A simulated sand dune graded drag reduction double-layer microrib structure

Abstract

This invention discloses a sand-ridge-like hierarchical drag-reducing double-layer microrib structure, relating to the field of surface drag reduction technology in fluid dynamics. The proposed sand-ridge-like hierarchical drag-reducing double-layer microrib structure includes primary drag-reducing microrib structures and secondary drag-reducing microrib structures arranged in a sequential array. The primary drag-reducing microrib structures are arranged on a material substrate layer, and the secondary drag-reducing microrib structures are placed between the primary drag-reducing microrib structures, with a pair of secondary microrib structures placed between each pair of primary drag-reducing microrib structures. The vortex structure size within the turbulent boundary layer continuously changes, and there are many small vortex structures inside. The sand-ridge-like hierarchical drag-reducing double-layer microrib structure stably controls the turbulent vortex structures at all scales, significantly reducing momentum exchange between the turbulent vortex structures and the wall surface, reducing the obstruction to internal flow, and decreasing frictional drag.

Description

Technical Field

[0001] This invention relates to the field of surface drag reduction technology in fluid dynamics, and in particular to a simulated sand dune graded drag reduction double-layer microrib structure, which improves the drag reduction performance of the microrib structure. Background Technology

[0002] In recent years, with the development of the aviation industry, especially the demand for high load capacity, long range, and low fuel consumption in the domestic large aircraft project, higher requirements have been placed on drag reduction technology. Various drag reduction technologies, such as aerodynamic shape optimization, surface modification, and lightweighting, have become increasingly mature, but their drag reduction improvement has reached a bottleneck stage.

[0003] For drag reduction in large aircraft, flow stagnation within the turbulent boundary layer near the aircraft surface is the primary source of frictional drag. Reducing frictional drag has always been a crucial and challenging problem. Achieving turbulent drag reduction by altering the microstructure of the aircraft surface offers a novel approach. Lee et al. proposed that the main principle of microrib drag reduction is: modifying the surface microstructure to control the flow within the boundary layer, stabilizing and lifting the vortex flow within the turbulent boundary layer, improving the stability of the near-wall flow to reduce momentum exchange between the vortex structure within the turbulent boundary layer and the wall, thereby reducing flow stagnation and ultimately lowering surface frictional drag.

[0004] In recent years, scholars both domestically and internationally have conducted extensive research on this topic. A series of patents, represented by patent publication number CN106585949A entitled "A Bionic Shark Skin Drag Reduction Structure" and patent publication number CN105644770B entitled "A Drag Reduction Method for a Shark Skin-Inspired Drag-Reducing Wing," represent the micro-rib drag reduction technology derived from shark skin biomimetic research. This technology is considered by the academic community to be a promising solution with significant engineering application potential. It is estimated that 50% of the frictional drag of large aircraft during cruise comes from surface frictional drag. However, previous research on shark skin-based micro-rib drag reduction technology has a relatively simple drag reduction structure and provides a limited drag reduction rate. Practical applications require greater drag reduction efficiency. Furthermore, the internal flow of turbulent boundary layer is a complex flow with multi-scale vortex structures. From the perspective of micro-rib drag reduction principles, controlling complex turbulent flow with a simple micro-rib structure cannot control the turbulent vortex structures at more scales, thus limiting its drag reduction performance. Therefore, the potential of micro-rib drag reduction technology remains to be explored, and its drag reduction performance needs further improvement. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing microrib structures in terms of drag reduction performance due to their single structure. In order to improve their drag reduction performance, based on sand dunes in nature with good drag reduction morphology, and after extensive screening and research, a sand dune-like graded drag reduction double-layer microrib structure is proposed, which can more effectively reduce flow resistance compared to existing microrib structures.

[0006] The proposed invention discloses a simulated sand dune-like graded drag-reducing double-layer microrib structure, comprising (a) a primary drag-reducing microrib structure and (b) a secondary drag-reducing microrib structure. The primary drag-reducing microrib structures are arranged on a material substrate layer, and the secondary drag-reducing microrib structures are placed between the primary drag-reducing microrib structures, with a pair of secondary microrib structures placed between each pair of primary drag-reducing microrib structures. These are arranged sequentially in an array to form the simulated sand dune-like graded drag-reducing double-layer microrib structure.

