Multi-parameter diagnosis method for blade surface separation flow transition characteristics of compression system under low Reynolds number

Abstract

The invention provides a multi-parameter diagnosis method for a blade surface separation flow transition characteristic of a compression system under a low Reynolds number, which is used for accurately judging a blade surface transition interval and supporting efficient regulation and control and design. The method comprises the following steps: firstly, performing high-fidelity numerical calculation on an internal flow field of a compression system under a low Reynolds number to obtain a convergence solution; numerical probes are uniformly arranged at different flow direction positions of a blade table along the normal direction, and average flow field parameters during near-wall time are extracted. And projecting the velocity component under the Cartesian coordinate along the local flow direction to obtain a boundary layer velocity type, and performing integral solution to obtain boundary layer characteristic parameters. In combination with a near-wall transient disturbance spatio-temporal evolution law and a linear stability theory, a compression system separation flow transition characteristic multi-parameter criterion is formed, and the transition starting and ending positions are accurate to be within 1% of the axial chord length. Compared with a traditional single criterion based on a shape factor, the method has the advantages that the diagnosis precision of the separation flow transition characteristic is improved, and support is provided for transition regulation and control and blade design of a compression system under a low Reynolds number.

Description

Technical Field

[0001] The present invention belongs to the technical field of flow diagnosis of aero-engine compression systems, and relates to an analysis of transition characteristics of a boundary layer of a compression system at low Reynolds numbers, and specifically to a multi-parameter diagnostic method for transition characteristics of a separation flow at low Reynolds numbers, which is used to accurately determine the transition interval of a blade surface of a compression system at low Reynolds numbers, and to provide support for efficient flow regulation and blade design. Background Art

[0002] Light aircraft engines are the main power unit for high-altitude UAVs. Due to the high ceiling and small size of UAVs, the Reynolds number at the inlet of the engine compression system is reduced by 1-2 orders of magnitude (~10 5 ). Compared with the ground operating conditions, the low Reynolds number effect of the compression system is prone to induce a series of problems such as pressure ratio drop, efficiency attenuation, and insufficient margin, which seriously affect the engine's high-altitude fuel consumption and working stability. When the Reynolds number is reduced to near the critical value, the laminar flow area on the surface of the compression system blade expands, and under the action of the adverse pressure gradient, it usually evolves into a fully developed turbulent state in the form of a separation flow transition (or separation bubble). When the separation flow transition occurs, the local turbulent pulsation is accelerated and the high-loss area expands rapidly; at the same time, the boundary layer thickens rapidly, inducing strong aerodynamic blockage, and the efficiency and margin of the compression system deteriorate. It can be considered that the separation flow transition is the root cause of inducing the low Reynolds number effect of the compression system and restricting the comprehensive performance of aircraft engines at high altitudes.

[0003] Accurate diagnosis of the transition characteristics of the separation flow (transition start / end position, transition interval length, and overall size of the separation bubble, etc.) is a prerequisite for efficient flow control and blade design of the compression system under low Reynolds number. Traditional transition diagnosis methods mainly rely on experimental measurements, such as hot wire anemometers and surface thermal films. However, with the gradual improvement of the thrust-to-weight ratio of modern aircraft engines, the compression system structure is more compact, the blades are shorter, and the size effect is significant. At this time, the common invasive transition test methods (such as surface thermal films, etc.) interfere more strongly with the flow on the blade surface, easily destroy the original flow field, and bring great uncertainty to the accuracy of the transition test. Moreover, the laying and installation process of the transition test components is complicated, and the test takes a long time, which makes it difficult to support the rapid iteration of transition control and blade design solutions.

[0004] Theoretically, by developing high-precision numerical calculation methods, it is possible to monitor all information about the separation flow transition of the compression system in a short period of time, without the problem of intrusive measurement equipment interfering with the flow field in a confined space. However, there are multi-scale secondary flow structures and strong vortex dynamics processes inside the compression system, and high turbulence non-equilibrium and anisotropy near the blades / end walls, which seriously interfere with local transition pattern recognition and transition interval diagnosis, resulting in poor accuracy and reliability of conventional numerical analysis methods in diagnosing separation flow transition characteristics.

