CN117566095A - A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure - Google Patents

Abstract

A three-dimensional turbulence drag reduction micro-rib structure with longitudinal change belongs to the technical field of hydrodynamic drag reduction. The invention aims to solve the problem that the multi-dimensional micro ribs may not have a drag reduction effect. The surface of the fluid wall material foundation is provided with a plurality of sawtooth structures which periodically undulate along the fluid flow direction, the surface of each sawtooth structure is provided with a plurality of micro-rib main bodies with equilateral triangle cross sections, the micro-rib main bodies are arranged along the fluid flow direction, and the micro-rib main bodies are sequentially propped against each other along the direction perpendicular to the fluid flow direction. The invention changes the mimicking structure in the turbulence boundary layer near the wall surface by limiting the outline dimension of the micro-rib structure, promotes the high-speed movement coherent vortex structure to be far away from the wall surface, reduces the contact area range of high-speed fluid and the wall surface, and replaces the significant reduction of friction resistance with the cost of slightly increasing pressure difference resistance, so that the total resistance is greatly reduced. Has important significance for the green sustainable development of energy conservation and emission reduction of the aircraft.

Description

A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure

Technical field

The invention belongs to the technical field of fluid mechanics drag reduction, and in particular relates to a longitudinally changing three-dimensional turbulent drag reduction micro-rib structure.

Background technique

The significant benefit of reducing the surface resistance of an aircraft is that it can save fuel. A lot of research work has been done in this regard. Among them, the application of micro-ribs is a simple and effective passive flow control method to reduce the surface resistance of the aircraft.

As early as the 1970s, Walsh and others from NASA in the United States studied the concept of microribs. By imitating the skin shape of sharks, a creature that can swim long distances and at high speeds, V-shaped microribs showed approximately 6% drag reduction, which changes people's traditional understanding of hydraulically smooth surfaces with minimal resistance. Afterwards, Bechert et al. conducted extensive experimental work including different micro-rib shapes and sizes. The effective protrusion height theory was proposed to explain the mechanism of micro-ribs reducing surface friction resistance. This theory believes that micro-ribs reduce the pulsating velocity component in the cross-flow direction, but there is no consensus in the academic community. Choi et al. used a direct numerical simulation method to simulate the drag reduction effect of triangular ribs, and pointed out a different mechanism, that is, the drag-reducing micro-ribs limit the displacement of near-wall flow vortices and only allow the tips of the micro-ribs to be exposed to high speeds. in the fluid. Therefore, only the micro-rib tips have higher resistance, while the rest of the wetted area has lower resistance. However, if the size of the micro-ribs is too large and causes the flow direction vortices to enter the interior of the micro-ribs, the resistance will increase. Martin et al. used a large eddy simulation method to study blade-type micro-ribs, focusing on the near-wall vortex results. The results show that when the flowwise vortex is about 1.5 times the micro-rib spacing, the best drag reduction effect of the microribs can be observed, while when the flowwise vortex size is smaller than the width of the microribs, the drag will increase. This conclusion is consistent with the explanation of the drag reduction mechanism by Choi et al. In fact, at this stage, people have formed some preliminary consensus on the drag reduction mechanism of micro-ribs, that is, the drag reduction or drag-increasing effect is achieved through the interaction between micro-ribs and near-wall flow vortices, where micro-ribs The geometry and dimensionless dimensions are key influencing factors.

An important reason that currently limits the further engineering application of micro-ribs is that the economic benefits of using micro-ribs to reduce drag on the outer surface of aircraft and other aircraft are not sufficient to support the cost of applying, maintaining and removing them as a coating or film. There are two ways to solve this dilemma: one is to use new manufacturing technology to print micro-ribs directly on the patent leather on the outer surface, thereby reducing costs; the other is to enhance the drag-reducing effect of micro-ribs. In the above related research work, it was found through numerical simulation or experimental research that compared with traditional micro-ribs, multi-dimensional micro-ribs are a better choice in terms of drag reduction performance, such as sinusoidal-shaped micro-ribs or herringbone-shaped micro-ribs. . However, the drag reduction effect of these multidimensional microribs is very sensitive to dimensional parameters and requires careful design, which suggests that some potential correlation between microrib geometry and near-wall turbulent flow dominates their drag characteristics, as if multidimensional microribs did not Choosing the appropriate dimensions may not have the drag reduction effect or may have the negative effect of increasing drag. Therefore, it is necessary to further explore micro-rib drag reduction technology to improve its drag reduction performance.