[0007] Specifically, the arrangement of the simulated sand-ridge graded drag-reducing double-layer microrib structure can be continuous, such as... Figure 1 As shown; and can be oscillating continuously in the flow direction, such as Figure 4 As shown; it can also have gaps, and the gaps can be arranged in a straight line or not in a straight line, such as... Figure 5 and Figure 6 As shown.

[0008] Specifically, the main stage drag-reducing microrib structure of the simulated sand dune graded drag-reducing double-layer microrib structure has a dimensionless height of h. + and dimensionless width t + The microribbed structure has no limitation on its shape, all structures are parallel along the flow direction, and the dimensionless spacing between the structures is s. + .in:

[0009]

[0010]

[0011]

[0012] Where, is the viscous scale in turbulence; u τ The wall friction speed; τ is the wall shear stress, which needs to be determined based on actual flow conditions; ρ is the fluid medium density; and υ is the kinematic viscosity. Microrib structures with good drag reduction performance generally require s + Between 15 and 18, approximately the average size D of the flow-directed vortex. m + (On average, it is about 30 viscous scales; according to Robinson, the diameter of the flow-directed vortex ranges from 5 to 115 viscous scales) 1 / 2 to 2 / 3, aspect ratio h + / s + =0.5, width t + The smaller the size, the better the drag reduction performance. In this way, each pair of main stage drag-reducing microrib structures forms a stable lifting control effect on the turbulent vortex structure inside the turbulent boundary layer, so that most turbulent vortex structures will not intrude due to their motion and cause momentum exchange between the main stage drag-reducing microrib structures and the wall, resulting in flow stagnation, thus forming the main stage drag reduction effect.

[0013] Specifically, the dimensionless dimension is specified to unify the dimensions of the simulated sand-ridge graded drag-reducing double-layer microrib structure under different flow conditions. When the simulated sand-ridge graded drag-reducing double-layer microrib structure is used for flow control under different flow conditions or different Reynolds numbers, the turbulent viscosity scale under the current flow condition needs to be determined, and then the actual dimensionless dimension s is used as the reference. + h + and t + After converting to actual dimensions, the superscript "+" indicates that the turbulent viscous scale has been used for dimensionless transformation. Generally, under flow conditions with high Reynolds numbers and ultra-low viscous scales, such as in aircraft, the actual size of the simulated sand-ridge graded drag-reducing double-layer microrib structure is within the range of tens to hundreds of micrometers.

[0014] The secondary drag-reducing microrib structure has a dimensionless height of h0. + and dimensionless width t0 + The structure has no fixed shape, is parallel to the flow direction, and has a dimensionless spacing of s0 between it. + Because the size of the vortex structure within the turbulent boundary layer is a changing process, energy is transferred within vortices of varying sizes, forming a series of energy levels. The energy dissipates from large vortices to form smaller vortices. Therefore, there exists a dimensionless spacing s smaller than that of the main stage drag-reducing microrib structure. + The turbulent vortex structure intrudes between the primary drag-reducing microrib structures. The secondary drag-reducing microstructures located between the primary drag-reducing microrib structures form stable lifting control flow-direction vortices with the same drag-reducing effect as the primary drag-reducing microstructures, thereby lifting and controlling the small-sized D0. + (Minimum dissipation scale of turbulent structures) <D0 + + The flow direction of the vortex. Ultimately, a sand-ridge-like, graded drag-reducing, double-layered microrib structure is formed, such as... Figure 1 As shown.

[0015] Preferably, the dimensionless dimension s of the primary drag-reducing microrib structure of the imitation sand dune graded drag-reducing double-layer microrib structure is s. + ≈18,h + / s + =0.5, t + =0.14s + The dimensionless spacing s0 of the secondary drag-reducing microrib structure + 1 / 2 to 2 / 3 s + dimensionless height h0 + 1 / 2s0 + dimensionless width t0 + Less than or equal to 1 / 2t + .