[0005] In summary, the transition process of separation flow under low Reynolds number is the core factor for the performance degradation of compression system. Traditional transition test methods such as hot film / hot wire have great limitations, and numerical analysis methods still face challenges in the accuracy and precision of transition criteria. Therefore, how to achieve a comprehensive and accurate analysis of transition characteristics under low Reynolds number conditions through precise numerical simulation and efficient multi-parameter transition diagnosis criteria is a technical problem that needs to be solved in the field of low Reynolds number effect research of compression systems. Summary of the invention

[0006] 1. Purpose of the invention In view of the above problems, the present invention proposes a multi-parameter diagnostic method for the transition characteristics of the separation flow on the blade surface of the compression system under low Reynolds numbers, starting from the spatiotemporal evolution law of the transient pulsation of the separation shear layer near the wall. First, based on the steady-state statistical data, the integral parameters of the boundary layer on the blade surface of the compression system are accurately calculated; then, the relationship between the vortex dynamics process at different transition stages and the rate of generation of turbulent pulsation near the wall is established, and combined with the linear stability theory, a multi-parameter criterion for the start / end position of the separation flow transition is formed. The above method significantly improves the diagnostic accuracy of the transition characteristics of the separation flow on the blade surface of the compression system, and provides basic support for the transition control and rapid iterative design of the compression system under low Reynolds numbers.

[0007] (II) Technical solution The purpose of the present invention is to propose a diagnostic method for the transition characteristics of the separation flow on the blade surface of the compression system under low Reynolds number, which is used to accurately determine the starting and ending positions of the transition and the length of the transition interval, and support efficient regulation and design. The key to the present invention is how to comprehensively utilize the time-averaged flow field data and the transient flow field data to achieve a panoramic diagnosis of the separation flow transition characteristics. The main steps and corresponding solutions are as follows: SS1. Numerical calculation of three-dimensional flow field: High-precision Large Eddy Simulation (LES) is used to numerically calculate the three-dimensional flow field inside the compression system under low Reynolds number conditions. After reaching statistical convergence, the load distribution at different blade heights, the total pressure loss coefficient of the outlet section, etc. are compared with the measurement results to verify the reliability of the numerical calculation method and obtain statistically converged three-dimensional flow field data. SS2. Extraction of blade surface boundary layer parameters: Based on the statistically converged three-dimensional flow field data, the thickness of the boundary layer on the blade surface under the current working conditions is preliminarily evaluated; the boundary layer numerical probes are evenly arranged along the local normal at different flow direction positions on the blade surface, and it is ensured that the numerical probes completely cover the boundary layer and do not exceed the calculation domain; the time-averaged flow field parameters in the area covered by each numerical probe are extracted, including at least the circumferential velocity in the Cartesian coordinate system. V y , axial speed Vz ,density ρ and turbulence statistical characteristic parameters; SS3. Calculation of integral parameters of blade surface boundary layer: Determine the flow angle at different locations on the blade surface α , the circumferential speed V y and axial speed V z Project along the local flow direction to obtain the local flow direction s - Wall Normal n Velocity components in the coordinate system V s and V n ; Based on the streamwise velocity component V s Determine the boundary layer velocity profile distribution at each location on the blade surface, and determine the boundary layer thickness based on the boundary layer velocity profile distribution δ , and integrate the boundary layer displacement thickness , momentum thickness θ And the form factor H 12 Boundary layer integration parameters included; SS4. Preliminary assessment of laminar separation, transition and reattachment locations: Preliminary evaluation of laminar separation, transition and turbulent reattachment locations in the blade surface boundary layer: When the shape factor H 12 When it increases to 4.2, laminar separation is determined to occur; when the momentum thickness of the boundary layer θ Start to grow rapidly or form factor H 12 When the shape factor starts to decrease from the peak value, it is determined as the spatial average transition position; H 12 When it drops from the peak point to about 3.5, it is determined that turbulent reattachment occurs; SS5. Accurate diagnosis of separation flow transition characteristics based on transient disturbance evolution characteristics in the near-wall region: On the basis of preliminary evaluation, the transition characteristics of separation flow in the near-wall area are further accurately diagnosed based on the evolution characteristics of transient disturbance in the near-wall area. First, monitoring points are arranged in the near-wall area of ​​the blade surface to extract the transient pulsation parameters at different flow directions, and the spatiotemporal development laws of transient disturbances at different transition stages are clarified. The time-averaged statistical results of the pulsation parameters at different flow directions are further extracted and organized into logarithmic form. Combined with the linear stability theory, a multi-parameter comprehensive criterion for the transition of separation flow in the near-wall area of ​​the blade surface is constructed: the position where the pulsation growth in the separation shear layer begins to deviate from the exponential path is determined as the starting position of the transition, and the position where the pulsation growth reaches the maximum value is determined as the ending position of the transition. The axial distance between the starting and ending positions of the transition is the length of the transition interval.

