A surface structure and design method applicable to the rotor of a low Reynolds number aircraft

Abstract

The present invention discloses a surface structure and design method of a rotor for a low Reynolds number aircraft applicable to the Martian environment, belonging to the technical field of aerospace aircraft. In the leading edge part of the airfoil, based on the initial low Reynolds number airfoil, by changing the control points, a serrated wrinkled surface is designed. The present invention can increase the turbulence intensity at low Reynolds numbers, form a turbulent boundary layer, realize drag reduction by laminar separation control at low Reynolds numbers, and reduce the shock wave drag, improve the lift-drag ratio of the rotor aircraft, thereby improving its aerodynamic efficiency, and the advantages are particularly obvious in Martian drones. The serrated wrinkled surface can fix the separation point, well control the laminar separation bubble, the change of the lift coefficient with the decrease of the Reynolds number is smaller than that of the smooth surface, and the stall characteristics are improved, and the performance is more excellent.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerospace vehicles, and particularly relates to a surface structure and design method of a rotor for a low Reynolds number aircraft applicable to the Martian environment. Background Art

[0002] Improving the lift coefficient of the rotor at low Reynolds numbers is an important task in the design of aircraft rotors. In future Mars exploration, Mars rotor unmanned aerial vehicles (UAVs) are suitable for collaborative exploration tasks with Mars rovers because they can explore high places and many areas inaccessible to Mars rovers, and can also point for Mars rovers. Therefore, they have become a research trend in various countries.

[0003] Factors such as different atmospheric densities, gravities, extreme temperatures, and energy sources on Mars and Earth determine that the design of Mars propellers faces great difficulties. The extremely low atmospheric density makes the rotor operate under low Reynolds number conditions, posing high requirements for the performance of the rotor. For a Mars UAV to take off, it needs to be light in weight. At the same time, the UAV should be able to be housed in a Mars rover, which requires the blades to be compact, light in weight and small in volume, and able to withstand high rotational speeds. In the Martian environment, when the rotor rotates at high speed, the interference between the shock wave and the boundary layer will cause strong shock wave drag and a large amount of unsteady motion. Reducing shock wave drag is an important part of aircraft design. To reduce this interference, vortex generators are often set on the blade surface to form a turbulent boundary layer, thereby suppressing boundary layer separation and improving the aerodynamic characteristics of the aircraft.

[0004] Generally, a smooth airfoil is superior to a wrinkled airfoil; however, when the Reynolds number is below 25,000, the lift coefficient of the smooth airfoil drops sharply with the decrease of the Reynolds number, and the lift performance of the rotor deteriorates sharply; while the lift coefficient of the wrinkled airfoil changes less significantly with the decrease of the Reynolds number. At this time, the lift performance of the rotor with a wrinkled airfoil is more superior than that of the rotor with a smooth airfoil. In addition, when the Reynolds number is below 25,000, as the angle of attack increases, the smooth airfoil is extremely prone to sudden stall, while the airfoil with wrinkles can maintain the surface airflow and make the stall occur more smoothly. Summary of the Invention

[0005] The present invention provides a surface structure and design method for a rotor of a low Reynolds number aircraft, which improves the lift-to-drag ratio of the rotor aircraft, thereby improving the aerodynamic efficiency of the rotor UAV operating between Reynolds numbers of 15,000 - 25,000.

[0006] To achieve the above objectives, the present invention adopts the following technical solutions:

[0007] A surface structure applicable to the rotor of a low Reynolds number aircraft, wherein the aircraft rotor is of a streamlined structure. In the leading edge part of the rotor, a corrugated surface is formed on the basis of the streamlined structure, and the corrugated surface is serrated. The serrations form fixed vortex generators to help form the boundary layer and contribute to improving the airfoil performance.

[0008] In the above-mentioned structure, the corrugated surface is provided with multiple serrated protrusions, and a smooth transition is achieved between the serrations based on the initial airfoil.

[0009] A design method for a surface structure applicable to the rotor of a low Reynolds number aircraft, comprising the following steps:

[0010] Step 1: Generate a serrated surface in the leading edge part of the initial airfoil by changing the control points.

