces respectively specifically includes:
[28] determining a distance between adjacent upper electrodes in the strip-shaped
DBD-VG by using a formula ,=200v/2z, where in the formula, A is the spanwise distance between adjacent upper electrodes, v represents kinematic viscosity coefficient, and 24 represents friction velocity; and
[29] determining a riblets width and a riblets depth of the riblets surfaces by using a formula s=/=151/4r, where in the formula, s is the riblets width, and #4 is the riblets 5 depth.
[30] Optionally, the manufacturing the strip-shaped DBD-VG according to the geometric parameters of the strip-shaped DBD-VG specifically includes:
[31] taking a polyimide film as an insulating dielectric layer, a double-conductive copper adhesive tape as lower electrodes and a lower common electrode, and a comb- shaped copper foil as upper electrodes and an upper common electrode, where a width of the double-conductive copper adhesive tape serving as the lower electrodes is equal to a distance between the adjacent upper electrodes;
[32] sequentially and uniformly arranging the lower electrodes in the insulating dielectric layer in a spanwise direction, and arranging the upper electrodes on the insulating dielectric layer according to the distance between the adjacent upper electrodes, such that the long sides of both the upper electrodes and the lower electrodes are parallel to the streamwise direction, the upper electrodes and the lower electrodes are sequentially and alternately arranged in the spanwise direction, the distance between the adjacent upper electrodes is equal to a width of the lower electrodes, and the lower electrodes are located under areas between the adjacent upper electrodes; and
[33] embedding the upper common electrode and the lower common electrode in a leading edge and a trailing edge of the insulating dielectric layer respectively.
[34] Optionally, the manufacturing the riblets surfaces by using a polydimethylsiloxane polymer mold overturning process according to the geometric parameters of the riblets surfaces specifically includes:
[35] a machining method for the riblets mold-overturning mold, which includes: when the riblets width of the riblets surfaces is greater than or equal to a width threshold (the threshold 1s about 0.5 mm, which is a limit size of large area machining), using CNC to machine a metal riblets mold-overturning mold, or manufacturing a non-metal riblets mold-overturning mold in a 3D printing manner, when the riblets width of the riblets surfaces is smaller than the width threshold, using a PVC finished grating film with a semi-circular surface as a mold overturning template, and fixing the PVC finished grating film at a bottom of the mold-overturning mold to form the riblets mold- overturning mold, where a size of the riblets mold-overturning mold conforms to the geometric parameters of the riblets surfaces;
[36] a preparation process for a riblets flexible film which includes:
[37] mixing polydimethylsiloxane colloid and an ethyl orthosilicate curing agent according to a weight of 10:1 to form a colloidal polymer;
[38] continuously vacuumizing the colloidal polymer in a vacuum drying oven at 0.1 MPa for 40 minutes so as to discharge bubbles visible to a naked eye in the colloidal polymer;
[39] pouring the colloidal polymer with bubbles discharged into the riblets mold- overturning mold, and filling the whole riblets mold-overturning mold with the colloidal polymer;
[40] vacuumizing the colloidal polymer in the riblets mold-overturning mold again until no bubbles are separated out from the surface, such that the colloidal polymer fills a riblets structure of the riblets mold-overturning mold;
[41] performing primary leveling on the riblets mold-overturning mold containing the colloidal polymer in the vacuum drying oven;
[42] continuously heating the colloidal polymer in the riblets mold-overturning mold at a temperature of 40°C in the vacuum drying oven until the surface of the colloidal polymer shows a semi-cured state and does not flow any more, and then taking out the mold;
[43] limiting a height of the colloidal polymer in each portion in the riblets mold- overturning mold by using a glass cover plate covered with a thermal shrinkage film so as to control a thickness of each portion of the colloidal polymer to be consistent;
[44] placing the riblets mold-overturning mold covered with the glass cover plate into the vacuum drying oven again for secondary leveling, and heating the mold again at an environment of 40°C until the surface of the colloidal polymer is completely cured; and
[45] separating the glass cover plate from the riblets mold-overturning mold, and cutting the colloidal polymer in the riblets mold-overturning mold to a required size to form the riblets flexible film.