[0008] SS6. Result verification and feedback optimization (optional step): The starting and ending positions of the separation flow transition and the length of the transition interval determined by the diagnosis are compared with the test data to evaluate and verify the accuracy and reliability of the transition diagnosis results. The separation flow transition diagnosis is carried out in combination with the flow field parameters under different working conditions to improve the adaptability and versatility of the diagnostic method.

[0009] (III) Technical Effect Compared with the prior art, the multi-parameter diagnostic method for the separation flow transition characteristics of the blade surface of the compression system under low Reynolds number provided by the present invention has the following advantages: (1) Based on high-precision large eddy simulation (LES) to obtain time-averaged and transient flow field data inside the compression system, this paper develops a multi-parameter diagnostic method for the separation flow transition characteristics, and realizes a panoramic diagnosis of the separation flow transition process under low Reynolds numbers. This method not only considers the boundary layer integral parameters in the time-averaged flow field, such as shape factor, displacement thickness and momentum thickness, but also considers the transient pulsation parameters in the near-wall region, such as transient pressure pulsation and Reynolds stress pulsation, so as to more comprehensively reflect the essential characteristics of the separation flow transition.

[0010] (2) Starting from the spatiotemporal evolution mechanism of the vortex system at different stages of the separation flow transition, the present invention clarifies the relationship between the above-mentioned vortex dynamics process and the rate of generation of near-wall turbulent pulsation. On this basis, by introducing the growth characteristics of transient pulsation in the near-wall region and combining with the linear stability theory, the start / end position criterion of the separation flow transition is strictly derived, which has clear physical meaning and high credibility.

[0011] (3) Compared with the existing single criterion based on boundary layer shape factor, the multi-parameter criterion of the present invention takes into account the spatiotemporal evolution of near-wall transient disturbances, and can provide richer transition information. The transition start and end positions are accurate to within 1% of the axial chord length, which significantly improves the diagnostic accuracy of the transition characteristics of the separation flow and can directly support the efficient control of the transition and blade design under low Reynolds numbers. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a flow chart for implementing a multi-parameter diagnostic method for the separation flow transition characteristics of the blade surface of a compression system at a low Reynolds number in an embodiment of the present invention; Figure 2 is a schematic diagram of the arrangement of the numerical calculation domain and the boundary layer numerical probe in an embodiment of the present invention; Figure 3 Schematic diagram of coordinate transformation and velocity projection in the boundary layer region according to an embodiment of the present invention; Figure 4 is the boundary layer shape factor in the embodiment of the present invention H 12 Position with dimensionless chord length x / C ax Schematic diagram of the distribution of x / C ax represents the dimensionless chord length position, the ordinate H 12 is the boundary layer shape factor; Figure 5 The different chord length positions in the embodiments of the present invention x / C ax Transient pressure pulsation p 'With dimensionless time Schematic diagram of the evolution law, the horizontal axis is is the dimensionless time, with reference speed U Chord Length c Normalization; ordinate p ' / p average It represents the ratio of pressure pulsation to time-averaged pressure. Different curves represent the time history of pressure pulsation at different chord length positions. Figure 6 is a schematic diagram of the linear form of Reynolds stress in an embodiment of the present invention, with the horizontal axis x / C ax is the dimensionless chord length position, the ordinate is the Reynolds normal stress (at reference speed U 1 squared dimensionless).

