A Subsonic Cylindrical Aircraft Body Surface Boundary Layer Artificial Thickening Structure and Design Method

Abstract

The present invention discloses a structure for artificially thickening the boundary layer on the surface of a subsonic cylindrical airframe and a design method thereof. The artificial thickening structure includes a cylindrical airframe and a trapezoidal ramp array arranged on the surface of the airframe. The trapezoidal ramp array includes a number of trapezoidal ramps evenly arranged along the circumferential direction of the airframe surface. The trapezoidal ramp includes a trapezoidal bottom surface, a trapezoidal top surface, and a connecting surface connecting the trapezoidal bottom surface and the trapezoidal top surface. The trapezoidal bottom surface is located on the airframe surface, and the trapezoidal bottom surface coincides with the lower base of the trapezoidal top surface. The trapezoidal top surface forms a certain inclination angle with the airframe surface to generate streamwise vortices on the airframe surface. The vertical distance from the highest point of the trapezoidal ramp to the airframe surface is not higher than the local free-stream boundary layer thickness on the airframe surface. The present invention generates streamwise vortex structures through the trapezoidal ramp structure array, promotes the mixing between the mainstream and the boundary layer airflow, effectively increases the boundary layer thickness downstream of the cylindrical airframe, and makes the boundary layer thickness closer to the actual flight situation of the airframe.

Description

Technical Field

[0001] The present invention relates to the technical field of aerodynamics, and particularly to an artificial thickening structure and design method for the boundary layer on the surface of a subsonic cylindrical airframe. Background Art

[0002] In the subsonic wind tunnel test of a scaled intake for a slender cylindrical airframe, blockage problems often occur, and it is necessary to modify model dimensions such as the fuselage length and diameter. In order to simulate the thick boundary layer of the airframe, it is necessary to control the boundary layer thickness of the modified model to match that of the original scaled model. At present, for the thickening of the boundary layer on the surface of a subsonic cylindrical airframe, it can be mainly divided into active control and passive control according to whether there is an injection of external energy. The backward ramp structure and the array of cylinder flow-around structures are currently common passive control thickening structures. The thickening principle of the backward ramp structure is that flow separation occurs behind the ramp to raise the streamline, thereby increasing the downstream boundary layer thickness. It has the advantages of obvious thickening and simple structure, etc., but the separation scale is difficult to control, resulting in difficulty in precisely controlling the boundary layer thickness. The thickening principle of the array of cylinder flow-around structures is that a recirculation vortex is generated behind the cylinder structure to raise the streamline, thereby increasing the downstream boundary layer thickness. It has the advantages of strong operability and small flow loss, etc., but its thickening effect still cannot fully simulate the thick boundary layer of the airframe. Summary of the Invention

[0003] Object of the Invention: Aiming at the above-mentioned disadvantages, the present invention provides an artificial thickening structure and design method for the boundary layer on the surface of a subsonic cylindrical airframe with good boundary layer thickening effect.

[0004] Technical Solution: To solve the above problems, the present invention adopts an artificial thickening structure for the boundary layer on the surface of a subsonic cylindrical airframe, including an airframe and a trapezoidal ramp array arranged on the surface of the airframe. The trapezoidal ramp array includes a plurality of trapezoidal ramps evenly arranged along the circumferential direction of the airframe surface; the trapezoidal ramp includes a trapezoidal bottom surface, a trapezoidal top surface, and a connecting surface connecting the trapezoidal bottom surface and the trapezoidal top surface. The trapezoidal bottom surface is located on the airframe surface, the upper bottom of the trapezoidal bottom surface coincides with the lower bottom of the trapezoidal top surface, and the trapezoidal top surface forms a certain inclination angle with the airframe surface to generate a streamwise vortex on the airframe surface. The vertical distance from the highest point of the trapezoidal ramp to the airframe surface is not higher than the local free-stream boundary layer thickness of the airframe surface.