Contents of the invention

The purpose of the present invention is to provide a longitudinally changing three-dimensional turbulent drag reduction micro-rib structure to solve the problem that if the multi-dimensional micro-ribs are not selected to a suitable external size, they may not have a drag reduction effect or may have the negative effect of increasing drag. . The technical solutions adopted by the present invention are as follows:

A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure, including a fluid wall material base. The surface of the fluid wall material base is provided with a number of periodically undulating sawtooth structures along the fluid flow direction. The surface of the sawtooth structure is provided with a number of equal cross-sections. A triangular-sided micro-rib main body is arranged along the fluid flow direction, and several micro-rib main bodies are arranged in order to offset each other in a direction perpendicular to the fluid flow direction;

Assuming that the dimensionless spacing of the micro-rib body is s ⁺ and the dimensionless height is h ⁺ , then s ⁺ is determined by the following formula:

h ⁺ is determined by:

In the formula:

δ _v is the viscosity scale of turbulence;

s is the actual spacing between adjacent micro-rib bodies;

h is the actual height of the micro-rib body;

δ _v is determined by:

In the formula:

v is the kinematic viscosity of the fluid;

u _τ is the friction speed of the fluid wall material base;

_uτ is determined by the following formula:

In the formula:

τ _w is the shear stress of the fluid wall material base;

ρ density of fluid;

Suppose the dimensionless length of the sawtooth structure is L ⁺ , then L ⁺ is determined by the following formula:

In the formula:

L is the actual length of the sawtooth structure.

Further, the range of s ⁺ is 10 to 25, and the range of h ⁺ is 8.66 to 21.65.

Furthermore, the sawtooth structure is composed of an ascending section and a descending section. The flow direction length _L1 of the ascending section accounts for 90% of L, and the flow direction length _L2 of the descending section accounts for 10% of L. The flow direction length of the ascending section and the free flow direction of the fluid are The angle α is 5°.

Further, the value of L ⁺ is taken to be 150.

Furthermore, the fluid wall material base may be the surface layer of an aircraft fuselage, aircraft wing, blade fluid machine, or a flexible polymer organic material film.

Compared with the prior art, the beneficial effects of the present invention are:

By changing the distribution structure of the existing micro-ribs along the flow direction, the present invention arranges several micro-rib bodies on a sawtooth structure that periodically fluctuates, so that the pseudo-sequential structure in the turbulent boundary layer near the wall is changed and promotes high-speed movement. The coherent vortex structure is far away from the wall, which reduces the contact area between the high-speed fluid and the wall. It also reduces the probability that the streamwise vortex structure will induce high shear stress and resistance near the wall due to sudden events, at the expense of a small increase in pressure differential resistance. In exchange for a significant reduction in frictional resistance, the total resistance can be significantly reduced. This overcomes the shortcoming of low drag reduction rate of traditional micro-ribs and can further enhance the drag-reduction effect of micro-ribs. This invention is applied to the outer surfaces of common aircraft and fluid machinery, such as aircraft fuselages, wings, propeller blades, underwater ship hulls, and automobile bodies, to reduce the resistance they receive when working and contribute to the green and sustainable development of energy conservation and emission reduction. has great significance.

Description of the drawings

Figure 1 is a schematic structural diagram of the present invention;

Figure 2 is a schematic cross-sectional view of the main body of several micro-ribs arranged on the surface of the sawtooth structure;

Figure 3 is a schematic structural diagram of the present invention along the vertical fluid flow direction;

Figure 4 is a longitudinal section instantaneous velocity distribution contour diagram of the present invention;

Figure 5 shows the horizontal cross-sectional velocity distribution and high/low speed strip structure of the traditional micro-rib structure at the dimensionless height y ⁺ = 36 near the wall;

Figure 6 shows the horizontal cross-sectional velocity distribution and high/low speed strip structure at the dimensionless height y ⁺ = 36 near the wall according to the present invention.

In the figure, 1. Fluid wall material foundation, 2. Sawtooth structure, 21. Rising section, 22. Descending section, 3. Micro-rib main body.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is described below through the specific embodiments shown in the drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the invention. Furthermore, in the following description, descriptions of well-known structures and techniques are omitted to avoid unnecessarily confusing the concepts of the present invention.