[0013] Figure 7 is a schematic diagram of the logarithmic form of Reynolds stress in an embodiment of the present invention, with the horizontal axis x / C ax is the dimensionless chord length position, and the ordinate is the Reynolds normal stress in logarithmic form. DETAILED DESCRIPTION

[0014] The purpose of the present invention is to propose a diagnostic method for the transition characteristics of separation flow at low Reynolds numbers, which is used to accurately determine the transition interval of the blades of the compression system at low Reynolds numbers, and to support efficient regulation and design. The key to the present invention is how to comprehensively utilize the time-averaged flow field data and the transient flow field data to achieve a panoramic diagnosis of the transition characteristics of the separation flow. In order to make the purpose, technical solutions and advantages of the implementation of the present invention clearer, the technical solutions in the embodiments of the present invention will be described in more detail below in conjunction with the drawings in the embodiments of the present invention. The described embodiments are part of the embodiments of the present invention, not all of the embodiments, and the described embodiments are exemplary and are intended to be used to explain the present invention, and cannot be understood as limitations on the present invention. Figure 1 As shown, the main steps and corresponding solutions of the present invention when implemented are as follows: SS1. Numerical calculation of three-dimensional flow field: For a certain compressor component, firstly, a high-precision large eddy simulation (LES) is used to carry out numerical calculation of the three-dimensional flow field under low Reynolds number. In order to capture the complex transition flow on the blade and end wall surface, the sub-grid model selects the wall adaptive local eddy viscosity model (WALE), and the grid resolution scale meets the LES calculation requirements. After the numerical calculation reaches statistical convergence, the numerical / experimental results of the load and total pressure loss coefficient at different blade heights are compared. Since the separation flow transition is the fundamental factor that determines the load distribution and loss size under low Reynolds number, when the load and total pressure loss at different blade heights are in good agreement with the experimental results, it is considered that the numerical calculation results of the separation flow transition are credible, and finally the statistically converged three-dimensional flow field data are obtained.

[0015] As a preference, when performing high-precision LES simulation, the grid resolution scale is set to meet the following conditions: the dimensionless grid size in the wall normal direction does not exceed 1, and the dimensionless grid sizes in the flow direction and circumferential direction do not exceed 15 and 20, respectively, to meet the analytical requirements of the LES turbulence integral scale; at the same time, the time step of the LES numerical solution is controlled to ensure that the CFL number corresponding to the time step is less than 1. The boundary conditions of the numerical calculation include: the inlet of the computational domain is 1.5 times the axial chord length from the leading edge of the blade, and the outlet is located 2 times the axial chord length downstream of the trailing edge of the blade; the total pressure, total pressure and airflow angle are given at the inlet, and the average static pressure is given at the outlet. The preset inlet Mach number and Reynolds number are achieved by adjusting the inlet total pressure and outlet back pressure; in order to improve the convergence of the calculation, the computational domain space discretization adopts bounded central difference, and the time advancement adopts the second-order backward Euler format; the internal iteration is 10 time steps to ensure that the residual is reduced to 10 after each step of calculation is completed. -4 ; Set multiple monitoring points at the front edge of the blade suction surface, separation shear layer and wake area. After the mean and standard deviation of the monitoring variables reach statistical convergence, continue to advance 10-15 flow cycles to obtain the flow field statistical parameters under this working condition.

[0016] SS2. Extraction of blade surface boundary layer parameters: After obtaining the statistically converged three-dimensional flow field, the thickness of the boundary layer on the blade surface under this working condition is preliminarily evaluated based on the time-averaged distribution of parameters such as blade surface velocity and pressure. At different flow positions on the blade surface, numerical probes are evenly arranged along the local normal. The numerical probes should completely cover the boundary layer while not exceeding the calculation domain. The arrangement of the numerical calculation domain and the boundary layer numerical probes is shown in the following figure. Figure 2 The time-averaged flow field parameters corresponding to the boundary layer numerical probe are extracted, including the circumferential velocity in the Cartesian coordinate system. V y , axial speed V z ,density ρ As well as turbulence statistical characteristic parameters such as Reynolds normal stress / shear stress.

