A method for analyzing shock low-frequency oscillation based on PIV instantaneous flow structure

Abstract

This invention relates to a shock wave low-frequency oscillation analysis method based on a PIV instantaneous flow structure: S1, acquire the instantaneous velocity field distribution (U(i,j), V(i,j)) in the particle image velocimetry area, where U(i,j) is the particle flow velocity vector, V(i,j) is the particle normal velocity vector, (i,j) is the particle coordinate in the flow field region, i is the flow direction coordinate, and j is the normal coordinate; S2, traverse all coordinates in the flow field region, and identify the location of the separation shock wave point based on the principle of velocity value reduction; S3, obtain the linear equation of the separation shock wave by fitting multiple separation shock wave points, obtain the angle of the separation shock wave from the slope of the separation shock wave, and determine the position of the separation shock wave foot from the intersection of the separation shock wave and the flow direction coordinate axis; S4, determine the location of the separation point based on the principle that the two-dimensional separation vortex is a closed ring.

Description

Technical Field

[0001] This invention relates to a shock wave low-frequency oscillation analysis method based on PIV instantaneous flow structure, belonging to the field of wind tunnel testing technology. Background Technology

[0002] Shock wave / boundary layer disturbance is a significant flow phenomenon in high-speed aircraft, widely present in the internal and external flows of various high-speed aircraft. Low-frequency shock wave oscillation is a typical unsteady flow phenomenon within shock wave / boundary layer disturbance, characterized by the reciprocating motion of the separated shock wave along the flow direction, which has a crucial impact on aircraft performance. The unsteady characteristics of shock wave and boundary layer disturbance lead to instability in the boundary layer flow, even resulting in regions of intense local heat flux growth, causing the aircraft to deviate from its original design conditions. By studying the unsteady characteristics of shock wave and boundary layer disturbance, the location of shock wave-induced separation can be predicted, allowing for the modification of the local shock wave's effect on the boundary layer by controlling the size of the separation zone. Current research on shock wave oscillation mainly focuses on numerical simulation. While experimental research is the core technology for obtaining shock wave oscillation frequencies, it largely concentrates on surface pressure sensor measurements, lacking experimental research on the low-frequency oscillation characteristics of spatial flow. This hinders a comprehensive and in-depth understanding of the flow characteristics of low-frequency oscillations.

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

[0004] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide a low-frequency oscillation analysis method based on instantaneous spatial flow structure, so as to realize the analysis of low-frequency oscillation of separated shock waves from the perspective of spatial flow.

[0005] The solution to the technical problem of this invention is: a method for analyzing low-frequency shock oscillations based on PIV instantaneous flow structures, the method comprising the following steps:

[0006] S1. Obtain the instantaneous velocity field distribution in the particle image velocimetry shooting area. ,in, The particle flow velocity vector, It is the particle's normal velocity vector. Let these be the coordinates of the particle in the flow field region. For flow direction coordinates, Normal coordinates;

[0007] S2. Traverse all coordinates within the flow field region and identify the location of the separation shock wave point based on the principle of velocity reduction;

[0008] S3. Obtain the linear equation of the separated shock wave by fitting multiple separated shock wave points, obtain the angle of the separated shock wave from the slope of the separated shock wave, and determine the position of the foot of the separated shock wave from the intersection of the separated shock wave and the flow direction coordinate axis.

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

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

[0011]

[0012] in, The slope representing the separating shock wave. To separate the shock wave intercept, y is the flow direction coordinate of the separation point, and y is the normal coordinate of the separation point.

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

[0014] .

[0015] Preferably, satisfying time The value represents the location of the break-off point.

[0016] Preferably, the number K of the instantaneous velocity fields being analyzed is generally no less than 2000.

[0017] Preferably, the flow velocity The location where the value decreases to 0.05 times the mainstream velocity is taken as the shock wave location.

[0018] Preferably, the flow velocity The decrease value is:

[0019]

[0020] in, This represents the mainstream velocity of the flow field.

[0021] The advantages of this invention compared to the prior art are:

[0022] (1) Based on the instantaneous velocity field of PIV, this invention performs quantitative analysis on the temporal changes of the separated shock wave and the separation point. Compared with the existing surface analysis method based on pressure sensor, it realizes the analysis of the low-frequency oscillation of the separated shock wave from the perspective of spatial flow.

[0023] (2) Compared with the spatial ripple generation method, the present invention does not have the problem of separating shock waves and locating separation points caused by the spanwise integral effect, thus ensuring the accuracy of the analysis results. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the analysis of an embodiment of the present invention. Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments.

[0026] Particle image velocimetry (PIV) is a non-contact measurement technique that can quantitatively acquire spatial velocity fields. By processing the velocity field, the spatial location of the separated shock wave and the separation zone can be displayed. Therefore, based on the PIV flow results, by establishing a method for identifying the separated shock wave and the separation point, the spatial position change of the separation point and the angular change of the separated shock wave can be effectively extracted. This allows for the analysis of the causal relationship between the low-frequency oscillation behavior of the shock wave and the flow separation, supporting the analysis of the physical mechanism of the low-frequency oscillation of the separated shock wave.