The connection mentioned in the present invention is divided into fixed connection and detachable connection. The fixed connection is a non-detachable connection, including but not limited to conventional fixed connection methods such as flange connection, rivet connection, adhesive connection and welding connection. Detachable connections include but are not limited to conventional disassembly methods such as bolt connections, snap connections, pin connections, and hinge connections. When the specific connection method is not clearly defined, by default, at least one connection method can be found among the existing connection methods to achieve this function. Those skilled in the art can make their own choices as needed. For example: choose welding connection for fixed connection, and choose bolt connection for detachable connection.

The present invention will be further described in detail below with reference to the accompanying drawings. The arrows in Figure 1 represent the fluid flow direction, the vortex lines in Figure 2 represent flow vortices, and the vortex lines in Figure 3 represent spanwise vortices. The following embodiments illustrate the present invention. explanation of the invention, but the invention is not limited to the following examples.

Embodiment: As shown in Figure 1-6, a longitudinally changing three-dimensional turbulent drag reduction micro-rib structure includes a fluid wall material base 1. The surface of the fluid wall material base 1 is provided with a number of periodically undulating saw teeth along the fluid flow direction. Structure 2, the surface of the sawtooth structure 2 is provided with a number of micro-rib bodies 3 with an equilateral triangle cross-section. The micro-rib bodies 3 are arranged along the direction of fluid flow, and the several micro-rib bodies 3 are arranged in order to offset each other in a direction perpendicular to the direction of fluid flow;

Assuming that the dimensionless spacing of the micro-rib body 3 is s ⁺ and the dimensionless height is h ⁺ , then s ⁺ is determined by the following formula:

h ⁺ is determined by:

In the formula:

δ _v is the viscosity scale of turbulence;

s is the actual spacing between adjacent micro-rib bodies 3;

h is the actual height of the micro-rib body 3;

δ _v is determined by:

In the formula:

v is the kinematic viscosity of the fluid;

u _τ is the friction speed of the fluid wall material base 1;

_uτ is determined by the following formula:

In the formula:

τ _w is the shear stress of the fluid wall material base 1;

ρ density of fluid;

Suppose the dimensionless length of the sawtooth structure 2 is L ⁺ , then L ⁺ is determined by the following formula:

In the formula:

L is the actual length of the sawtooth structure 2.

The range of s ⁺ is 10 to 25, and the range of h ⁺ is 8.66 to 21.65. The range of the dimensionless spacing s ⁺ at which the micro-rib main body 3 in the present invention exhibits excellent drag reduction performance is preferably 10 to 25, that is, the maximum drag reduction effect occurs within this range. When exceeding this dimensionless size range, the micro-rib body 3 in the present invention may lose its drag reduction effect or have the negative effect of increasing drag. This is because the optimal dimensionless spacing s ⁺ and h ⁺ are restricted by the diameter of the near-wall turbulent high/low speed strip structure and the closely related streamwise vortex structure. According to research, the general average dimensionless diameter of near-wall flow vortices is between 20 and 30. When the dimensionless spacing s ⁺ of the micro-rib body 3 is within the preferred range, the average dimensionless diameter of the near-wall flow vortex is larger than the dimensionless spacing of the micro-rib body 3 , and the flow vortex is located above the micro-rib body 3 . When the dimensionless spacing of the micro-rib bodies 3 exceeds the optimal range, the near-wall high-speed flow vortex motion originally pushed away by the micro-rib bodies 3 will drop close to the wall micro-ribs, or even completely fall between the two micro-rib bodies 3. Inside the groove between them, the high-speed fluid contacts more wall surface area and generates more resistance, thus causing the drag reduction effect to deteriorate or even increase the resistance. Therefore, when processing, manufacturing and using the micro-rib body 3, it should be ensured that it is within the above-mentioned dimensionless size range. Correspondingly, since the cross-sectional shape of the micro-rib body 3 is an equilateral triangle, the dimensionless height h ⁺ of the micro-rib body 3 is 0.866s ⁺ , that is, the range of h ⁺ is 8.66 to 21.65.

The sawtooth structure 2 is composed of an ascending section 21 and a descending section 22. The flow length L ₁ of the ascending section 21 accounts for 90% of L, and the flow length L ₂ of the descending section 22 accounts for 10% of L. The ascending section 21 is free from the fluid. The angle α between the flow directions is 5°.