[0005] Further, the connecting surface includes a first side surface and a second side surface connecting the side edges of the trapezoidal bottom surface and the trapezoidal top surface, and a top surface connecting the upper bottom of the trapezoidal bottom surface and the upper bottom of the trapezoidal top surface. One side of the top surface coincides with one side of the first side surface, and the other side of the top surface coincides with one side of the second side surface.

[0006] Further, the trapezoidal bottom surface, the trapezoidal top surface, and the top surface are all isosceles trapezoids, and the upper bottom length of the trapezoidal bottom surface is less than the upper bottom length of the trapezoidal top surface; the plane where the top surface is located is perpendicular to the airframe axis.

[0007] Further, the first side surface and the second side surface are triangular, and the planes where the first side surface and the second side surface are located intersect the axis of the body at the same point.

[0008] Further, the length of the upper base of the trapezoidal top surface is 1 / 3 of the length of the lower base of the trapezoidal top surface.

[0009] Further, the included angle between the trapezoidal top surface and the body surface is 20°.

[0010] Further, the several trapezoidal slopes include several trapezoidal top surfaces, the lower bases of the several trapezoidal top surfaces are arranged on the same circumference, and the distance between the lower bases of adjacent trapezoidal top surfaces is 60% of the length of the lower base of the trapezoidal top surface.

[0011] Further, a downstream air inlet is provided on the body, and the flow distance between the trapezoidal slope array and the downstream air inlet is a, and the axial length of the body is b, and a / b ≥ 0.125.

[0012] The present invention also provides a design method for the artificial thickening structure of the surface boundary layer of the above subsonic cylindrical body, including the following steps:

[0013] Step 1: According to the difference ε between the boundary layer thickness required by the downstream air inlet and the thickness of the free development boundary layer of the body surface without the trapezoidal slope array, initially determine the trapezoidal slope height h, h = fε, f < 1;

[0014] Step 2: Obtain the axial length l of the trapezoidal slope according to the height h of the trapezoidal slope, l = h÷tanα, where α is the included angle between the trapezoidal top surface and the body surface;

[0015] Step 3: Determine the length a of the lower base of the trapezoidal top surface according to the axial length l of the trapezoidal slope, a = l÷sin60°;

[0016] Step 4: Determine that the length b of the upper base of the trapezoidal top surface is 1 / 3 of the length a of the lower base according to the length of the lower base of the trapezoidal top surface;

[0017] Step 5: Determine that the distance between adjacent two trapezoidal slopes is 60% of the length a of the lower base of the trapezoidal top surface according to the length of the lower base of the trapezoidal top surface;

[0018] Step 6: Determine the shape of the connecting surface to connect the trapezoidal top surface and the trapezoidal bottom surface;

[0019] Step 7: Determine the number of trapezoidal slopes in the trapezoidal slope array;

[0020] Step 8: Conduct CFD simulation verification according to the design Mach number and angle of attack;

[0021] Step 9: Adjust the coefficient f according to the CFD simulation results and repeat Steps 2 - 8 until the error between the boundary layer thickness at the inlet of the downstream inlet duct and the actual required boundary layer thickness is less than the preset value.

[0022] Further, Step 7 is specifically: Determine the total circumferential length of the trapezoidal ramp array according to the range where the boundary layer needs to be thickened on the airframe. The quotient of the total circumferential length of the trapezoidal ramp array divided by the sum of the lower base length of the trapezoidal top surface of the trapezoidal ramp and the spacing between adjacent trapezoidal ramps is taken as an integer, which is the number of trapezoidal ramps in the trapezoidal ramp array.

[0023] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are: (1) Generate streamwise vortex structures through the trapezoidal ramp array, promote the mixing between the mainstream and the boundary layer airflow, effectively increase the boundary layer thickness downstream of the cylindrical airframe, making the boundary layer thickness closer to the actual flight situation of the airframe; (2) The trapezoidal ramp design fits the airflow movement well, has a good fit with the along - path pressure distribution curve of the freely developing boundary layer with the same thickness, and has little influence on the drag characteristics of the aircraft. Description of the Drawings

[0024] Figure 1 It is a schematic diagram of the overall structure of the artificial boundary layer thickening structure of the present invention;