[0027] The purpose of this invention is to establish a low-frequency oscillation analysis method based on instantaneous spatial flow structure, starting from quantitative experimental measurement data of PIV spatial flow fields, to overcome the shortcomings of traditional methods that rely on wall pressure pulsation information to study low-frequency oscillation behavior. This invention is achieved through the following technical solution: First, based on the velocity field distribution, the separation shock wave angle and the position of the separation point are calculated using methods for identifying and calculating the separation shock wave and separation point. Then, the position of the separation shock wave foot is calculated based on the separation shock wave angle.

[0028] The present invention will now be described in further detail step by step with reference to the accompanying drawings.

[0029] S1. Obtain the instantaneous velocity field distribution in the particle image velocimetry shooting area. ,in, The particle flow velocity vector, It is the particle's normal velocity vector. Let these be the coordinates of the particle in the flow field region. For flow direction coordinates, Normal coordinates;

[0030] The number of instantaneous velocity fields used for analysis, K, should be no less than 2000 to ensure the validity of the statistical data;

[0031] S2. Traverse all coordinates within the flow field region and identify the location of the separation shock wave point based on the principle of velocity reduction;

[0032] Since the flow velocity decreases when passing through a shock wave, the location of the shock wave can be identified based on the principle of velocity reduction, and the flow velocity can be defined. The location where the shock wave position is reached when the value decreases to 0.05 times the mainstream velocity is taken as the shock wave position. Therefore, the shock wave position is determined by the flow velocity. The value is reduced to calculate the shock wave location.

[0033] Flow velocity The decrease value is:

[0034] .

[0035] In the formula, The mainstream velocity of the flow field, for The change in the flow velocity at a given location.

[0036] S3. Obtain the linear equation of the separated shock wave by fitting multiple separated shock wave points, obtain the angle of the separated shock wave from the slope of the separated shock wave, and determine the position of the foot of the separated shock wave from the intersection of the separated shock wave and the flow direction coordinate axis.

[0037] The linear equation of the separated shock wave is:

[0038]

[0039] in, The slope representing the separating shock wave. To separate the shock wave intercept, y is the flow direction coordinate of the separation point, and y is the normal coordinate of the separation point.

[0040] Angle of separation shock wave for:

[0041] .

[0042] S4. Based on the principle that the two-dimensional separation vortex is a closed ring, determine the location of the separation point.

[0043] Since the two-dimensional separated vortex is a closed ring, the sign of the flow velocity will change within the separation region. Based on this principle, and considering possible errors in the instantaneous velocity field calculation, the following conditions must be met: time The value represents the location of the separation point, that is, the positive and negative values ​​of the flow velocity at four consecutive points in the flow direction. The values ​​at the first two points are opposite in sign to those at the last two points, indicating that the direction of the streamline has changed. The physical location of the point is the location of the separation point.

[0044] Based on the results of all instantaneous fields, the positional relationship between the separation shock foot and the separation point is analyzed, and the correlation between their positional changes before and after is studied, thereby confirming the causal relationship between the oscillations before and after the separation shock and the changes in the separation vortex.

[0045] Finally, statistical analysis can be performed on all instantaneous velocity field results to establish the time correspondence between the separation shock foot and the separation point position, thereby analyzing the physical mechanism of the low-frequency shock oscillation.

[0046] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for analyzing low-frequency shock oscillations based on PIV instantaneous flow structures, characterized in that... Includes the following steps: S1. Obtain the instantaneous velocity field distribution in the particle image velocimetry shooting area. ,in, The particle flow velocity vector, It is the particle's normal velocity vector. Let these be the coordinates of the particle in the flow field region. For flow direction coordinates, Normal coordinates; S2. Traverse all coordinates within the flow field region and identify the location of the separation shock wave point based on the principle of velocity reduction; S3. Obtain the linear equation of the separated shock wave by fitting multiple separated shock wave points, obtain the angle of the separated shock wave from the slope of the separated shock wave, and determine the position of the foot of the separated shock wave from the intersection of the separated shock wave and the flow direction coordinate axis. S4. Based on the principle that the two-dimensional separation vortex is a closed ring, determine the location of the separation point; satisfy time The value represents the location of the break-off point.

2. The shock wave low-frequency oscillation analysis method based on PIV instantaneous flow structure according to claim 1, characterized in that, The linear equation of the separated shock wave is: in, The slope representing the separating shock wave. To separate the shock wave intercept, y 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 instantaneous flow structure according to claim 1, characterized in that, Angle of separation shock wave for: 。 4. The shock wave low-frequency oscillation analysis method based on PIV instantaneous flow structure according to claim 1, characterized in that, The number of instantaneous velocity fields K analyzed is no less than 2000.

5. The shock wave low-frequency oscillation analysis method based on PIV instantaneous flow structure according to claim 1, characterized in that, Flow velocity The location where the value decreases to 0.05 times the mainstream velocity is taken as the shock wave location.

6. The shock wave low-frequency oscillation analysis method based on PIV instantaneous flow structure according to claim 1, characterized in that, Flow velocity The decrease value is: in, This represents the mainstream velocity of the flow field.

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