[0011] Step 2: Quickly perform a flow field simulation in a CFD program and observe the control effect of the serrated surface on the laminar separation bubble in the flow field diagram.

[0012] Step 3: While observing the control effect of the serrated surface on the laminar separation bubble in the flow field diagram in Step 2, analyze the influence of the designed serrated surface on the lift, drag coefficients of the airfoil and their ratio by calculating the Navier - Stokes equations, and then optimize the serrated surface using the method in Step 1.

[0013] Step 4: Repeat the above steps to obtain the optimal solution.

[0014] In the above-mentioned steps, in Step 1, the overall geometric shape of the initial airfoil is described by a continuous curve. By changing the control points on the curve, a serrated surface is generated in the leading edge part of the initial airfoil, so that the laminar transition region is completely controlled within the serrations.

[0015] The flow field simulation in Step 2 is a viscous boundary layer flow field simulation.

[0016] In Step 3, the k - kl - ω transition model is used, and the specific formula of the equation is as follows:

[0017]

[0018]

[0019]

[0020] In the formula: D() / Dt is the derivative of the term in the brackets with respect to time; ρ is the atmospheric density; k is the turbulent kinetic energy; ω is the unit dissipation rate; μ is the viscosity coefficient; u represents the velocity, x represents the coordinate axis system, and its subscripts i and j both represent the directions of each axis system; σk, σω, σω,2, γ, β, β*, a1 are all constants; τ ij is the Reynolds stress term; μt is the eddy viscosity coefficient; Ω is the vorticity; F1 and F2 are mixing effect functions, and their specific expressions are:

[0021]

[0022]

[0023]

[0024]

[0025]

[0026] where: y is the height from the wall surface; ν is the velocity scale;

[0027] With the above model, in step 3, the airfoil is aerodynamically analyzed. According to the results of the aerodynamic analysis, the effects of sawtooth protrusions at different positions, different numbers of sawteeth, and different sawtooth spacings on the aerodynamic performance of the airfoil are analyzed. Using the method in step 1, the serrated surface is optimized. On the basis of ensuring the ability to maintain the control of the laminar separation bubble, by adjusting the position of the sawtooth protrusions, the number of sawteeth, and the distance between sawteeth, better performance can be obtained.

[0028] Beneficial effects: The present invention provides a surface structure suitable for the rotor of a low Reynolds number aircraft and its design method. Multiple protruding serrated structures are arranged along the initial airfoil arc on the rotor surface, and the streamline structure of the initial airfoil is still maintained between the sawteeth. The involved serrated wrinkled surface can fix the separation point, promote transition, increase the turbulence intensity, form a turbulent boundary layer, and well achieve laminar separation control and drag reduction at low Reynolds numbers and reduce shock wave drag, and prevent sudden stall at high angles of attack. It can improve the lift-drag ratio of the rotor aircraft at a lower Reynolds number, thereby improving its aerodynamic efficiency, significantly improving the stall characteristics, providing good performance, and meeting the performance requirements of Mars unmanned aircraft and other rotor unmanned aircraft operating at lower Reynolds numbers. Description of the Drawings

[0029] Figure 1 is a schematic diagram of laminar separation occurring on the upper surface of a smooth airfoil at low Reynolds numbers;

[0030] Figure 2 is a schematic diagram of laminar separation occurring on the upper surface of a serrated airfoil at low Reynolds numbers;

[0031] Figure 3 is a schematic diagram of the surface structure of a serrated airfoil;

[0032] Figure 4 is a detailed view of the wrinkled surface of a serrated airfoil;

[0033] Figure 5 Grid schematic diagram of the serrated airfoil involved in the invention;

[0034] Figure 6 Lift coefficient - angle of attack curve of the serrated airfoil;

[0035] Figure 7 Drag coefficient - angle of attack curve of the serrated airfoil. Specific implementation mode

[0036] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments:

[0037] As Figure 1 shown, at low Reynolds numbers, when the airflow passes over the upper surface of a smooth airfoil, laminar separation occurs, forming a laminar separation bubble. Among them, the laminar flow leaves the upper surface and transforms into a turbulent flow state. In this case, the laminar separation bubble destroys the streamlined structure of the airfoil and has a negative impact on the airfoil performance.

