A multi-parameter diagnostic method for the transition characteristics of blade surface separation flow in a compression system at low Reynolds number.

Abstract

This invention proposes a multi-parameter diagnostic method for the transition characteristics of separated flow on the blade surface of a compression system at low Reynolds numbers. This method accurately determines the transition interval on the blade surface, supporting efficient control and design. First, the method performs high-fidelity numerical calculations on the internal flow field of the compression system at low Reynolds numbers to obtain a convergent solution. Numerical probes are uniformly arranged along the normal direction at different flow directions on the blade surface to extract near-wall time-averaged flow field parameters. The velocity components in Cartesian coordinates are projected along the local flow direction to obtain the boundary layer velocity profile, and the boundary layer characteristic parameters are obtained by integration. Combining the spatiotemporal evolution law of near-wall transient disturbances and linear stability theory, a multi-parameter criterion for the transition characteristics of separated flow in the compression system is formed, with the transition start and end positions accurate to within 1% of the axial chord length. Compared with the traditional single criterion based on shape factor, this method improves the diagnostic accuracy of separated flow transition characteristics, providing support for transition control and blade design in compression systems at low Reynolds numbers.

Description

Technical Field

[0001] This invention belongs to the field of flow diagnostic technology for aero-engine compression systems, and relates to the analysis of boundary layer transition characteristics of compression systems at low Reynolds numbers. Specifically, it relates to a multi-parameter diagnostic method for the transition characteristics of separated flow at low Reynolds numbers, which is used to accurately determine the transition range of the blade surface of the compression system at low Reynolds numbers, providing support for efficient flow control and blade design. Background Technology

[0002] Lightweight aircraft engines are the primary power source for high-altitude unmanned aerial vehicles (UAVs). Due to the high service ceiling and small size of UAVs, the Reynolds number at the engine compression system inlet is reduced by 1-2 orders of magnitude (~10) compared to ground conditions. 5 Compared to ground operating conditions, the low Reynolds number effect in compression systems easily induces a series of problems such as decreased pressure ratio, efficiency degradation, and insufficient margin, severely affecting the engine's high-altitude fuel consumption and operational stability. When the Reynolds number drops to near the critical value, the laminar flow region on the blade surface of the compression system expands, and under the influence 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, local turbulent pulsation accelerates, and the high-loss region expands rapidly; at the same time, the boundary layer thickens rapidly, inducing strong aerodynamic blockage, and deteriorating the efficiency and margin of the compression system. It can be considered that the separation flow transition is the fundamental cause of the low Reynolds number effect in compression systems and the constraint on the overall high-altitude performance of aero-engines.

[0003] Accurate diagnosis of the transition characteristics of the separated flow (transition start / end positions, transition interval length, and overall size of the separated bubble, etc.) is a prerequisite for efficient flow control and blade design in low Reynolds number compression systems. Traditional transition diagnostic methods mainly rely on experimental measurements, such as hot-wire anemometers and surface hot films. However, with the increasing thrust-to-weight ratio of modern aero-engines, compression system structures are becoming more compact, blades are shorter, and size effects are significant. In this case, common invasive transition testing methods (such as surface hot films) interfere more strongly with the flow on the blade surface, easily disrupting the original flow field and introducing significant uncertainty into the accuracy of transition testing. Moreover, the installation and setup of transition testing components are complex and time-consuming, making it difficult to support rapid iteration of transition control and blade design schemes.

[0004] Theoretically, by developing high-precision numerical calculation methods, it is possible to comprehensively monitor all information related to the transition of the separated flow in a compression system within a short period of time, without the problems of intrusive measurement equipment interfering with the flow field in a confined space. However, the compression system contains multi-scale secondary flow structures and intense vortex dynamics processes, and exhibits highly turbulent non-equilibrium and anisotropy near the blades / endwalls, which severely interferes with the identification of local transition patterns and the diagnosis of transition intervals. This results in poor accuracy and reliability of conventional numerical analysis methods in diagnosing the transition characteristics of the separated flow.

