Shock wave low-frequency oscillation analysis method based on PIV instantaneous flow structure

Abstract

The invention relates to a shock wave low-frequency oscillation analysis method based on a PIV instantaneous flow structure. The method comprises the steps that S1, distribution (U (i, j) and V (i, j)) of an instantaneous velocity field in a particle image velocity measurement shooting area is obtained, U (i, j) is a particle flow velocity vector, V (i, j) is a particle normal velocity vector, (i, j) is coordinates of particles in a flow field area, and i is flow coordinates and j normal coordinates; s2, traversing all coordinates in the flow field area, and identifying the position of a separated shock wave point according to the principle that the speed value is reduced; s3, fitting according to the plurality of separated shock wave points to obtain a linear equation of the separated shock wave, obtaining an angle of the separated shock wave according to the slope of the separated shock wave, and determining a separated shock wave foot position according to an intersection point of the separated shock wave and a flow direction coordinate axis; and S4, determining the position of the separation point based on the principle that the two-dimensional separation vortex is in a closed ring shape.

Description

Technical Field

[0001] The invention relates to a shock wave low-frequency oscillation analysis method based on a PIV instantaneous flow structure, and belongs to the technical field of wind tunnel tests. Background Art

[0002] Shock wave / boundary layer interference is an important flow in the flight of high-speed aircraft, which is widely present in the internal and external flows of various high-speed aircraft. The low-frequency oscillation of shock waves is a typical unsteady flow phenomenon in shock wave / boundary layer interference, that is, the separation shock wave moves back and forth along the flow direction, which has a very important impact on the performance of the aircraft. The unsteady characteristics of the shock wave and boundary layer interference will lead to the instability of the boundary layer flow, and even the local heat flow will increase violently, causing the aircraft to deviate from the original design conditions. Through the study of the unsteady characteristics of the shock wave and boundary layer interference, the shock wave induced separation position can be predicted, so as to change the effect of the local shock wave of the aircraft on the boundary layer by controlling the size of the separation zone to enhance or weaken it. At present, the research on shock wave oscillation is mainly focused on numerical simulation. Although experimental research is the core technical means to obtain the shock wave oscillation frequency, it is mostly focused on the measurement of surface pressure sensor means. There is a lack of experimental research on the low-frequency oscillation characteristics of spatial flow characteristics, which makes it difficult to fully understand the flow characteristics of low-frequency oscillation.

[0003] The traditional method for analyzing low-frequency oscillation processes from spatial flow results is the schlieren method. However, the spatial wave structure obtained by the schlieren method has a spanwise integration effect and cannot quantitatively reflect the spatial flow structure information of a specific cross section. Summary of the invention

[0004] The technical problem solved by the present invention is: to overcome the deficiencies of the prior art, to provide a low-frequency oscillation analysis method based on instantaneous spatial flow structure, and to realize the analysis of the low-frequency oscillation of the separation shock wave from the perspective of spatial flow.

[0005] The solution to the technical problem of the present invention is: a shock wave low-frequency oscillation analysis method based on PIV transient flow structure, the method comprising the following steps:

[0006] S1. Obtain the distribution of the instantaneous velocity field in the particle image velocimetry shooting area (U(i,j), V(i,j)), where U(i,j) is the particle flow velocity vector, V(i,j) is the particle normal velocity vector, (i,j) is the coordinate of the particle in the flow field area, i is the flow coordinate, and j is the normal coordinate;

[0007] S2, traverse all coordinates in the flow field area, and identify the position of the separation shock wave point according to the principle of velocity value reduction;

[0008] S3, obtaining a linear equation of the separation shock wave by fitting multiple separation shock wave points, obtaining an angle of the separation shock wave by the slope of the separation shock wave, and determining a position of the separation shock wave foot by the intersection of the separation shock wave and the flow direction coordinate axis;

[0009] S4. Based on the principle that the two-dimensional separation vortex is a closed ring, the position of the separation point is determined.