[0025] Figure 2 It is a schematic diagram of the overall structure of the trapezoidal ramp array of the present invention;

[0026] Figure 3 It is a partial enlarged schematic diagram of the trapezoidal ramp array of the present invention;

[0027] Figure 4 It is a schematic diagram of the circumferential cross - section streamline distribution after the trapezoidal ramp array of the airframe in the second embodiment is simulated by CFD. Detailed Embodiments

[0028] Embodiment 1

[0029] As Figure 1 shown, an artificial boundary layer thickening structure on the surface of a subsonic cylindrical airframe in this embodiment includes an airframe 1 and a trapezoidal ramp array arranged on the surface of the airframe 1. The trapezoidal ramp array includes a plurality of trapezoidal ramps 101 arranged evenly in the circumferential direction on the surface of the airframe 1. As Figure 2 and Figure 3As shown, the trapezoidal ramp 101 includes a trapezoidal bottom surface 107, a trapezoidal top surface 102, and a connecting surface connecting the trapezoidal bottom surface 107 and the trapezoidal top surface. Both the trapezoidal bottom surface 107 and the trapezoidal top surface 102 are isosceles trapezoids, and the upper base length of the trapezoidal bottom surface 107 is less than the upper base length of the trapezoidal top surface 102. The trapezoidal bottom surface 107 is located on the surface of the body 1, the lower base of the trapezoidal bottom surface 107 coincides with the lower base of the trapezoidal top surface 102, and the trapezoidal top surface 102 forms a certain inclination angle with the surface of the body 1, such that the airflow generates a flow vortex 106 on the surface of the body 1. In order to ensure that the downstream boundary layer can fully recover to the fully developed turbulent state, the ratio of the flow direction distance between the downstream inlet 108 and the trapezoidal ramp array to the axial length of the cylindrical body 1 should be not less than 0.125.

[0030] The design method of the boundary layer artificial thickening structure in this embodiment specifically includes the following steps:

[0031] Step 1: According to the difference ε between the boundary layer thickness required at the downstream inlet 108 and the thickness of the free developing boundary layer on the surface of the body 1 without the trapezoidal ramp array, preliminarily determine the height h of the trapezoidal ramp 101, h = fε, where f < 1. In this embodiment, the body 1 is cylindrical, with a body radius of 51.11 mm and a length of 1200 mm. At Ma 0.5 and an angle of attack of 3°, the boundary layer thickness required at the downstream inlet 108 is 20.428 mm, and the thickness of the free developing boundary layer on the surface of the cylindrical body 107 without the trapezoidal ramp array is 16.510 mm. The difference ε between the two is 3.918 mm. According to the existing literature and experimental requirements, preliminarily determine the coefficient f = 0.854. Thus, the height of the trapezoidal ramp 101 is determined to be h = 0.854ε = 3.35 mm, which is less than the thickness of the local free developing boundary layer of the trapezoidal ramp array, 8.277 mm, meeting the requirements.

[0032] Step 2: Obtain the axial length l of the trapezoidal ramp 101 according to its height h, l = h÷tanα, where α is the angle between the trapezoidal top surface 102 and the surface of the body 1. In this embodiment, α = 20°, and thus l = 9.20 mm is calculated.

[0033] Step 3: Determine the lower base length a of the trapezoidal top surface 102 according to the axial length l of the trapezoidal ramp 101, a = l÷sin60° = 10.62 mm.

[0034] Step 4: Determine that the upper base length b of the trapezoidal top surface 102 is 1 / 3 of the lower base length a according to the lower base length of the trapezoidal top surface 102, b = 3.54 mm.

[0035] Step 5: Determine that the distance between two adjacent trapezoidal ramps 101 is 60% of the lower base length a of the trapezoidal top surface 102 according to the lower base length of the trapezoidal top surface 102, and the distance is 6.37 mm.