[0005] In summary, the transition process of the separated flow at low Reynolds numbers is a core factor contributing to the performance degradation of compression systems. Traditional transition testing methods such as hot film / hot wire have significant limitations, and numerical analysis methods still face challenges in terms of 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 diagnostic criteria is a pressing technical problem to be solved in the field of low Reynolds number effect research on compression systems. Summary of the Invention

[0006] (a) Purpose of the invention

[0007] To address the aforementioned problems, this invention proposes a multi-parameter diagnostic method for the transition characteristics of the separated flow on the blade surface of a compression system at low Reynolds numbers, starting from the spatiotemporal evolution of near-wall transient fluctuations in the separated shear layer. First, based on steady-state statistical data, the integral parameters of the boundary layer on the blade surface of the compression system are accurately calculated. Then, the correlation between the eddy dynamics process and the near-wall turbulent fluctuation generation rate at different transition stages is established, and combined with linear stability theory, a multi-parameter criterion for the start / end position of the separated flow transition is formed. This method significantly improves the diagnostic accuracy of the transition characteristics of the separated flow on the blade surface of the compression system, providing fundamental support for the transition control and rapid iterative design of compression systems at low Reynolds numbers.

[0008] (II) Technical Solution

[0009] The purpose of this invention is to propose a diagnostic method for the transition characteristics of separated flow on the blade surface of a compression system at low Reynolds numbers. This method accurately determines the start and end points of transition and the length of the transition interval, supporting efficient control and design. The key to this invention lies in how to comprehensively utilize time-averaged flow field data and transient flow field data to achieve a panoramic diagnosis of the transition characteristics of separated flow. The main steps and corresponding solutions are as follows:

[0010] SS1. Numerical Calculation of Three-Dimensional Flow Field:

[0011] High-precision large eddy simulation (LES) was used to numerically calculate the three-dimensional flow field inside the compression system under low Reynolds number conditions. After statistical convergence, the load distribution at different blade heights and the total pressure loss coefficient at the outlet section were compared with the measurement results to verify the reliability of the numerical calculation method and obtain statistically converged three-dimensional flow field data.

[0012] SS2. Extraction of boundary layer parameters on blade surface:

[0013] Based on statistically converged three-dimensional flow field data, the boundary layer thickness on the blade surface under current operating conditions is preliminarily assessed. Boundary layer numerical probes are uniformly deployed along the local normal at different flow direction locations on the blade surface, ensuring that the numerical probes completely cover the boundary layer and do not exceed the computational domain. Time-averaged flow field parameters within the coverage area of ​​each numerical probe are extracted, including at least the circumferential velocity in Cartesian coordinates. V y axial velocity V z ,density ρ and turbulence statistical characteristic parameters;

[0014] SS3. Calculation of boundary layer integral parameters on the blade surface:

[0015] Determine the flow direction angle at different locations on the blade surface α Circumferential velocity V y and axial velocity V z Projecting along the local flow direction yields the local flow direction. s - Wall normal n velocity components in coordinate system V s and V n Based on flow velocity components V s Determine the boundary layer velocity profile distribution at various locations on the blade surface, and then determine the boundary layer thickness based on the boundary layer velocity profile distribution. δ And integral calculation including boundary layer displacement thickness Momentum thickness θ and shape factor H 12 Boundary layer integral parameters included;

[0016] SS4. Preliminary assessment of laminar flow separation, transition, and reattachment locations:

[0017] Preliminary assessment of laminar separation, transition, and turbulent reattachment locations in the blade surface boundary layer: when the shape factor H 12 When the value is increased to 4.2, laminar separation is determined to have occurred; when the boundary layer momentum thickness... θ Start to grow rapidly or shape factor H 12 The point where the price begins to decline from its peak is identified as the spatial average transition point; when the shape factor... H 12 When the value drops from the peak to around 3.5, turbulent reattachment is considered to have occurred.

[0018] SS5. Accurate diagnosis of separation flow transition characteristics based on the evolution features of transient perturbations in the near-wall region:

[0019] Based on the preliminary assessment, the transition characteristics of the separation flow are further precisely diagnosed based on the evolution characteristics of transient disturbances in the near-wall region. First, monitoring points are set up in the near-wall region of the blade surface to extract transient pulsation parameters at different flow directions, clarifying the spatiotemporal development law of transient disturbances at different transition stages. Then, the time-averaged statistical results of pulsation parameters at different flow directions are extracted and organized into logarithmic form. Combined with linear stability theory, a multi-parameter comprehensive criterion for the transition of the separation flow in the near-wall region 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, the position where the pulsation growth reaches its maximum value is determined as the transition end position, and the axial distance between the transition start position and the transition end position is the transition interval length.