[0016] ​Compared with existing micro-rib drag reduction technology, the beneficial effects of this invention are:

[0017] When the incoming flow within the turbulent boundary layer passes through the simulated sand dune-like staged drag-reducing double-layer microrib structure, the larger turbulent vortex structures are stably lifted and controlled by the paired primary drag-reducing microrib structures, preventing excessive momentum exchange with the wall. As the turbulent vortex structures move and transfer energy, smaller turbulent vortex structures dissipate and intrude between the primary drag-reducing microrib structures. At this point, the paired secondary drag-reducing microrib structures located between the primary drag-reducing microrib structures come into play, stably lifting and controlling the smaller turbulent vortex structures, preventing excessive momentum exchange between the smaller turbulent vortex structures and the wall. Compared to the shark-like microrib structure studied previously, this further improves the control of vortex structures within the turbulent boundary layer. Figure 2 As shown, the size of the vortex structure in the turbulent boundary layer changes continuously, and there are many small vortex structures inside. The sand-ridge-like graded drag-reducing double-layer microrib structure stably controls the turbulent vortex structure at all scales, greatly reducing the momentum exchange between the turbulent vortex structure and the wall, reducing the obstruction to the internal flow, and reducing frictional drag. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the drag reduction mechanism of the simulated sand dune graded drag reduction double-layer microrib structure.

[0019] Figure 2 This is a schematic diagram of a stable control vortex structure based on a graded drag reduction double-layer microrib structure mimicking sand dunes.

[0020] Figure 3 This is a schematic diagram of a simulated sand dune graded drag reduction double-layer microrib structure, as shown in Implementation Case 1.

[0021] Figure 4 This is a schematic diagram of the simulated sand dune graded drag reduction double-layer microrib structure in specific implementation case 2.

[0022] Figure 5 , 6 This is a schematic diagram of the simulated sand dune graded drag reduction double-layer microrib structure in specific implementation case 3. Detailed Implementation Plan

[0023] The following embodiments further describe and demonstrate preferred embodiments within the scope of the present invention. These embodiments are merely illustrative and should not be construed as limiting the invention.

[0024] Specific implementation case 1:

[0025] The primary drag-reducing microrib structure of the simulated sand dune hierarchical drag-reducing double-layer microrib structure has dimensions of s = 140 μm, h = 70 μm, t = 20 μm and s0 = 70 μm, h0 = 35 μm, t0 = 10 μm. Both the primary and secondary drag-reducing microrib structures are continuously arranged. Both the simulated sand dune hierarchical drag-reducing double-layer microrib structure and the substrate material layer are made of silicon-based material, with a smooth plane on the upper surface of the substrate material layer. Figure 3 As shown. Under the flow condition with a turbulent viscosity scale of 7.6 μm, the dimensionless dimension s corresponding to the main stage drag-reducing microrib structure of the sand-ridge stage drag-reducing double-layer microrib structure. + =18.4, h + =9.2, t + =2.62. The secondary drag-reducing microrib size is s0. + =9.2, h0 + =4.6, t0 + =1.31. Testing and analysis revealed that this specific implementation case, with its simulated sand-ridge graded drag-reducing double-layer micro-rib structure, achieves an 86.66% improvement in drag reduction compared to the traditional micro-rib structure.

[0026] Specific Implementation Case 2:

[0027] The primary drag-reducing microrib structure of the simulated sand dune-like graded drag-reducing double-layer microrib structure has dimensions of s = 140 μm, h = 70 μm, t = 20 μm and s0 = 70 μm, h0 = 35 μm, t0 = 10 μm. Both the primary and secondary drag-reducing microrib structures are continuously arranged, but oscillate along the flow direction with a certain amplitude. Both the simulated sand dune-like graded drag-reducing double-layer microrib structure and the substrate material layer are made of UV adhesive (ultraviolet-curing adhesive). The upper surface of the substrate material layer is a smooth plane, such as... Figure 4 As shown. Under the flow condition with a turbulent viscosity scale of 8.3 μm, the dimensionless dimension s corresponding to the main stage drag-reducing microrib structure of the sand-ridge stage drag-reducing double-layer microrib structure. + =16.9, h + =8.5, t + =2.4. The secondary drag-reducing microrib size is s0. + =8.5, h0 + =4.3, t0 + =1.2. Testing and analysis revealed that this specific implementation case, featuring a sand-ridge-like graded drag-reducing double-layer micro-rib structure, achieves an 87.70% improvement in drag reduction compared to the traditional micro-rib structure that oscillates with a certain amplitude.