[0017] As a preferred method, the time-averaged parameter distribution of blade surface velocity and pressure is obtained by time-averaging the statistically converged three-dimensional flow field data, and the boundary layer development state and boundary layer thickness under the current working condition are preliminarily evaluated according to the near-wall velocity (or pressure) distribution, and then the normal arrangement range of the boundary layer numerical probe is determined; when the time-averaged flow field parameters at each numerical probe position are extracted, the turbulence statistical characteristic parameters at least include the Reynolds normal stress ( u ' u '、 v ' v '、 w ' w ') and Reynolds shear stress (- u ' v '、- u ' w '、- v ' w '), and record the spatial coordinates of each numerical probe ( x , y , z ), which is used for subsequent coordinate transformation and parameter calculation. In addition, the arrangement of boundary layer numerical probes is as follows: a set of numerical probe sequences is arranged at least every 1% of the axial chord length from the leading edge to the trailing edge, and the density of numerical probes is appropriately increased in the transition region of the separation shear layer. At a fixed flow direction position, the probes are arranged at a rate of 0.02 δ The numerical probes are arranged along the local normal direction at intervals of , and the positions of the outermost numerical probes should ensure that the boundary layer area is fully covered while not exceeding the calculation domain.

[0018] SS3. Calculation of integral parameters of blade surface boundary layer: Determine the flow angle at different locations on the blade surface α , respectively, the circumferential speed V y and axial speed V z Project along the local flow direction to obtain the local flow directions - Wall Normal n Velocity components in the coordinate system V s and V n , complete the boundary layer region coordinate transformation and velocity projection, such as Figure 3 As shown, the relevant formula is as follows: By using the above method, the boundary layer velocity distribution at each position on the blade surface can be obtained, that is, the streamwise velocity component V s At different wall normal distances V s The maximum value is the mainstream speed , and the normal distance from the wall at 99% of the mainstream velocity is the boundary layer thickness δ According to the boundary layer velocity type and boundary layer thickness, the boundary layer displacement thickness is obtained by integral solution , momentum thickness θ And the form factor H 12 The specific definitions are as follows: SS4. Preliminary assessment of laminar separation, transition and reattachment locations: Based on the above three boundary layer integration parameters, the laminar separation, transition and reattachment positions on the blade surface can be preliminarily evaluated, such as Figure 4 shown. Figure 4 The blade suction surface boundary layer shape factor is shown H 12 With axial position x / C ax The distribution of x / C ax <0.3 area, H 12 Maintaining a stable value of about 2.5, it indicates that the boundary layer is in a laminar state. H 12 When it increases to 4.2, laminar separation is considered to occur, and the boundary layer displacement thickness is The growth rate has obviously accelerated, as shown by H 12 When the separation shear layer becomes unstable and triggers the transition, the momentum thickness of the boundary layer θ Started to grow rapidly, leading to H 12 It starts to drop from the peak position. Therefore, θ Start to grow rapidly or H12 The point where the decline starts from the peak is the turning point. H 12 When the peak value drops to about 3.5, it is considered that turbulent reattachment occurs. H 12 It gradually stabilizes, indicating that the boundary layer transforms into a fully developed turbulent state.

[0019] Theoretically, as long as θ or H 12 The transition position can be determined by the distribution. However, for compressor blades with large curvature and strong adverse pressure gradient, the transition of the separated flow at low Reynolds number is completed within a certain flow distance, that is, there is a transition interval. However, the transition position obtained by the above criterion is the spatial average transition position, and it is difficult to determine the transition start / end position and the length of the transition interval. The transition start / end position is crucial to the determination of the fine flow control scheme and loss analysis at low Reynolds number.