[46] Optionally, the sequentially arranging the machined strip-shaped DBD-VG and the riblets surfaces alternately in a streamwise direction to form the friction drag reduction device specifically includes:
[47] manufacturing stepped riblets having a depth equal to a bottom thickness of riblets in the riblets surfaces in each of the leading edge and the trailing edge of the insulating dielectric layer of the strip-shaped DBD-VG; and
[48] covering the stepped riblets at the trailing edges of the strip-shaped DBD-VG with the machined leading edges of the riblets surfaces.
[49] According to particular examples provided by the present invention, the technical effects disclosed in the present invention are as follows:
[50] Disclosed in the present invention are a friction drag reduction device for multi-scale flow control in turbulent boundary layer and a manufacturing method. The strip-shaped DBD-VG and the riblets surfaces are sequentially and alternately arranged in the streamwise direction, the strip-shaped DBD-VG form the streamwise vortex array arranged in the spanwise direction, and the streamwise vortex array limits a spanwise irregular movement of the large-scale flow structures in the turbulent boundary layer and further reduces friction drag components caused by the large-scale flow structures under a medium-high Reynolds number. The riblets surfaces reduce riblets surface friction drag caused by the flow structures of near-wall region in the turbulent boundary layer, and control the small-scale flow structures with a size smaller than a thickness of the boundary layer in the near-wall region. A region for controlling the multi-scale flow structures in the turbulent boundary layer is formed in a combined action range of the streamwise vortex array generated by the DBD-VG and the riblets surfaces. According to the present invention, control is jointly applied to the irregular movement of the large-
scale flow structures in and out of the turbulent boundary layer and the irregular movement of the small-scale flow structures in the near-wall region, such that the corresponding friction drag components are reduced, the total drag is reduced, and a drag reduction effect is improved.
[51] To describe the technical solutions in the examples of the present invention or in the prior art more clearly, the accompanying drawings required for the examples are briefly described below. Apparently, the accompanying drawings in the following description show merely some examples of the present invention, and those of ordinary skill in the art may still derive other accompanying drawings from these accompanying drawings without creative efforts.
[52] FIG. 1 is a structural diagram of a friction drag reduction device for multi- scale flow control provided by the present invention;
[53] FIG. 2 is a schematic diagram of a friction drag reduction device for multi- scale flow control provided by the present invention;
[54] FIG. 3 1s an axially structural diagram of strip-shaped DBD-VG provided by the present invention;
[55] FIG. 4 is a cutaway view of strip-shaped DBD-VG provided by the present invention;
[56] FIG. 5 is a structural diagram of a riblets surface provided by the present invention;
[57] Fig. 6 is a flow chart of a manufacturing method for a friction drag reduction device provided by the present invention;
[58] FIG. 7 is a schematic diagram for mold overturning of a riblets flexible film provided by the present invention;
[59] FIG. 8 is a schematic structural diagram for preparation of strip-shaped DBD-
VG provided by the present invention;
[60] FIG. 9 is a schematic diagram for assembly of strip-shaped DBD-VG and riblets surface provided by the present invention;
[61] FIG. 10 is a schematic diagram for arrangement of bimodal riblets provided by an example of the present invention; and
[62] FIG. 11 is a result diagram for a low-speed wind tunnel experiment of bimodal riblets provided by an example of the present invention.
[63] Reference numerals: 1-strip-shaped Dielectric-Barrier-Discharge Vortex
Generators (DBD-VG), 2-riblets surface, 3-streamwise vortex, 4-alternating current power supply, 5-voltage measurement device, 6-riblets flexible film, 7-finished grating film, 8-mold-overturning mold, 9-friction drag measurement sensor, 10-wind tunnel, 11- upper electrode, 12-lower electrode, and 13-insulating dielectric layer.
[64] The technical solutions in the examples of the present invention will be clearly and completely described below with reference to the accompanying drawings in the examples of the present invention. Obviously, the described examples are merely some examples rather than all examples of the present invention. All the other examples obtained by those of ordinary skill in the art based on the examples in the present invention without creative efforts shall fall within the scope of protection of the present invention.
[65] An objective of the present invention is to provide a friction drag reduction device for multi-scale flow control and a manufacturing method, which jointly apply control over two modes of large-scale flow structures and small-scale flow structures in turbulent boundary layer, thereby improving a drag reduction effect.