[0036] Step 6. Determine the shape of the connection surface to connect the trapezoidal top surface 102 and the trapezoidal bottom surface 107. The connection surface includes a first side surface 103 and a second side surface 104 that connect the sides of the trapezoidal bottom surface 107 and the sides of the trapezoidal top surface 102. Both the first side surface 103 and the second side surface 104 are triangles. The planes where the first side surface 103 and the second side surface 104 are located intersect the axis of the body 1 at the same point C(x c , y c ). The top surface 105 connects the upper base of the trapezoidal bottom surface 107 and the upper base of the trapezoidal top surface 102. One side of the top surface 105 coincides with one side of the first side surface 103, and the other side of the top surface 105 coincides with one side of the second side surface 104. The top surface 105 is an isosceles trapezoid, and the plane where the top surface 105 is located is perpendicular to the axis of the body 1.

[0037] Step 7. Determine the number of the trapezoidal ramps 101 in the trapezoidal ramp array. According to the geometric structure of the cylindrical body 1 and the range where the boundary layer needs to be thickened, determine the total circumferential length of the trapezoidal ramp array. In this embodiment, only the boundary layer within the range of the downstream inlet 108 needs to be thickened. Therefore, the total circumferential length of the trapezoidal ramp array does not exceed 120 mm. The length of the lower base of the trapezoidal top surface 102 of the trapezoidal ramp 101 and the distance between two adjacent trapezoidal ramps 101 are 16.99 mm. The quotient of the two is rounded up to obtain that the number of the trapezoidal ramps 101 in the trapezoidal ramp array is 7.

[0038] Step 8. Conduct CFD simulation verification according to the design Mach number and the angle of attack. For the Ma0.5, 3° angle of attack condition, the thickening effect can be seen in Table 1. After thickening, the boundary layer thickness in front of the downstream inlet 108 is 19.985 mm. Compared with the expected boundary layer thickness of 20.428 mm, the error is 2.17%, meeting the requirements.

[0039] Step 9. If the thickening structure designed according to the initial coefficient f does not meet the requirements of the experiment, reset the coefficient f and repeat steps 2 - 8 until the boundary layer thickness at the downstream inlet meets the requirements.

[0040] Embodiment 2

[0041] Design the thickening structure for the Ma0.6, 3° angle of attack condition according to the above method. At Ma0.6, 3° angle of attack, the required thickness of the boundary layer at the downstream inlet 108 is 20.641 mm, and the thickness of the free - developing boundary layer on the surface of the cylindrical body 1 without the trapezoidal ramp array is 16.714 mm. The difference ε between the two is 3.927 mm. Determine that the height h of the trapezoidal ramp 101 is h = 0.854ε = 3.35 mm. The obtained height of the trapezoidal ramp 101 is the same as that at Ma0.5, 3° angle of attack. Therefore, the trapezoidal ramp array structure is the same as that in Embodiment 1. The thickening effect of the finally designed thickening structure can be seen in Table 1. As Figure 4As shown, after passing through the trapezoidal ramp array, the boundary layer thickness increases significantly. The boundary layer thickness in front of the inlet 108 of the downstream inlet duct after thickening is 20.065 mm, and the error compared with the expected boundary layer thickness of 20.641 mm is 2.79%, meeting the requirements.

[0042] Table 1 Thickness increase results of the embodiments under the conditions of Ma0.5, 3° angle of attack and Ma0.6, 3° angle of attack

[0043]