[0020] SS5. Accurate diagnosis of separation flow transition characteristics based on transient disturbance evolution characteristics in the near-wall region: Next, based on the spatiotemporal evolution of transient disturbances in the near-wall region, a multi-parameter diagnostic method for the separation flow transition zone (transition start / end position) is proposed. Monitoring points are arranged in the near-wall region of the blade surface to extract transient pulsation parameters at different flow directions, such as Figure 5 When laminar separation does not occur, the near-wall parameter pulsation is very weak; when the laminar boundary layer separates from the wall under the action of the adverse pressure gradient to form a separation shear layer, the near-wall parameter pulsation begins to grow; when the separation shear layer becomes unstable and triggers the transition, the near-wall parameter pulsation fluctuates greatly, indicating that the prominent feature of the separation flow transition process is accompanied by strong momentum exchange; when the transition ends and the separation shear layer reattaches, the turbulent pulsation in the near-wall region drops back to a lower level.

[0021] In order to more clearly reflect the growth law of pulsation at different transition stages, the pulsation parameters at different flow positions are organized into logarithmic form. Based on the linear stability theory, the pulsation in the near-wall zone initially shows an exponential rapid growth; when secondary instability phenomena such as vortex pairing appear in the separation shear layer, the three-dimensional effect begins to induce the pulsation growth path to deviate from the exponential law, indicating that the transition is triggered here; when large-scale hairpin vortices break in the separation shear layer at the end of the transition, the "upward spray-down sweep" of the near-wall low-energy fluid occurs, the pulsation further grows to the maximum value, and the transition is completed. Therefore, the position where the pulsation growth in the separation shear layer begins to deviate from the exponential path is the transition starting point; when the pulsation grows to the maximum value, it is the transition end point, and the transition interval length is between the two. Therefore, starting from the eddy dynamics process in different transition stages and the underlying physical mechanism of its influence on disturbance growth, combined with linear stability theory, a multi-parameter criterion for the start, end and length of the transition interval is formed, laying the foundation for transition regulation and loss analysis.

[0022] SS6. Result verification and feedback optimization (optional step): The starting and ending positions of the separation flow transition and the length of the transition interval determined by the diagnosis are compared with the test data to evaluate and verify the accuracy and reliability of the transition diagnosis results. The separation flow transition diagnosis is carried out in combination with the flow field parameters under different working conditions to improve the adaptability and versatility of the diagnostic method.

[0023] Take a compressor cascade as an example. Figure 4 The low Reynolds number (Re=1.5×10 5 ) Blade suction surface boundary layer shape factor H 12 Distribution. H 12 From the peak value, it can be seen that the spatial average transition position is located at 59% of the axial chord length ( x / C ax =0.59). In order to accurately determine the starting / ending position of the blade suction surface transition and the length of the transition interval, Figure 5-7 The transient pressure pulsation of the suction surface under this condition, the linear form of Reynolds stress and the exponential form of Reynolds stress distribution are given respectively. The laminar boundary layer is at 32.2% of the axial chord length ( x / C ax =0.322) separation occurs, after which the disturbance begins to grow slowly. At 52% axial chord length ( x / C ax = 0.52), the pulsation in the separation shear layer begins to increase exponentially, and then at 56% of the axial chord length ( x / C ax=0.56) deviates from the exponential growth path. According to the linear stability theory, the secondary instability mechanism such as vortex pairing in the separated shear layer triggers the onset of transition. During the downstream movement, the vortex dynamics such as hairpin vortex breaking and the "upward spray-downward sweep" of the low-energy fluid near the wall cause the pulsation to grow further until it reaches 68% of the axial chord length ( x / C ax =0.68) reaches a peak, indicating that the transition is completed at this point and the boundary layer evolves into a fully developed turbulent state. Through the above multi-parameter diagnostic method, not only the spatial average transition position is determined, but also the transition start / end position is further clarified, and the corresponding transition interval is 12% of the axial chord length. The above results have important guiding significance for transition control and blade design under low Reynolds numbers.

[0024] Through the above embodiments, the purpose of the present invention is fully and effectively achieved. Those skilled in the art can understand that the present invention includes but is not limited to the contents described in the drawings and the above specific embodiments. Although the present invention has been described with respect to the most practical and preferred embodiments currently considered, it should be understood that the present invention is not limited to the disclosed embodiments, and any modification that does not deviate from the functional and structural principles of the present invention will be included in the scope of the claims.