[66] To make the above-mentioned objective, features and advantages of the present invention clearer and more comprehensible, the following will further describe the present invention in detail with reference to the accompanying drawing and particular embodiments.
[67] The present invention provides a friction drag reduction device for multi-scale flow control. As shown in FIG. 1, the friction drag reduction device includes: strip-
shaped DBD-VG 1 and riblets surfaces 2. The number of the strip-shaped DBD-VG 1 and the number of the riblets surfaces 2 are the same and at least one, and the strip- shaped DBD-VG 1 and the riblets surfaces 2 are sequentially and alternately arranged in a streamwise direction. The strip-shaped DBD-VG 1 are configured to form a streamwise vortex array arranged in a spanwise direction so as to reduce riblets surface friction drag caused by flow structures in and out of the turbulent boundary layer. The riblets surfaces 2 are configured to reduce riblets surface friction drag caused by flow structures of near-wall region in the turbulent boundary layer.
[68] In FIG. 1, an x direction indicates a streamwise direction, a y direction indicates a normal direction, and a z direction indicates a spanwise direction. The strip- shaped DBD-VG 1 limit a spanwise irregular movement of a strip structure at near riblets by generating a streamwise vortex array that is arranged in the spanwise direction, thereby reducing riblets surface friction drag caused by the occurrence of quasi- streamwise vortexes 3 and generation of a burst phenomenon. In the turbulent boundary layer, riblets bottoms of the riblets surfaces 2 are filled by low-speed flow structures or secondary vortexes, and high-speed flow structures such as the streamwise vortexes 3 are lifted on flow layers above the riblets surfaces 2, thereby achieving drag reduction by reducing direct contact between a solid riblets surface and a high-speed flow and limiting irregular movements of the flow structures above the flow layers. With reference to FIG. 2, the DBD-VG and the riblets surfaces 2 are sequentially and alternately arranged in the streamwise direction, which lies in that the streamwise vortex array formed by the DBD-VG still has a certain control effect on the large-scale flow structures in a downstream region of the electrodes. Meanwhile, riblets are simultaneously arranged downstream of the DBD-VG to control the small-scale flow structures of the near-wall region, and therefore, “bimodal riblets” which jointly control the large- and small-scale flow structures may be formed in the region. When the large- scale structures of the region fails to be effectively controlled due to dissipation of the streamwise vortex array along the process, a “DBD-VG-riblets” array is arranged in the streamwise direction again, and a portion which may control the large and small-scale flow structures in the turbulent boundary layer is formed in each region in which the riblets are laid in an alternating relay manner.
[69] Preferably, in a normal direction, the strip-shaped DBD-VG 1 and the riblets surfaces 2 are arranged at the same solid riblets surface height, in the streamwise direction, the strip-shaped DBD-VG 1 are arranged upstream of airflow, the riblets surfaces 2 are arranged downstream of the airflow, leading edges of the riblets surfaces 2 are tightly attached to trailing edges of the strip-shaped DBD-VG 1, and in the spanwise direction, a spanwise width of the strip-shaped DBD-VG 1 are consistent with a spanwise width of the riblets surfaces 2.
[70] Illustratively, as shown in FIG. 3 and FIG. 4, each of the strip-shaped DBD-
VG 1 includes: an insulating dielectric layer 13, a plurality of upper electrodes 11 and a plurality of lower electrodes 12. The plurality of lower electrodes 12 are arranged inside the insulating dielectric layer 13, the plurality of lower electrodes 12 are sequentially and uniformly arranged in the spanwise direction, and the plurality of upper electrodes 11 are located on the insulating dielectric layer 13. Long sides of both the plurality of upper electrodes 11 and the plurality of lower electrodes 12 are parallel to the streamwise direction, the plurality of upper electrodes 11 and the plurality of lower electrodes 12 are sequentially and alternately arranged in the spanwise direction, a distance between the adjacent upper electrodes 11 is equal to a width of the lower electrodes 12, and the lower electrodes 12 are located under regions between the adjacent upper electrodes 11. The plurality of upper electrodes 11 and the plurality of lower electrodes 12 are configured to form an electric field, and under a joint action of the formed electric field and an incoming flow, near-riblets surface gas between the adjacent upper electrodes 11 forms reverse-arranged streamwise vortexes 3, thereby forming a streamwise vortex array arranged in the spanwise direction.