[0010] Preferably, the linear equation of the separation shock wave is:

[0011] y=kx+b

[0012] Among them, k represents the slope of the separation shock wave, b is the intercept of the separation shock wave, x is the flow direction coordinate of the separation point, and y is the normal coordinate of the separation point.

[0013] Preferably, the angle α of the separation shock wave is:

[0014] α=arctan(k)

[0016] Preferably, the i value that satisfies u(i-1,1)>0&u(i,1)>0&u(i+1,1)<0&u(i+2,1)<0 is the position of the separation point.

[0017] Preferably, the number K of instantaneous velocity fields analyzed is generally not less than 2000.

[0018] Preferably, the position where the stream velocity U(i,j) decreases to 0.05 times the mainstream velocity is taken as the shock wave position.

[0019] Preferably, the flow velocity U(i,j) is reduced by:

[0020] ΔU(i,j)=U(i,j)-U ∞

[0021] Among them, U ∞ is the mainstream velocity of the flow field.

[0022] The beneficial effects of the present invention compared with the prior art are:

[0023] (1) The present invention quantitatively analyzes the temporal changes of the separation shock wave and the separation point based on the PIV instantaneous velocity field. Compared with the existing surface analysis method based on pressure sensors, it realizes the analysis of the low-frequency oscillation of the separation shock wave from the perspective of spatial flow.

[0024] (2) Compared with the low-frequency shock wave oscillation characteristics obtained by the spatial schlieren emission method, the present invention does not have the problem of separation shock wave and separation point positioning caused by the spanwise integration effect, thereby ensuring the accuracy of the analysis results. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Analyze the flow chart of the embodiment of the present invention; DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with the embodiments.

[0027] Particle image velocimetry (PIV) technology is a non-contact measurement technology that can quantitatively obtain spatial velocity fields. The spatial position of the separation shock wave and the separation zone can be displayed by processing the velocity field. Therefore, on the basis of the PIV flow results, by establishing a separation shock wave and separation point identification method, the spatial position change of the separation point and the angle change of the separation shock wave can be effectively extracted, and then the causal relationship between the low-frequency oscillation behavior of the shock wave and flow separation can be analyzed, supporting the analysis of the physical mechanism of the low-frequency oscillation of the separation shock wave.

[0028] The purpose of the present invention is to establish a low-frequency oscillation analysis method based on instantaneous spatial flow structure based on quantitative test measurement data of PIV spatial flow field, so as to make up for the deficiency of traditional research on low-frequency oscillation behavior based on wall pressure pulsation information. The present invention is realized by the following technical scheme. First, according to the distribution of velocity field, the separation shock wave angle and the position of the separation point are calculated according to the identification calculation method of the separation shock wave and the separation point, and then the position of the separation shock wave foot is obtained according to the angle calculation of the separation shock wave.

[0029] The present invention will be further described in detail step by step in conjunction with the accompanying drawings.

[0030] S1. Obtain the distribution of the instantaneous velocity field in the particle image velocimetry shooting area (U(i,j), V(i,j)), where U(i,j) is the particle flow velocity vector, V(i,j) is the particle normal velocity vector, (i,j) is the coordinate of the particle in the flow field area, i is the flow coordinate, and j is the normal coordinate;

[0031] The number K of instantaneous velocity fields used for analysis is no less than 2000 to ensure the validity of statistical data;

[0032] S2, traverse all coordinates in the flow field area, and identify the position of the separation shock wave point according to the principle of velocity value reduction;

[0033] Since the velocity of the flow field decreases when passing through the shock wave, the position of the shock wave can be identified based on the principle of velocity value reduction. The position when the flow velocity U(i,j) decreases to 0.05 times the mainstream velocity is defined as the shock wave position. Therefore, the shock wave position is calculated by reducing the velocity U(i,j).

[0034] The reduction value of the flow velocity U(i,j) is:

[0035] ΔU(i,j)=U(i,j)-U ∞ .

[0036] Where U ∞ is the mainstream velocity of the flow field, and ΔU(i,j) is the change in the flow velocity value at the position (i,j).