Claims

1. A multi-parameter diagnostic method for the transition characteristics of the separation flow on the blade surface of a compression system under low Reynolds number, characterized in that: The method mainly comprises the following steps when implemented: SS1. Use the high-precision LES method to numerically calculate the three-dimensional flow field inside the compression system under low Reynolds number conditions. After reaching statistical convergence, compare the load distribution at different blade heights, the total pressure loss coefficient of the outlet section, etc. with the measurement results to verify the reliability of the numerical calculation method and obtain statistically converged three-dimensional flow field data; SS2. Based on the statistically converged three-dimensional flow field data, preliminarily evaluate the thickness of the boundary layer on the blade surface under the current working conditions; evenly arrange boundary layer numerical probes at different flow direction positions on the blade surface along the local normal direction, and ensure that the numerical probes completely cover the boundary layer and do not exceed the calculation domain; extract the time-averaged flow field parameters in the area covered by each numerical probe, including at least the circumferential velocity in the Cartesian coordinate system V y , axial speed V z ,density ρ and turbulence statistical characteristic parameters; SS3. Determine the flow angle at different locations on the blade surface α , the circumferential speed V y and axial speed V z Project along the local flow direction to obtain the local flow direction s - Wall Normal n Velocity components in the coordinate system V s and V n ; Based on the streamwise velocity component V s Determine the boundary layer velocity profile distribution at each location on the blade surface, and determine the boundary layer thickness based on the boundary layer velocity profile distribution δ , and integrate the boundary layer displacement thickness , momentum thickness θ And the form factor H 12 Boundary layer integration parameters included; SS4. Preliminary evaluation of laminar separation, transition and turbulent reattachment locations in the boundary layer on the blade surface: when the shape factor H 12 When it increases to 4.2, laminar separation is determined to occur; when the momentum thickness of the boundary layer θ Start to grow rapidly or form factor H 12 When the shape factor starts to decrease from the peak value, it is determined as the spatial average transition position; H 12 When it drops from the peak point to about 3.5, it is determined that turbulent reattachment occurs; SS5. Monitoring points are arranged in the near-wall area of ​​the blade surface to extract the transient pulsation parameters at different flow positions, and to clarify the spatiotemporal development laws of transient disturbances at different transition stages. The time-averaged statistical results of the pulsation parameters at different flow positions are further extracted and organized into a logarithmic form. Combined with the linear stability theory, a multi-parameter comprehensive criterion for the transition of the separation flow in the near-wall area of ​​the blade surface is constructed: the position where the pulsation growth in the separation shear layer begins to deviate from the exponential path is determined as the transition start position, and the position where the pulsation growth reaches the maximum value is determined as the transition end position. The axial distance between the transition start position and the transition end position is the transition interval length.

2. The multi-parameter diagnostic method for the separation flow transition characteristics of the blade surface of the compression system under low Reynolds number according to claim 1 is characterized in that: It also includes step SS6 for verifying the results and providing feedback for optimization. By comparing the separation flow transition start and end positions and transition interval length determined by the diagnosis with the test data, the accuracy and reliability of the transition diagnosis results are evaluated and verified, and the separation flow transition diagnosis is carried out in combination with the flow field parameters under different working conditions to improve the adaptability and versatility of the diagnostic method.

3. The multi-parameter diagnostic method for the separation flow transition characteristics of the blade surface of the compression system under low Reynolds number according to claim 1, characterized in that: In the above step SS1, the sub-grid model uses the wall-adaptive local eddy viscosity WALE model, and the grid resolution scale is set to meet the following conditions: the dimensionless grid size in the wall normal direction does not exceed 1, and the dimensionless grid sizes in the stream and circumferential directions do not exceed 15 and 20, respectively, to meet the analytical requirements of the LES turbulence integral scale; at the same time, the time step of the LES numerical solution is controlled to ensure that the CFL number corresponding to the time step is less than 1.