[0028] Specific Implementation Case 3:

[0029] The primary drag-reducing microrib structure of the simulated sand dune hierarchical drag-reducing double-layer microrib structure has dimensions of s = 140 μm, h = 70 μm, t = 20 μm and s0 = 70 μm, h0 = 35 μm, t0 = 10 μm. Both the primary and secondary drag-reducing microrib structures are discontinuously arranged, with the gaps between them either aligned in a straight line or not. Both the simulated sand dune hierarchical drag-reducing double-layer microrib structure and the substrate material layer are made of PDMS (polydimethylsiloxane), and the upper surface of the substrate material layer is a smooth plane. Figure 5 , 6 As shown. Under the flow condition with a turbulent viscosity scale of 9.1 μm, the dimensionless dimension s corresponding to the main stage drag-reducing microrib structure of the sand-ridge stage drag-reducing double-layer microrib structure. + =15.5, h + =7.7, t + =2.2. The secondary drag-reducing microrib size is s0. + =7.7, h0 + =3.8, t0 + =1.1. Testing and analysis revealed that this specific implementation case, featuring a sand-ridge-like graded drag-reducing double-layer micro-rib structure, achieves a drag reduction effect of 90.45% compared to the traditional micro-rib structure that oscillates with a certain amplitude.

Claims

1. A sand dune imitating hierarchical drag-reducing double-layer micro-rib structure, characterized in that, The micro-riblet structure comprises (a) a primary drag-reducing micro-riblet structure and (b) a secondary drag-reducing micro-riblet structure; the primary drag-reducing micro-riblet structure is arranged on a material substrate layer, and the secondary drag-reducing micro-riblet structure is arranged between the primary drag-reducing micro-riblet structures; one pair of secondary drag-reducing micro-riblet structures is arranged between each pair of primary drag-reducing micro-riblet structures; The primary drag-reducing micro-riblet structures and the secondary drag-reducing micro-riblet structures are arranged in an array in sequence to form the sand-ridge-imitating hierarchical drag-reducing double-layer micro-riblet structure; The arrangement mode of the sand-ridge-imitating hierarchical drag-reducing double-layer micro-riblet structure is one of the following: linear continuous arrangement, oscillatory continuous arrangement in a flow direction, or gap arrangement. The main drag-reducing micro-rib structure of the sand-ridge-imitating hierarchical drag-reducing double-layer micro-rib structure has a dimensionless height h + and a dimensionless width t + , all the structures are parallel along the flow direction, and the dimensionless distance between the structures is s + ; wherein: wherein v / u τ is the viscous scale in turbulent flow; u τ is the wall friction velocity; τ is the wall shear stress, p is the fluid medium density; v is the kinematic viscosity, h is the actual height of the primary drag-reducing micro-rib structure, t is the actual width of the primary drag-reducing micro-rib structure, s is the actual pitch of the primary drag-reducing micro-rib structure; + is between 15 and 18; The secondary drag-reducing micro-rib structure is a structure with a dimensionless height h0 + and a dimensionless width t0 + , and the dimensionless spacing between the structures is s0 + , which is defined by analogy with the primary drag-reducing micro-rib structure; the parameter relationship is as follows: the dimensionless spacing s0 + of the secondary drag-reducing micro-rib structure is 1 / 2-2 / 3s + , the dimensionless height h0 + is 1 / 2s0 + , and the dimensionless width t0 + is less than or equal to 1 / 2t + .

2. A sand dune emulating hierarchical drag-reducing double-layer micro- riblet structure as claimed in claim 1, wherein, The primary drag reducing micro rib structure has a dimensionless size s + is 18, the aspect ratio h + / s + = 0.5, t + = 0.14s + .

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