[0020] SS6. Result Validation and Feedback Optimization (Optional Step):

[0021] By comparing the diagnosed start and end points of the transition and the length of the transition interval with experimental data, the accuracy and reliability of the transition diagnosis results are evaluated and verified. Furthermore, the transition diagnosis of the transition is carried out in combination with the flow field parameters under different operating conditions, thereby improving the adaptability and versatility of the diagnostic method.

[0022] (III) Technical Effects

[0023] Compared with existing technologies, the multi-parameter diagnostic method for the surface separation flow transition characteristics of compression system blades at low Reynolds numbers provided by this invention has the following advantages:

[0024] (1) Based on high-precision large eddy simulation (LES) to obtain time-averaged and transient flow field data inside the compression system, this invention develops a multi-parameter diagnostic method for the transition characteristics of separated flow, realizing a panoramic diagnosis of the transition process of separated flow at 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 fluctuation parameters in the near-wall region, such as transient pressure fluctuations and Reynolds stress fluctuations, thus enabling a more comprehensive reflection of the essential characteristics of the transition of separated flow.

[0025] (2) Starting from the spatiotemporal evolution mechanism of the vortex system at different stages of the separation flow transition, this invention clarifies the correlation between the above-mentioned vortex dynamics process and the near-wall turbulent fluctuation generation rate. On this basis, by introducing the growth characteristics of transient fluctuations in the near-wall region and combining linear stability theory, the starting / ending position criterion of the separation flow transition is rigorously derived, with clear physical meaning and high reliability.

[0026] (3) Compared with the existing single criterion based on boundary layer shape factor, the present invention considers the multi-parameter criterion of near-wall transient disturbance spatiotemporal evolution, which can provide richer transition information. The start and end positions of the transition are accurate to within 1% of the axial chord length, which significantly improves the diagnostic accuracy of the transition characteristics of the separated flow and can directly support efficient control of transition and blade design at low Reynolds number. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the implementation of a multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system at low Reynolds number, as described in this invention.

[0028] Figure 2 This is a schematic diagram of the numerical computation domain and boundary layer numerical probe arrangement in an embodiment of the present invention;

[0029] Figure 3 This is a schematic diagram of the coordinate transformation and velocity projection of the boundary layer region in an embodiment of the present invention;

[0030] Figure 4 This is the boundary layer shape factor in the embodiments of the present invention. H 12 Position of dimensionless chord x / C ax A schematic diagram showing the distribution of [something], with the horizontal axis [something]. x / C ax Represents the position of a dimensionless chord, y-axis H 12 This is the boundary layer shape factor;

[0031] Figure 5 These are different chord length positions in the embodiments of the present invention. x / C ax Transient pressure pulsation p 'With dimensionless time A schematic diagram of the evolutionary pattern, with the horizontal axis... For dimensionless time, refer to the speed. U chord length c Normalization; ordinate p ' / p average It represents the ratio of pressure pulsation to average pressure over time, and different curves represent the time history of pressure pulsation at different chord lengths.

[0032] Figure 6 This 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 The dimensionless chord length position, ordinate Reynolds normal stress (at reference velocity) U (1 squared dimensionless).

[0033] Figure 7 This 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 The coordinates represent the dimensionless chord length position, and the vertical axis represents the Reynolds normal stress in logarithmic form. Detailed Implementation

[0034] The purpose of this invention is to propose a diagnostic method for the transition characteristics of separated flow at low Reynolds numbers, used to accurately determine the blade transition range of a compression system at low Reynolds numbers, supporting efficient control and design. The key to this invention lies in how to comprehensively utilize time-averaged flow field data and transient flow field data to achieve a panoramic diagnosis of the transition characteristics of separated flow. To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be described in more detail below with reference to the accompanying drawings. The described embodiments are some, but not all, embodiments of this invention, and are exemplary, intended to explain the invention, and should not be construed as limiting the invention. Figure 1 As shown, the main steps and corresponding solutions for implementing this invention are as follows:

[0035] SS1. Numerical Calculation of Three-Dimensional Flow Field:

[0036] For a specific compressor component, high-precision Large Eddy Simulation (LES) was first used to conduct three-dimensional flow field numerical calculations at low Reynolds numbers. To capture the complex transition flows on the blade and endwall surfaces, a wall-adaptive local eddy viscosity (WALE) model was selected as the subgrid model, and the mesh analytical scale met the requirements of LES calculations. After the numerical calculations achieved statistical convergence, the numerical / experimental results of the loads and total pressure loss coefficients at different blade heights were compared. Since the separated flow transition is the fundamental factor determining the load distribution and loss magnitude at low Reynolds numbers, the numerical calculation results of the separated flow transition were considered reliable when the loads and total pressure loss at different blade heights agreed well with the experimental results. Finally, statistically converged three-dimensional flow field data were obtained.