4. The multi-parameter diagnostic method for the separation flow transition characteristics of the blade surface of the compression system under low Reynolds number according to claim 1, characterized in that: In the above step SS1, the boundary conditions of the numerical calculation include: the distance between the inlet of the computational domain and the leading edge of the blade is 1.5 times the axial chord length, and the outlet is located at 2 times the axial chord length downstream of the trailing edge of the blade; the total pressure, total pressure and airflow angle are given at the inlet, and the average static pressure is given at the outlet. The preset inlet Mach number and Reynolds number are achieved by adjusting the inlet total pressure and the outlet back pressure; in order to improve the convergence of the calculation, the spatial discretization of the computational domain adopts bounded central difference, and the time advancement adopts the second-order backward Euler format; the inner iteration is 10 time steps, ensuring that the residual error is reduced to 10 after each step of calculation is completed -4 ; Set multiple monitoring points at the front edge of the blade suction surface, separation shear layer and wake area. After the mean and standard deviation of the monitoring variables reach statistical convergence, continue to advance 10-15 flow cycles to obtain the flow field statistical parameters under this working condition.

5. The multi-parameter diagnostic method for the separation flow transition characteristics of the blade surface of the compression system under low Reynolds number according to claim 1, characterized in that: In the above step SS2, the arrangement of the boundary layer numerical probes is as follows: a set of numerical probe sequences is arranged at least every 1% of the axial chord length from the leading edge to the trailing edge, and the density of numerical probes is appropriately increased in the transition region of the separation shear layer, and at a fixed flow direction position, the probes are arranged at a rate of 0.02 δ The numerical probes are arranged along the local normal direction at intervals of , and the positions of the outermost numerical probes should ensure that the boundary layer area is fully covered while not exceeding the calculation domain.

6. The multi-parameter diagnostic method for the transition characteristics of the separation flow on the blade surface of the compression system under low Reynolds number according to claim 1, characterized in that: In the above step SS2, the time-averaged parameter distribution of blade surface velocity and pressure is obtained by averaging the statistically converged three-dimensional flow field data, and the boundary layer development state and boundary layer thickness under the current working conditions are preliminarily evaluated based on the near-wall velocity or pressure distribution, and then the normal arrangement range of the boundary layer numerical probe is determined; when the time-averaged flow field parameters at each numerical probe position are extracted, the turbulence statistical characteristic parameters include at least Reynolds normal stress and Reynolds shear stress, and the spatial coordinates of each numerical probe are recorded at the same time.

7. The multi-parameter diagnostic method for the separation flow transition characteristics of the blade surface of the compression system under low Reynolds number according to claim 1, characterized in that: In the above step SS3, the local flow angle α is determined according to the blade surface velocity vector, and the circumferential velocity in the Cartesian coordinate system is V y and axial speed V z Projection to local flow s and wall normal n In the new orthogonal coordinate system, the stream velocity component is obtained V s and the normal velocity component V n , complete the boundary layer area coordinate transformation and velocity projection, the algorithm formula is as follows: The flow angle α Spatial orientation determination based on local numerical probe sequences.

8. The multi-parameter diagnostic method for the transition characteristics of the separation flow on the blade surface of the compression system under low Reynolds number according to claim 1, characterized in that: In the above step SS3, based on the flow velocity component V s Determine the boundary layer velocity distribution at various locations on the blade surface, and V s The maximum value is the mainstream speed , at 99% mainstream speed The normal distance from the wall is taken as the boundary layer thickness δ ; Boundary layer displacement thickness , momentum thickness θ and form factor H 12 The algorithm formula is as follows: V s is the streamwise velocity component, For mainstream speed, is the mainstream density, δ is the boundary layer thickness, n is the wall normal coordinate.

9. The multi-parameter diagnostic method for the transition characteristics of the separation flow on the blade surface of the compression system under low Reynolds number according to claim 1, characterized in that: In the above step SS5, the monitoring points in the near-wall area are arranged at a distance no greater than 5 times the unit distance of the wall surface from the blade surface. y + At the position, a monitoring point is arranged every 1% of the axial chord length along the flow direction, and the points are denser in the area where transition may occur. The spacing is reduced to 0.5% of the axial chord length, and the monitoring time is not less than 10 flow cycles; the transient pulsation parameters include transient pressure pulsation and velocity pulsation in three directions. The time-averaged statistical results of pulsation parameters at different flow directions are extracted, and the time-averaged pulsation parameters are logarithmically transformed by a mathematical transformation method based on the natural logarithm.

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