[71] Each of the strip-shaped DBD-VG 1 further includes: an upper common electrode, a lower common electrode, an alternating current power supply 4 and a voltage measurement device 5. A leading edge of the insulating dielectric layer 13 is provided with the upper common electrode extending in the spanwise direction, and a trailing edge of the insulating dielectric layer 13 is provided with the lower common electrode extending in the spanwise direction. The plurality of upper electrodes 11 are all connected to the upper common electrode, and the plurality of lower electrodes 12 are all connected to the lower common electrode. The upper common electrode and the lower common electrode are connected to two ends of the alternating current power supply 4 respectively, and two ends of the voltage measurement device 5 are connected to the upper common electrode and the lower common electrode respectively.
[72] The insulating dielectric layer 13 is divided into two layers, namely an upper layer insulating dielectric layer and a lower layer insulating dielectric layer. The upper layer insulating dielectric layer is arranged on an upper surface of the lower layer insulating dielectric layer, and the plurality of upper electrodes 11 are arranged on the upper layer insulating dielectric layer. The lower electrodes 12 are clamped between the upper layer insulating dielectric layer and the lower layer insulating dielectric layer.
[73] Long sides of both the strip-shaped upper electrodes 11 and lower electrodes 12 of the DBD-VG are parallel to a streamwise direction (x direction). The lower electrodes 12 are wrapped around the upper and lower layer insulating dielectric layer 13, and the upper electrodes 11 located at a top of the insulating dielectric layer 13 are exposed in incoming flow air. The upper electrodes 11 are arranged on two sides of the lower electrodes 12 in the spanwise direction (z direction), the long sides of the upper electrodes overlap one another in the spanwise direction, in the normal direction (v direction), the upper electrodes are spaced apart by a distance equal to a thickness of the insulating dielectric layer 13 (as shown in FIG. 4), and the plurality of upper electrodes and the lower electrodes 12 are alternately arranged in the spanwise direction according to the rule. The plurality of upper electrodes 11 are connected by one common electrode extending in the spanwise direction at the leading edge of the insulating dielectric layer 13, and the plurality of lower electrodes 12 are also connected by one common electrode extending in the spanwise direction at the trailing edge of the insulating dielectric layer 13. The common electrode portions of the upper electrodes 11 and the lower electrodes 12 are connected to two ends of the alternating current power supply 4 respectively, and are connected to an oscilloscope or a voltmeter to obtain voltage information of two segments of each electrode (as shown in FIG. 3).
[74] The upper and lower electrodes of the DBD-VG are separated by the insulating dielectric layer 13 and are not in conduction. After the upper and lower electrodes are connected to positive and negative electrodes of the high-voltage alternating current power supply 4 respectively, an alternating electric field with sufficient strength is generated between the upper electrodes and the lower electrodes, such that the alternating electric field generates a dielectric barrier discharge phenomenon. Under the action of the electric field formed by the upper electrodes 11 exposed in the air and arranged in the spanwise direction and the lower electrodes 12 tow sides of which are covered with the dielectric layers, the near-riblets surface gas forms a spanwise opposite- blowing jet (as shown in FIG. 4) structure which is reverse-arranged, and furthermore, streamwise vortexes 3 are formed under the action of the incoming flow.
[75] Illustratively, the riblets surfaces 2 are of a riblets surface structure formed by a plurality of riblets which extend in the streamwise direction and are sequentially arranged in the spanwise direction. Traditional single-scale riblets are used to control the small-scale flow structures of near-wall region in the turbulent boundary layer. The single-scale riblets are composed of two-dimensional protruding ribs or channels extending in the streamwise direction on riblets surfaces of an object and alternately arranged in the spanwise direction, and the surface morphology is shown in FIG. 5.