Claims

1. A subsonic cylindrical body surface boundary layer artificial thickening structure, characterized in that: The invention comprises a cylindrical body (1) and a trapezoidal slope array arranged on the surface of the body (1), wherein the trapezoidal slope array comprises a plurality of trapezoidal slopes (101) uniformly arranged along the circumferential direction of the surface of the body (1); the trapezoidal slope (101) comprises a trapezoidal bottom surface (107), a trapezoidal top surface (102), and a connecting surface connecting the trapezoidal bottom surface (107) and the trapezoidal top surface (102); the trapezoidal bottom surface (107) is located on the surface of the body (1); the lower base of the trapezoidal bottom surface (107) and the lower base of the trapezoidal top surface (102) overlap; The trapezoidal top surface (102) forms a certain inclination angle with the surface of the body (1) so as to generate a streamwise vortex on the surface of the body (1); the vertical distance between the highest point of the trapezoidal slope (101) and the surface of the body (1) is not higher than the thickness of the local free development boundary layer on the surface of the body (1); the plurality of trapezoidal slopes (101) include a plurality of trapezoidal top surfaces (102); the lower bases of the plurality of trapezoidal top surfaces (102) are arranged on the same circumference, and the distance between the lower bases of adjacent trapezoidal top surfaces (102) is 60% of the length of the lower bases of the trapezoidal top surfaces (102); The connecting surface comprises a first side surface (103) and a second side surface (104) connecting the side edge of the trapezoidal bottom surface (107) and the side edge of the trapezoidal top surface (102), and a top surface (105) connecting the upper base of the trapezoidal bottom surface (107) and the upper base of the trapezoidal top surface (102); one side of the top surface (105) overlaps with one side of the first side surface (103), and the other side of the top surface (105) overlaps with one side of the second side surface (104); the first side surface (103) and the second side surface (104) are triangular, and the plane where the first side surface (103) and the plane where the second side surface (104) are located intersect the axis of the body (1) at the same point; The angle between the trapezoidal top surface (102) and the surface of the body (1) is 20 degrees, a downstream air inlet (108) is provided on the body (1), a flow distance between the trapezoidal slope array and the downstream air inlet (108) is a, an axial length of the body (1) is b, and a / b≥0.

125.

2. The subsonic cylindrical body surface boundary layer artificial thickening structure according to claim 1, characterized in that: The trapezoidal bottom surface (107), the trapezoidal top surface (102), and the top surface (105) are all isosceles trapezoids; the upper base length of the trapezoidal bottom surface (107) is shorter than the upper base length of the trapezoidal top surface (102); and the plane where the top surface (105) is located is perpendicular to the axis of the machine body (1).

3. The subsonic cylindrical body surface boundary layer artificial thickening structure according to claim 2, characterized in that: The length of the upper base of the trapezoidal top surface (102) is 1 / 3 of the length of the lower base of the trapezoidal top surface (102).

4. A method for designing an artificially thickened structure of a subsonic cylindrical body surface boundary layer according to any one of claims 1 to 3, characterized in that: The following steps are involved: Step 1: Based on the difference between the required boundary layer thickness at the downstream air inlet entrance (108) and the freely developed boundary layer thickness on the surface of the body (1) without the trapezoidal ramp array, , preliminarily determine the height of the trapezoidal slope (101) , , f<1; Step 2: According to the height of the trapezoidal slope (101) Find its axial length , is the angle between the trapezoidal top surface (102) and the surface of the body (1); Step 3: According to the axial length of the trapezoidal slope (101) Determine the length of the lower base of the trapezoidal top surface (102) ; Step 4: Determine the length of the upper base of the trapezoidal top surface (102) based on the length of the lower base of the trapezoidal top surface (102). is the bottom length 1 / 3 of Step 5: According to the length of the lower base of the trapezoidal top surface (102), determine that the distance between two adjacent trapezoidal slopes (101) is the length of the lower base of the trapezoidal top surface (102). 60%; Step 6, determining the shape of the connection surface so that the trapezoidal top surface (102) is connected to the trapezoidal bottom surface (107); Step 7, determining the number of trapezoidal slopes (101) in the trapezoidal slope array; Step 8: Perform CFD simulation verification based on the designed Mach number and angle of attack; Step 9: Adjust the coefficient f according to the CFD simulation results and repeat steps 2-8 until the error between the boundary layer thickness at the downstream inlet entrance and the actual required boundary layer thickness is less than a preset value.

5. The design method according to claim 4, characterized in that: The step 7 specifically comprises: determining the total circumferential length of the trapezoidal slope array according to the range of the boundary layer that needs to be thickened of the body (1), and taking an integer as the quotient of the total circumferential length of the trapezoidal slope array and the sum of the length of the lower base of the trapezoidal top surface (102) of the trapezoidal slope (101) and the spacing between two adjacent trapezoidal slopes (101) as the number of the trapezoidal slopes (101) in the trapezoidal slope array.

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