[0037] As a preferred approach, when performing high-precision LES simulations, the analytical grid 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, satisfying the analytical requirements of the LES turbulence integral scale; simultaneously, the time step of the LES numerical solution is controlled to ensure that the CFL number corresponding to this time step is less than 1. The boundary conditions for the numerical calculation include: the computational domain inlet is 1.5 times the axial chord length from the blade leading edge, and the outlet is located 2 times the axial chord length downstream of the blade trailing edge; the inlet is given a total pressure, total pressure, and airflow angle, and the outlet is given a mean static pressure, with the preset inlet Mach number and Reynolds number achieved by adjusting the inlet total pressure and outlet back pressure; to improve computational convergence, the computational domain spatial discretization adopts bounded central difference, and the time advancement adopts a second-order backward Euler scheme; the inner iteration is 10 time steps, ensuring that the residual is reduced to 10 after each calculation step. -4 Multiple monitoring points were set up at the front edge of the blade suction, the separation shear layer, and the wake region. After the mean and standard deviation of the monitored variables reached statistical convergence, the flow was continued for 10-15 flow cycles to obtain the flow field statistical parameters under this condition.

[0038] SS2. Extraction of boundary layer parameters on blade surface:

[0039] After obtaining the statistically convergent three-dimensional flow field, the boundary layer thickness on the blade surface under this operating condition is preliminarily assessed based on the time-averaged parameter distributions of the blade surface velocity, pressure, etc. Numerical probes are uniformly deployed along the local normal at different flow directions on the blade surface. The numerical probes must completely cover the boundary layer while not exceeding the computational domain. The numerical computational domain and the arrangement of the boundary layer numerical probes are as follows: Figure 2 As shown. The time-averaged flow field parameters corresponding to the boundary layer numerical probe are extracted, including the circumferential velocity in Cartesian coordinates. V y axial velocity V z ,density ρ And turbulence statistical characteristic parameters such as Reynolds normal stress / shear stress.

[0040] Preferably, by performing time-averaging processing on the statistically converged three-dimensional flow field data, the time-averaged parameter distributions of velocity and pressure on the blade surface are obtained. Based on the near-wall velocity (or pressure) distribution, the boundary layer development state and boundary layer thickness under the current operating conditions are preliminarily assessed, and then the normal arrangement range of the boundary layer numerical probes is determined. When extracting the time-averaged flow field parameters at each numerical probe location, the turbulent statistical characteristic parameters include at least the Reynolds normal stress (…). u ' u '、 v ' v '、 w ' w ') and Reynolds shear stress (- u ' v'、- u ' w '、- v ' w '), and simultaneously record the spatial coordinates of each numerical probe ( x , y , z This is used for subsequent coordinate transformation and parameter calculation. Furthermore, the boundary layer numerical probes are arranged as follows: a sequence of numerical probes is placed at least every 1% of the axial chord length from the leading edge to the trailing edge, with the density of numerical probes appropriately increased in the transition region of the separated shear layer. At a fixed flow direction location, at a density of 0.02... δ Numerical probes are arranged at intervals along the local normal, and the location of the outermost numerical probe must ensure that it completely covers the boundary layer region while not exceeding the computational domain.

[0041] SS3. Calculation of boundary layer integral parameters on the blade surface:

[0042] Determine the flow direction angle at different positions on the blade surface α Circumferential velocity V y and axial velocity V z Projecting along the local flow direction yields the local flow direction. s - Wall normal n velocity components in coordinate system V s and V n Complete the coordinate transformation and velocity projection of the boundary layer region, such as... Figure 3 As shown, the relevant formulas are as follows:

[0043]

[0044] Using the above method, the boundary layer velocity profile distribution at various locations on the blade surface can be obtained, which is the flow velocity component. V s At different wall normal distances V s The maximum value is the mainstream speed The normal distance from the wall at 99% of the mainstream velocity is the boundary layer thickness. δ Based on the boundary layer velocity profile and boundary layer thickness, the boundary layer displacement and thickness are obtained by integral solution. Momentum thickness θ and shape factor H 12 The specific definition is as follows:

[0045]

[0046] SS4. Preliminary assessment of laminar flow separation, transition, and reattachment locations:

[0047] Based on the above three boundary layer integration parameters, the laminar separation, transition, and reattachment locations on the blade surface can be preliminarily assessed, such as... Figure 4 As shown. Figure 4 The boundary layer shape factor of the blade suction surface is shown. H 12 With axial position x / C ax The distribution of [something] can be seen in the figure. x / C ax <0.3 area, H 12 A stable value of around 2.5 indicates that the boundary layer is in a laminar state; when H 12 When the value increases to 4.2, laminar separation is considered to have occurred, at which point the boundary layer displacement thickness... The growth has accelerated significantly, as shown in H 12 The rapid increase in boundary layer momentum thickness. When the separating shear layer becomes unstable and triggers a transition, the boundary layer momentum thickness... θ It began to grow rapidly, leading to H 12 It begins to decline from its peak. Therefore, θ Started to grow rapidly or H 12 The point where the decline begins from the peak is the turning point. After the turning point ends, H 12 A drop from the peak value to around 3.5 is considered an indication of turbulent reattachment. H 12 The gradual stabilization indicates that the boundary layer has transitioned into a fully developed turbulent state.

[0048] Theoretically speaking, as long as it passes θ or H 12 The distribution of flow can be used to determine the transition location. However, for compressor blades with large curvature and strong adverse pressure gradients, the transition of separated flow at low Reynolds numbers is completed within a certain flow direction distance, i.e., a transition interval exists. However, the transition location obtained by the above criteria is a spatially averaged transition location, making it difficult to determine the start / end position of the transition and the length of the transition interval. The start / end position of the transition is crucial for determining fine flow control schemes and loss analysis at low Reynolds numbers.

[0049] SS5. Accurate diagnosis of separation flow transition characteristics based on the evolution features of transient perturbations in the near-wall region:

[0050] Next, based on the spatiotemporal evolution of transient disturbances in the near-wall region, a multi-parameter diagnostic method is proposed to separate the transition interval (transition start / end position). Monitoring points are arranged in the near-wall region of the blade surface, and transient pulsation parameters at different flow directions are extracted, such as... Figure 5 As shown, when laminar separation does not occur, the near-wall parameter fluctuations are very weak; when the laminar boundary layer detaches from the wall under the influence of the adverse pressure gradient to form a separation shear layer, the near-wall parameter fluctuations begin to increase; when the separation shear layer becomes unstable and triggers transition, the near-wall parameter fluctuations exhibit large fluctuations, indicating that the prominent feature of the separation transition process is the strong momentum exchange; after the transition ends and the separation shear layer reattaches, the turbulent fluctuations in the near-wall region decrease back to a lower level.

[0051] To more clearly reflect the growth pattern of pulsations at different transition stages, the pulsation parameters at different flow directions are organized into logarithmic form. Based on linear stability theory, the pulsations in the near-wall region initially exhibit rapid exponential growth. When secondary instabilities such as vortex pairing occur in the separated shear layer, three-dimensional effects begin to induce the pulsation growth path to deviate from the exponential pattern, indicating the onset of transition. At the end of the transition, when large-scale hairpin vortex breakup and near-wall low-energy fluid "upward jet-downward sweep" processes occur in the separated shear layer, the pulsations further increase to their maximum value, and the transition is complete. Therefore, the location where the pulsation growth in the separated shear layer begins to deviate from the exponential path is the transition initiation point; when the pulsations reach their maximum value, it is the transition end point, and the distance between the two is the length of the transition interval. Therefore, starting from the eddy dynamics process at different transition stages and the underlying physical mechanism of its impact on disturbance growth, and combining 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 control and loss analysis.

[0052] SS6. Result Validation and Feedback Optimization (Optional Step):

[0053] By comparing the diagnosed start and end points of the transition and the length of the transition interval with experimental data, the accuracy and reliability of the transition diagnosis results are evaluated and verified. Furthermore, the transition diagnosis of the transition is carried out in combination with the flow field parameters under different operating conditions, thereby improving the adaptability and versatility of the diagnostic method.