[76] The present invention has the technical thinking as follows: on the basis of using the single-scale riblets to control friction reduction of the small-scale flow structures in the turbulent boundary layer, the streamwise vortex array generated by the strip-shaped DBD-VG 1 is also used for limiting the irregular movement of the large- scale flow structures, such that the friction drag components caused by the large-scale flow structures under a medium-high Reynolds number are further reduced, thereby obtaining a higher frictional drag reduction rate compared with a frictional drag reduction rate when only the single-scale flow structure is controlled. The device of the present invention is implemented as follows: the DBD-VG and the riblets are alternately arranged in the streamwise direction, and a region for controlling the multi-scale flow structures in the turbulent boundary layer is formed in a combined action range of the streamwise vortex array generated by the DBD-VG and the riblets surfaces 2. The device of the present invention is of a flexible film in design and machining and example, has the advantages of being easy to apply, low in shape drag, low in additional mass, and small in influence on the appearance and structure of an original solid riblets surface, and greatly improves engineering practicability of the present invention while presenting a good friction reduction effect.
[77] Compared with a friction reduction effect generated by conventional riblets which only control the small-scale flow structures of near-wall region in the turbulent boundary layer, the present invention has the advantages as follows:
[78] (1) The control over the large-scale structures changes the local flow of the near riblets, and the control of the streamwise vortex array generated by the DBD-VG over near-wall spanwise flowing reduces the influence of an incoming flow deflection angle on a drag reduction rate of the riblets to a certain extent, thereby improving drag reduction capacity of the present invention under a non-design working condition.
[79] (2) The friction drag components caused by the large-scale structures in the medium-high Reynolds number are increased, and in this case, the frictional drag reduction benefits jointly controlled by the large and small-scale structures are obvious, and the possibility of drag reduction application in an aircraft with a large feature size and a large Reynolds number span is provided.
[80] (3) A momentum and substance exchange between the near-wall region of the turbulent boundary layer and an outer region is suppressed while drag reduction is achieved by means of control over the large-scale flow, such that the possibility that dust particles in the incoming flow enter the near riblets, flow and block the riblets is reduced, and practicability of the friction drag reduction technology is improved by reducing requirements for a use environment.
[81] (4) The present invention inherits the advantages of the DBD-VG and the riblets of being easy to apply, light in weight, low in shape drag, and free of damage to the surface topography and an internal structure of a solid riblets surface. Particular embodiments of the flexible film of the present invention further reduce a use threshold of the friction drag reduction technology, which facilitates direct covering and use on the basis of an existing model.
[82] The present invention further provides a manufacturing method for the friction drag reduction device mentioned above. As shown in FIG. 6, the manufacturing method includes:
[83] step S1, determine geometric parameters of the strip-shaped DBD-VG 1 and the riblets surfaces 2 respectively.
[84] Illustratively, a method for determining the geometric parameters of the strip- shaped DBD-VG 1 is as follows:
[85] The upper electrodes and the lower electrodes are alternately arranged in the spanwise direction. A pair of reverse rotation streamwise vortex 3 appear on both sides of a spanwise direction of the single upper electrode, and a pair of reverse rotation streamwise vortexes appear between adjacent upper electrodes in the spanwise direction, and therefore, the relationship between an upper electrode distance and a streamwise vortex diameter ZD, (as shown in FIG. 3) is shown in formula (1):
[86] A=D,. (1)
[87] Since the spanwise distance of the streamwise strip structure is 100/%(2°=v/4, v represents kinematic viscosity coefficient , 2 represents friction velocity, /* represents the minimum scale of vortex in turbulent boundary layer ) and the large-scale flow structures is often controlled by a lower flow layer between the adjacent streamwise vortexes, selection is made:
[88] Dus=1007 (2)
[89] Furthermore, the spanwise distance of the strip-shaped upper electrodes may be selected as shown in formula (3):
[90] A=2000"=200vu. (3)
[91] Therefore, 2 and v are calculated according to the incoming flow turbulent boundary layer, that is, the upper electrode distance 4 of the DBD-VG for controlling the large-scale flow structures may be determined. For a lower electrode distance, there is no special requirement. In general, the long sides of the upper electrodes and the lower electrodes are overlapped in the spanwise direction, a width of the strip-shaped lower electrodes may be selected to be equal to the distance 4 of the upper electrodes, and the distance between the lower electrodes is equal to a width w of the upper electrodes (as shown in FIG. 3).
[92] There is no specific requirement for a streamwise length / of the upper and lower electrodes and a peak-to-peak value #;p of the power supply, which depends on the circumstances. Generally, larger | and £,, may generate a streamwise vortex 3 with higher circulation, and the flow control capability and drag reduction effect thereof are also better.