[0038] Based on the above situation, a surface structure of the rotor of a low Reynolds number aircraft applicable to the Martian environment is designed, including the following steps:

[0039] The first step: Describe the overall geometric shape of the initial airfoil with a continuous curve. By selectively changing the control points on the curve, a serrated surface is generated at the leading edge part of the initial airfoil, so as to be able to control the entire laminar - turbulent transition region within the serrations;

[0040] The second step: Quickly perform a viscous boundary - layer flow field simulation in the CFD program to observe the control effect of the serrated surface on the laminar separation bubble;

[0041] The third step: According to the results obtained from the simulation, while observing the control effect of the serrated surface on the laminar separation bubble in the flow field diagram, analyze the influence of the designed serrated surface on the lift and drag coefficients of the airfoil and their ratio by calculating the Navier - Stokes equation. The k - kl - ω transition model is used, and the specific formula of the equation is as follows:

[0042]

[0043]

[0044]

[0045] In the formula: D() / Dt represents the derivative of the term inside the parentheses with respect to time; ρ is the atmospheric density; k is the turbulent kinetic energy; ω is the unit dissipation rate; μ is the viscosity coefficient; u represents velocity, x represents the coordinate axis system, and its subscripts i and j both represent the directions of each axis system; σk, σω, σω, 2, γ, β, β*, a1 are all constants; τij is the Reynolds stress term; μt is the eddy viscosity coefficient; Ω is the vorticity; F1 and F2 are mixing effect functions, and the specific expressions are as follows:

[0046]

[0047]

[0048]

[0049]

[0050]

[0051] In the formula: y is the height from the wall surface; ν is the velocity scale.

[0052] With the above model, the airfoil is aerodynamically analyzed. According to the results of the aerodynamic analysis, the effects of the sawtooth protrusions at different positions, different numbers of sawteeth, and different sawtooth spacings on the aerodynamic performance of the airfoil are analyzed. In the first step, the serrated surface is optimized. On the basis of ensuring the ability to maintain the control of the laminar separation bubble, the position, number, and distance between the sawtooth protrusions are adjusted to obtain better performance. Repeat the above steps to finally obtain the optimal design scheme.

[0053] As Figure 3 shown, the structure designed by the above method is: the rotor of the aircraft is of a streamlined structure. At the leading edge of the rotor, a wrinkled surface is formed on the basis of the streamlined structure. As Figure 4 shown, the wrinkled surface is four sawtooth protrusions, and the sawteeth are smoothly transitioned based on the initial airfoil. The sawteeth form a fixed vortex generator, which helps to form the boundary layer and improve the airfoil performance.

[0054] As Figure 1 shown, at low Reynolds numbers, when the air flow passes through the upper surface of the smooth airfoil, laminar transition occurs on the airfoil surface, forming a laminar separation bubble, which causes a large resistance and destroys the aerodynamic performance of the aircraft; as Figure 2 shown, when the air flow passes through the upper surface of the serrated airfoil, vortices are formed inside the sawteeth. At this time, the equivalent geometric shape composed of the vortices and the airfoil is smooth, and no obvious laminar separation bubble is seen on the upper surface of the airfoil, and the drag reduction effect is good.

[0055] As Figure 5 shown, to analyze the performance of the designed airfoil, the corresponding grid is drawn for aerodynamic analysis in the flow field simulation;

[0056] As Figure 6 and Figure 7 shown, the variation trends of the lift coefficient and the drag coefficient of the serrated airfoil with the increase of the angle of attack are respectively shown at Reynolds numbers of 15000, 18000, 21000 and Mach numbers of 0.45, 0.65, 0.85. It can be seen from the figure that the lift coefficient and the drag coefficient generally increase with the increase of the angle of attack. Among them, with the decrease of the Reynolds number, the lift coefficient of the serrated airfoil does not decrease significantly, especially at high Mach numbers; while at lower Mach numbers, when the Reynolds number drops to 15000, the lift coefficient of the serrated airfoil increases instead, indicating that the serrated airfoil has good adaptability at low Reynolds numbers. Compared with general low-Reynolds-number airfoils, the serrated airfoil has a smaller drag coefficient, and the change of the lift coefficient with the decrease of the Reynolds number is less than that of a smooth surface, indicating that the designed serrated surface has a certain drag reduction effect and improves the stall characteristics, and the performance is more excellent.