The value of L ⁺ is taken as 150. In the present invention, the dimensionless length L ⁺ of the sawtooth structure 2 showing excellent drag reduction performance is preferably 150. When the above size range is changed, the micro-rib main body 3 in the present invention may lose the drag reduction effect or have the negative effect of increasing drag. This is because the optimal dimensionless length L ⁺ is restricted by the length of the near-wall turbulent high/low speed strip structure and the closely related streamwise vortex structure. According to research, the general average dimensionless length of near-wall flowwise vortices is about 150. When the dimensionless length L ⁺ of the sawtooth structure 2 is the preferred size of 150, the near-wall turbulence can form an effective drag-reducing flow structure similar to a rolling bearing in a wide range, that is, the flow velocity is formed in the periodically undulating depressions of the sawtooth structure 2 The lower spanwise vortex structure acts like a rolling bearing, causing the high-speed fluid above it to break away from the wall and move on the rolling bearing-like structure, thereby reducing drag.

The dimensionless dimensions s ⁺ , h ⁺ and L ⁺ mentioned above are a common concept in the field of fluid mechanics and can be used to simplify the complexity of physical problems and transform them into a more concise form. By introducing the concept of dimensionless size, micro-ribs and flow states of different scales can be summarized and unified, thereby obtaining a more universal and concise expression result. The micro-rib body 3 in the present invention can be applied to different flow environments including different densities, flow speeds, fluid viscosity, etc. The actual size of the micro-rib body 3 itself also varies with the different flow environments used. Generally speaking, for medium and large passenger aircraft in a common cruising state in the atmosphere, the actual size of the micro-rib body 3 in the present invention should be between 1-1000 μm according to the flow density and viscosity of the aircraft and the characteristic size of the aircraft. When the flow environment in which the micro-rib body 3 in the present invention is located changes, it is necessary to calculate the new turbulent viscosity scale (or internal scale) δ _v according to the changed new flow environment, and then calculate the new turbulent viscosity scale (or internal scale) according to the present invention. The preferred dimensionless dimensions s ⁺ , h ⁺ and L ⁺ are calculated, and the new actual dimensions s, h and L of the micro-rib structure with excellent drag reduction effect are calculated.

The fluid wall material base 1 can be the surface layer of aircraft fuselages, aircraft wings and bladed fluid machinery on which laser etching can be performed. The laser etching process can also be replaced by a new, lower-cost spray painting process, or can be attached The flexible polymer organic material film on the outer surface of the metal. The size of the film can be adjusted according to the conditions of the processing equipment. Generally, it is formed by splicing multiple films and then covering and pasting them on the outer surface.

This embodiment gives a practical application example of the present invention. When working in water as a fluid medium, that is, the fluid density ρ is 998.2kg/m ³ , the kinematic viscosity v is 1.0048×10 ^-6 Ns/m ² , and the wall friction speed u _τ is 1.8×10 ^-2 m/s, the inner scale of turbulence or the viscosity scale δ _v is 55.56 μm. The fluid wall material base 1 of the micro-rib structure is a flexible material attached to the outer surface of its fluid mechanical metal. The polymer organic material film is formed by splicing multiple small-sized films and then covering and pasting them on the outer surface of the fluid machinery metal. On the fluid wall material base 1, a number of periodically undulating sawtooth structures 2 are continuously arranged along the fluid flow direction. The longitudinal cross-sectional shape of the sawtooth structure 2 is a sawtooth shape. The actual length L of an independent unit sawtooth structure 2 is 8.333mm. An independent unit sawtooth structure 2 consists of an ascending section 21 and a descending section 22. The flow length L ₁ of the ascending section 21 accounts for 90% of the total length L of the unit, and the flow length L ₂ of the descending section 22 accounts for 10% of the total length L of the unit. %, the angle between the ascending section 21 and the free flow direction of the fluid, that is, the angle of attack α is 5°. A number of micro-rib bodies 3 are arranged on the zigzag structure 2 along the vertical fluid flow direction. The cross-sectional shape of the micro-rib body 3 is an equilateral triangle. All micro-rib bodies 3 are equidistant and parallel along the flow direction, and their dimensions are: The actual distance s between two adjacent micro-rib bodies 3 is 1 mm, and the actual height h of the triangular cross-section of the micro-rib body 3 is 0.866 mm. The fluid wall material base 1, a plurality of sawtooth structures 2 and a plurality of micro-rib bodies 3 constitute the micro-rib structure of the present invention. The parameters of the micro-rib structure correspond to the dimensionless dimensions: the dimensionless spacing s ⁺ is 18, the dimensionless The dimensionless height h ⁺ is 15.588 and the dimensionless length L ⁺ is 150, both of which are within the preferred parameter range. After testing and analysis, it was found that in the area far away from the wall of the micro-rib structure, the instantaneous velocity distribution contours show that the speed is large and chaotic, but in the area near the wall of the micro-rib structure, especially in the two sawtooth structures 2 The flow is quieter near the depression in the connecting area. It can be seen from the velocity distribution and high/low speed strip structure of the h ⁺ = 36 horizontal section near the wall that the high/low speed strip structure of the turbulent flow near the wall can be clearly identified. Compared with the traditional micro-rib structure, the present invention Microrib structures exhibit longer ribbon structures that can extend to more than 1,000 internal scales. This indicates that the high-speed vortex motion is further pushed from the viscous bottom layer to the outer layer, and due to the interaction between turbulence and the micro-rib structure, the turbulent boundary layer above the micro-rib structure is quieter at the same dimensionless normal height. , which is conducive to further reduction of resistance. Compared with the traditional micro-rib structure, the longitudinally changing three-dimensional turbulent drag reduction micro-rib structure of this specific embodiment can increase the drag reduction rate from 6.55% to 9.74% in the same state, achieving an additional drag reduction rate of 3.19%.