[93] Illustratively, a method for determining the geometric parameters of the riblets surfaces 2 is as follows:
[94] Take single-scale inverted U-shaped riblets as an example, optimal drag reduction feature sizes Sop: and A, of the riblets are generally selected as shown in formula (4):
[95] Sopt=hop=15=15vat;, (4)
[96] where s and h represent a center distance and a rib height respectively of the riblets ribs (as shown in FIG. 5). Therefore, 2 and v may be calculated according to the condition of the incoming flow turbulent boundary layer, thereby calculating the riblets size under the optimal drag reduction condition. In this case, the frictional drag reduction rate of the single-scale two-dimensional riblets may reach the maximum.
[97] Step S2, manufacture the strip-shaped DBD-VG 1 according to the geometric parameters of the strip-shaped DBD-VG 1;
[98] Illustratively, a polyimide film is used as the insulating dielectric layer 13. The thickness of the insulating layer covering the lower electrodes 12 is selected according to the electrode peak-to-peak value (Ep) to prevent the upper and lower electrodes of the insulating layer from being broken down to form a path at a higher alternating voltage. A double-conductive copper adhesive tape is used as the lower electrodes 12 and the lower common electrode, and a comb-shaped copper foil cut by a die is used as the upper electrodes 11 and the upper common electrode. The common portions of the upper and lower electrodes had better be buried in the leading edge and the trailing edge of the dielectric layer respectively so as to prevent point discharge at the joint of the common electrodes from breaking down the insulating layer, and also prevent an unnecessary streamwise jet at the upper common electrode. Specific manufacturing steps are as follows: 1, Take the polyimide film as the insulating dielectric layer 13, the double-conductive copper adhesive tape as the lower electrodes 12 and the lower common electrode, and the comb-shaped copper foil as the upper electrodes 11 and the upper common electrode, where a width of the double-conductive copper adhesive tape serving as the lower electrodes 12 is equal to the distance between the adjacent upper electrodes 11; 2, sequentially and uniformly arrange the lower electrodes 12 in the insulating dielectric layer 13 in the spanwise direction, and arrange the upper electrodes 11 on the insulating dielectric layer 13 according to the distance between the adjacent upper electrodes, such that the long sides of both the upper electrodes 11 and the lower electrodes 12 are parallel to the streamwise direction, the upper electrodes 11 and the lower electrodes 12 are sequentially and alternately arranged in the spanwise direction, the distance between the adjacent upper electrodes is equal to the width of the lower electrodes 12, and the lower electrodes 12 are located under regions between the adjacent upper electrodes; and 3, embed the upper common electrode and the lower common electrode in the leading edge and the trailing edge of the insulating dielectric layer 13 respectively.
[99] Step S3, manufacture the riblets surfaces 2 by using a polydimethylsiloxane polymer mold overturning process according to the geometric parameters of the riblets surfaces 2.
[100] The polydimethylsiloxane (PDMS) polymer mold overturning process is used for obtaining a riblets flexible film. Machining of a riblets mold and preparation of the riblets flexible film are included.
[101] (1) a machining method for the riblets mold-overturning mold includes: when the riblets width of the riblets surfaces 2 is greater than or equal to a width threshold (the threshold is about 0.5 mm, which is a limit size of large area machining), use CNC to machine the metal riblets mold-overturning mold, or manufacture a non-metal riblets mold-overturning mold in a 3D printing manner, when the riblets width of the riblets surfaces 2 is smaller than the width threshold, use a PVC finished grating film 7 with a semi-circular surface as a mold overturning template, and fix the PVC finished grating film 7 at a bottom of the mold-overturning mold 8 to form the riblets mold-overturning mold, where a size of the riblets mold-overturning mold conforms to the geometric parameters of the riblets surfaces 2. The grating film has a semi-circular surface, and inverted U-shaped riblets used in the present invention may be obtained after mold overturning is performed on the grating film (as shown in FIG. 5).