[0057] The above is the preferred embodiment of the present invention, but the present invention is not limited to the above embodiments. For those of ordinary skill in the art in this technical field, within the scope of their knowledge and without departing from the principle of the present invention, several improvements can be made to the present invention, and these are also regarded as the protection scope of the present invention.

Claims

1. A design method for the surface structure of a rotor of a low Reynolds number aircraft applicable to the Martian environment, characterized in that, It includes the following steps: Step 1: Generate a serrated surface at the leading edge part of the initial airfoil by changing the control points. Step 2: Quickly conduct a flow field simulation in the CFD program to observe the advantages and disadvantages of the control effect of the serrated surface on the laminar separation bubble in the flow field diagram. Step 3: While observing the advantages and disadvantages of the control effect of the serrated surface on the laminar separation bubble in the flow field diagram in Step 2, analyze the influence of the designed serrated surface on the lift coefficient, drag coefficient and their ratio of the airfoil by calculating the Navier-Stokes equation, and then optimize the serrated surface by using the method in Step 1. Step 4: Repeat the above steps to obtain the best solution.

2. The design method of the surface structure of the rotor of a low Reynolds number aircraft applicable to the Martian environment according to claim 1, characterized in that In Step 1, the overall geometric shape of the initial airfoil is described by a continuous curve. By changing the control points on the curve, a serrated surface is generated at the leading edge part of the initial airfoil, so that the laminar transition region is completely controlled inside the serrations.

3. The design method of the surface structure of the rotor of a low Reynolds number aircraft applicable to the Martian environment according to claim 1, characterized in that, In Step 2, the flow field simulation is a viscous boundary layer flow field simulation.

4. The design method of the surface structure of the rotor of a low Reynolds number aircraft applicable to the Martian environment according to claim 1, characterized in that, The transition model analysis used in Step 3 is as follows. The specific formula of the equation is as follows: ， ， ， where: D() / Dt is the derivative of the term in the parentheses with respect to time; ρ is the atmospheric density; k is the turbulent kinetic energy; ω is the unit dissipation rate; μ is the viscosity coefficient; u represents velocity, x represents the coordinate axis system, and its subscripts i and j both represent the directions of each axis system; σ ｋ , σ ω , σ ω，２ , γ, β, β*, a1 are all constants; τ ij is the Reynolds stress term; μ t is the eddy viscosity coefficient; Ω is the vorticity; F1, F2 are the mixing effect functions, and the specific expressions are: ， ， ， ， ， In the formula: y is the height from the wall surface; ν is the velocity scale.

5. The design method of the surface structure of the rotor of a low Reynolds number aircraft applicable to the Martian environment according to claim 1 or 4, characterized in that In Step 3, analyze the influence of the serration protrusions at different positions, different numbers of serrations, and different serration spacings on the aerodynamic performance of the airfoil. Use the method in Step 1 to optimize the design of the serrated surface. On the basis of ensuring the ability to maintain the control of the laminar separation bubble, adjust the position of the serration protrusions, the number of serrations, and the serration distance to obtain better performance.

6. The surface structure of the rotor of a low Reynolds number aircraft applicable to the Martian environment designed by the method according to any one of claims 1-5, characterized in that, The rotor of the aircraft is of a streamlined structure. At the leading edge part of the rotor, a wrinkled surface is formed on the basis of the streamlined structure, and the wrinkled surface is serrated. The serrations form fixed vortex generators to help form the boundary layer and contribute to improving the airfoil performance.

7. The surface structure of the rotor of a low Reynolds number aircraft applicable to the Martian environment according to claim 6, characterized in that, The wrinkled surface is provided with multiple serrated protrusions, and the serrations are smoothly transitioned based on the initial airfoil.

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