By changing the distribution structure of the existing micro-ribs along the flow direction, the present invention arranges several micro-rib bodies 3 on the sawtooth structure 2 that periodically fluctuates, so that the pseudo-sequential structure in the turbulent boundary layer near the wall is changed, promoting The high-speed moving coherent vortex structure is far away from the wall, which reduces the contact area between the high-speed fluid and the wall. It also reduces the probability that the streamwise vortex structure will induce high shear stress and resistance near the wall due to sudden events, thereby increasing the pressure difference resistance by a small amount. The price is exchanged for a significant reduction in frictional resistance, resulting in a significant reduction in total resistance. This overcomes the shortcoming of low drag reduction rate of traditional micro-ribs and can further enhance the drag-reduction effect of micro-ribs. This invention is applied to the outer surfaces of common aircraft and fluid machinery, such as aircraft fuselages, wings, propeller blades, underwater ship hulls, and automobile bodies, to reduce the resistance they receive when working and contribute to the green and sustainable development of energy conservation and emission reduction. has great significance.

The above embodiments are only illustrative illustrations of the present invention and do not limit its protection scope. Those skilled in the art can also make partial changes to them. As long as the spirit and essence of the present invention are not exceeded, they are all within the protection scope of the present invention.

Claims (5)

1. A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure, characterized by: including a fluid wall material base (1), and the surface of the fluid wall material base (1) is provided with a number of periodically undulating sawtooth structures along the fluid flow direction. (2), the surface of the sawtooth structure (2) is provided with a number of micro-rib bodies (3) with equilateral triangle cross-sections. The micro-rib bodies (3) are arranged along the direction of the fluid flow. Several micro-rib bodies (3) are arranged along the direction perpendicular to the fluid flow. The directions of flow are arranged in order to offset each other; Assuming that the dimensionless spacing of the micro-rib body (3) is s ⁺ and the dimensionless height is h ⁺ , then s ⁺ is determined by the following formula: h ⁺ is determined by: In the formula: δ _v is the viscosity scale of turbulence; s is the actual spacing between adjacent micro-rib bodies (3); h is the actual height of the micro-rib body (3); δ _v is determined by: In the formula: v is the kinematic viscosity of the fluid; u _τ is the friction speed of the fluid wall material base (1); _uτ is determined by the following formula: In the formula: τ _w is the shear stress of the fluid wall material foundation (1); ρ density of fluid; Suppose the dimensionless length of the sawtooth structure (2) is L ⁺ , then L ⁺ is determined by the following formula: In the formula: L is the actual length of the sawtooth structure (2). 2. A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure according to claim 1, characterized in that: the range of s ⁺ is 10 to 25, and the range of h ⁺ is 8.66 to 21.65. 3. A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure according to claim 1, characterized in that: the sawtooth structure (2) consists of an ascending section (21) and a descending section (22), and the ascending section (22) The flow direction length L ₁ of 21) accounts for 90% of L, the flow direction length L ₂ of the descending section (22) accounts for 10% of L, and the angle α between the ascending section (21) and the free flow direction of the fluid is 5°. 4. A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure according to claim 3, characterized in that: the value of L ⁺ is 150. 5. A longitudinally changing three-dimensional turbulent drag reduction micro-rib structure according to any one of claims 1 to 4, characterized in that: the fluid wall material base (1) can be an aircraft fuselage, an aircraft wing, a blade type The surface layer or flexible polymer organic material film of fluid machinery.