[0037] S3, obtaining a linear equation of the separation shock wave by fitting multiple separation shock wave points, obtaining an angle of the separation shock wave by the slope of the separation shock wave, and determining a position of the separation shock wave foot by the intersection of the separation shock wave and the flow direction coordinate axis;

[0038] The linear equation of the separation shock wave is:

[0039] y=kx+b

[0040] Among them, k represents the slope of the separation shock wave, b is the intercept of the separation shock wave, x is the flow direction coordinate of the separation point, and y is the normal coordinate of the separation point.

[0041] The angle α of the separation shock wave is:

[0042] α=arctan(k)

[0044] S4. Based on the principle that the two-dimensional separation vortex is a closed ring, the position of the separation point is determined.

[0045] Since the two-dimensional separation vortex is a closed ring, the positive and negative values ​​of the flow velocity will change in the separation area. According to this principle and taking into account the possible errors in the calculation of the instantaneous velocity field, the value of i that satisfies u(i-1,1)>0&u(i,1)>0&u(i+1,1)<0&u(i+2,1)<0 is the position of the separation point. That is, from the positive and negative values ​​of the flow velocity at four consecutive points, the values ​​of the first two points are opposite to those of the last two points, indicating that the streamline direction has changed. At this time, the physical position at point i is the position of the separation point.

[0046] According to the results of all instantaneous fields, the positional relationship between the separation shock foot and the separation point is analyzed, and the before-and-after correlation of their position changes is studied, thereby confirming the causal relationship between the before-and-after oscillation of the separation shock and the change of the separation vortex.

[0047] Finally, the results of all instantaneous velocity fields can be statistically analyzed to establish the time correspondence between the separation shock wave foot and the separation point position, thereby analyzing the physical mechanism of the low-frequency oscillation of the shock wave.

[0048] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A shock wave low-frequency oscillation analysis method based on PIV transient flow structure, characterized in that The steps include: S1. Obtain the distribution of the instantaneous velocity field in the particle image velocimetry shooting area (U(i,j), V(i,j)), where U(i,j) is the particle flow velocity vector, V(i,j) is the particle normal velocity vector, (i,j) is the coordinate of the particle in the flow field area, i is the flow coordinate, and j is the normal coordinate; S2, traverse all coordinates in the flow field area, and identify the position of the separation shock wave point according to the principle of velocity value reduction; S3, obtaining a linear equation of the separation shock wave by fitting multiple separation shock wave points, obtaining an angle of the separation shock wave by the slope of the separation shock wave, and determining a position of the separation shock wave foot by the intersection of the separation shock wave and the flow direction coordinate axis; S4. Based on the principle that the two-dimensional separation vortex is a closed ring, the position of the separation point is determined.

2. The shock wave low-frequency oscillation analysis method based on PIV transient flow structure according to claim 1 is characterized in that: The linear equation of the separation shock wave is: y=kx+b Among them, k represents the slope of the separation shock wave, b is the intercept of the separation shock wave, x is the flow direction coordinate of the separation point, and y is the normal coordinate of the separation point.

3. The shock wave low-frequency oscillation analysis method based on PIV transient flow structure according to claim 1 is characterized in that: The angle α of the separation shock wave is: α = arctan(k).

4. The shock wave low-frequency oscillation analysis method based on PIV transient flow structure according to claim 1 is characterized in that: The i value that satisfies u(i-1,1)>0&u(i,1)>0&u(i+1,1)<0&u(i+2,1)<0 is the position of the separation point.

5. The shock wave low-frequency oscillation analysis method based on PIV transient flow structure according to claim 1 is characterized in that: The number K of instantaneous velocity fields analyzed is generally not less than 2000.

6. The shock wave low-frequency oscillation analysis method based on PIV transient flow structure according to claim 1 is characterized in that: The position where the streamwise velocity U(i,j) decreases to 0.05 times the mainstream velocity is taken as the shock wave position.

7. The shock wave low-frequency oscillation analysis method based on PIV transient flow structure according to claim 1 is characterized in that: The reduction value of the flow velocity U(i,j) is: ΔU(i,j)=U(i,j)-U ∞ Among them, U ∞ is the mainstream velocity of the flow field.

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