[102] (2) With reference to FIG. 7, a preparation process for the riblets flexible film includes:
[103] mix polydimethylsiloxane colloid and an ethyl orthosilicate curing agent according to a weight of 10:1 to form a colloidal polymer;
[104] © continuously vacuumize the colloidal polymer in a vacuum drying oven at 0.1 MPa for 40 minutes so as to discharge bubbles visible to a naked eye in the colloidal polymer;
[105] © pour the colloidal polymer with bubbles discharged into the riblets mold- overturning mold, and fill the whole riblets mold-overturning mold with the colloidal polymer;
[106] @ vacuumize the colloidal polymer in the riblets mold-overturning mold again until no bubbles are separated out from the surface, such that the colloidal polymer fills a riblets structure of the riblets mold-overturning mold,
[107] © perform primary leveling on the riblets mold-overturning mold containing the colloidal polymer in the vacuum drying oven;
[108] (© continuously heat the colloidal polymer in the riblets mold-overturning mold at a temperature of 40°C in the vacuum drying oven until the surface of the colloidal polymer shows a semi-cured state and does not flow any more, and then take out the mold, where continuous heating time is about 2 hours;
[109] © limit a height of the colloidal polymer in each portion in the riblets mold- overturning mold by using a glass cover plate covered with a thermal shrinkage film so as to control a thickness of each portion of the colloidal polymer to be consistent, where in order to solve the problem of difficult demoulding after contact between the colloidal polymer and glass, the glass cover plate is wrapped around the thermal shrinkage film, wrinkles on a surface of the cover plate caused by the thermal shrinkage film may be eliminated by heating, and the bubbles in the flexible film may be driven away by applying force to the cover plate for fine adjustment during limiting;
[110] © place the riblets mold-overturning mold covered with the glass cover plate into the vacuum drying oven again for secondary leveling, and heat the mold again at an environment of 40°C until the surface of the colloidal polymer is completely cured, where secondary heating time is about 4 hours; and
[111] © separate the glass cover plate from the riblets mold-overturning mold, and cut the colloidal polymer in the riblets mold-overturning mold to a required size to form the riblets flexible film.
[112] Step S4, sequentially and alternately arrange the machined strip-shaped DBD-
VG 1 and the riblets surfaces 2 in the streamwise direction to form the friction drag reduction device.
[113] For the DBD-VG controlling the large-scale flow structure, the upper electrodes 11 are exposed in air, and the lower electrodes 12 are embedded in the insulating dielectric layer 13. In order to prevent the situation that the regions where the spanwise jet fails to be generated in the upper electrodes 11 and the dielectric layer region are exposed too much in the incoming flow, such that shape drag is increased and consequently, a drag reduction effect of the device is influenced, the present invention combines the DBD-VG and the single-scale riblets flexible film 6 according to the following form (as shown in FIG. 8): stepped riblets with a width of about 1 cm and a depth about equal to a bottom thickness of the riblets film riblets is reserved at each of the leading edge and the trailing edge of the dielectric layer of the DBD-VG, and the machined riblets flexible film 6 covers the steps (as shown in FIG. 9). When arrangement is performed in this way, the upper surface of the DBD-VG is flush with a bottom of a riblets channel of the riblets flexible film, and the riblets flexible film also covers the upper electrode portion region where the upper surface of the DBD-VG extends to the stepped riblets of the dielectric layer and fails to generate the spanwise jet, thereby reducing an area of a non-drag-reduction region in the device to the maximum to reduce drag caused by the device itself. Based on this, a leading edge of the downstream riblets flexible film 6 is connected to the stepped riblets at the trailing edge of the upstream
DBD-VG, a trailing edge of the riblets flexible film 6 is connected to the stepped riblets located at the leading edge of the next DBD-VG located at the downstream of the riblets flexible film, and the “bimodal” riblets for jointly controlling large and small-scale flow structures in the turbulent boundary layer is formed by means of a DBD-VG and riblets relay form combination.
[114] The streamwise arrangement region of the riblets is determined according to an effective action range of the streamwise vortex array generated by the upstream DBD-
VG on the large-scale structure. The region covered by the riblets is a region for jointly controlling friction reduction of the multi-scale flow structures of the turbulent boundary layer, and intensity of the streamwise vortexes and action ranges of the streamwise vortexes on large-scale flow control may be improved by improving the electrode voltage peak-to-peak value Ep, increasing the strip-shaped electrode streamwise length 1 and other measures, such that an area proportion of the friction reduction region in a single group of DBD-VG-riblets streamwise array is improved, and the friction reduction effect of the device is further improved.
[115] The control range of the DBD-VG downstream streamwise vortex is limited, and when the large-scale structures of the region fail to be effectively controlled due to dissipation of the streamwise vortex array along the process, a “DBD-VG-riblets” array is arranged in the streamwise direction again. By periodically and alternately arranging the DBD-VG-riblets, regions which jointly apply control over the large and small-scale flow structures in the turbulent boundary layer may be arranged as much as possible in the whole region to be subjected to drag reduction, thereby further improving the frictional drag reduction effect of the present invention.
[116] The present invention will be described hereafter in conjunction with a specific application scenario. In the example, a fully developed flat turbulent boundary layer 1s used as a control object, Re~2700 ~ 8200 (Re: is the friction Reynolds number,
Re=u:0/v, where ò is a nominal thickness of the turbulent boundary layer, v represents kinematic viscosity coefficient, and u; represents friction velocity), and 0=10cm. The spanwise distance of the upper electrodes of the DBD-VG is 4=2.5 cm, the streamwise length /=ò, and the electrode voltage peak-to-peak value Erp=5 k~20 kV. The section of the used single-scale riblets is semi-circular, and the rib spanwise center distance is s=15"~30/", and the rib height is 4=7? — 15/". The device entity uses two sets of “vortex generator-riblets” arrays to be arranged in a wind tunnel 10 in the streamwise direction, and as shown in FIG. 10, a friction drag measurement sensor 9 (a double-layer thermal film sensor) is arranged close to the trailing edge spanwise center of the device to determine drag at this position. A drag reduction rate (DR) is defined as shown in formula (5):
Smooth wall surface friction~ [UT] pp = Yale mena aby pot ein 00, (5)
[118] In an Re~8200 experiment, the maximum frictional drag reduction rate of the present invention may be reach 6% (as shown in FIG. 11). In FIG. 11, Z1 represents the frictional drag reduction rate when the DBD-VG electrode voltage peak-to-peak value
Epp=15 KV, and Z2 represents the frictional drag reduction rate when E&pr=20 KV.
[119] Various examples in the description are described in a progressive manner, differences between each example and other examples are mainly described, and the same and similar portions among various examples are seen from each other for reference.
[120] In the specification, particular embodiments are used for illustration of the principles and implementations of the present disclosure. The description of the foregoing embodiments is used to help illustrate the method of the present disclosure and the core principles thereof. In addition, those of ordinary skill in the art can make any modification in terms of particular implementations and scope of application in accordance with the teachings of the present disclosure.
In conclusion, the content of the present description shall not be construed as a limitation to the present invention.
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| US20110120980A1 (en) * | 2005-10-17 | 2011-05-26 | Thomas Corke | System and Method for Aerodynamic Flow Control |
| US8220754B2 (en) * | 2009-06-03 | 2012-07-17 | Lockheed Martin Corporation | Plasma enhanced riblet |
| US20220055739A1 (en) * | 2006-09-08 | 2022-02-24 | Delos Aerospace Llc | Method and apparatus for mitigating trailing vortex wakes of lifting or thrust generating bodies |
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| US8460779B2 (en) * | 2011-03-30 | 2013-06-11 | General Electric Company | Microstructures for reducing noise of a fluid dynamic structure |
| CN203222109U (en) * | 2013-04-18 | 2013-10-02 | 北京航空航天大学 | Plasma vortex generator |
| CN103287575B (en) * | 2013-06-07 | 2016-01-13 | 上海交通大学 | Based on the method for the minimizing skin resistance that plasma exciter realizes |
| CN203743140U (en) * | 2014-02-23 | 2014-07-30 | 中国科学院工程热物理研究所 | Resistance reduction rib |
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| US20110120980A1 (en) * | 2005-10-17 | 2011-05-26 | Thomas Corke | System and Method for Aerodynamic Flow Control |
| US20220055739A1 (en) * | 2006-09-08 | 2022-02-24 | Delos Aerospace Llc | Method and apparatus for mitigating trailing vortex wakes of lifting or thrust generating bodies |
| US8220754B2 (en) * | 2009-06-03 | 2012-07-17 | Lockheed Martin Corporation | Plasma enhanced riblet |
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