[0054] Taking a compressor blade cascade as an example, Figure 4 The results are given for low Reynolds numbers (Re=1.5×10⁻⁶). 5 Blade suction surface boundary layer shape factor H 12 Distribution. By H 12 The peak value indicates that the spatial average transition point is located at 59% of the axial chord length. x / C ax =0.59). To accurately determine the start / end position of the blade suction surface transition and the length of the transition interval, Figure 5-7 The transient pressure fluctuations, linear Reynolds stress, and exponential Reynolds stress distributions of the suction surface under this working condition are presented respectively. The laminar boundary layer is located at 32.2% of the axial chord length ( x / C ax Separation occurs at 0.322, after which the perturbation begins to grow slowly. At 52% of the axial chord length ( x / C ax =0.52), the pulsations in the separated shear layer begin to show an exponential and rapid increase, and then at 56% axial chord length ( x / C ax =0.56) deviates from the exponential growth path. According to linear stability theory, secondary instability mechanisms such as vortex pairing in the separated shear layer at this location trigger the transition initiation. During downstream movement, vortex dynamics processes such as hairpin vortex breaking and near-wall low-energy fluid "upward jet-downward sweep" lead to further increase in pulsation until it reaches 68% of the axial chord length ( x / C ax The peak value (=0.68) indicates that the transition is complete at this point, and the boundary layer evolves into a fully developed turbulent state. Through the aforementioned multi-parameter diagnostic method, not only was the spatially average transition location determined, but the start / end points of the transition were also clarified, with the corresponding transition interval being 12% of the axial chord length. These results have significant guiding implications for transition control and blade design at low Reynolds numbers.

[0055] The objectives of this invention have been fully and effectively achieved through the above embodiments. Those skilled in the art will understand that this invention includes, but is not limited to, the contents described in the accompanying drawings and the specific embodiments described above. Although the invention has been described with reference to what is currently considered the most practical and preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments, and any modifications that do not depart from the functional and structural principles of the invention will be included within the scope of the claims.

Claims

1. A multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system blade at low Reynolds numbers, characterized in that, The method includes the following steps when implemented: SS1. The high-precision LES method is used to numerically calculate the three-dimensional flow field inside the compression system under low Reynolds number conditions. After statistical convergence, the load distribution at different blade heights and the total pressure loss coefficient at the outlet section 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. Based on statistically converged three-dimensional flow field data, preliminarily assess the boundary layer thickness on the blade surface under current operating conditions; uniformly deploy boundary layer numerical probes along the local normal at different flow direction locations on the blade surface, ensuring that the numerical probes completely cover the boundary layer and do not exceed the computational domain; extract the time-averaged flow field parameters within the coverage area of ​​each numerical probe, including at least the circumferential velocity in Cartesian coordinates. V y axial velocity V z ,density ρ and turbulence statistical characteristic parameters; SS3. Determine the flow direction angle at different locations on the blade surface. α Circumferential velocity V y and axial velocity V z Projecting along the local flow direction yields the local flow direction. s - Wall normal n velocity components in coordinate system V s and V n Based on flow velocity components V s Determine the boundary layer velocity profile distribution at various locations on the blade surface, and then determine the boundary layer thickness based on the boundary layer velocity profile distribution. δ And integral calculation including boundary layer displacement thickness Momentum thickness θ and shape factor H 12 Boundary layer integral parameters included; SS4. Preliminary assessment of laminar separation, transition, and turbulent reattachment locations in the blade surface boundary layer: when the shape factor H 12 When the value is increased to 4.2, laminar separation is determined to have occurred; when the boundary layer momentum thickness... θ Start to grow rapidly or shape factor H 12 The point where the price begins to decline from its peak is identified as the spatial average transition point; when the shape factor... H 12 When the value drops from the peak to around 3.5, turbulent reattachment is considered to have occurred. SS5. Monitoring points are set up in the near-wall region of the blade surface to extract transient pulsation parameters at different flow directions, clarifying the spatiotemporal development law of transient disturbances at different transition stages; further, the time-averaged statistical results of pulsation parameters at different flow directions are extracted and organized into logarithmic form, and combined with linear stability theory, a multi-parameter comprehensive criterion for the transition of separated flow in the near-wall region of the blade surface is constructed: the position where the pulsation growth in the separated shear layer begins to deviate from the exponential path is determined as the transition start position, the position where the pulsation growth reaches its maximum value is determined as the transition end position, and 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 surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, It also includes step SS6 for verifying and optimizing the results. By comparing the start and end positions of the separation flow transition and the length of the transition interval determined by the diagnosis with the experimental data, the accuracy and reliability of the transition diagnosis results are evaluated and verified. Furthermore, the separation flow transition diagnosis is carried out in combination with the flow field parameters under different operating conditions to improve the adaptability and versatility of the diagnosis method.

3. The multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, In step SS1 above, the subgrid model uses the wall-adaptive local eddy viscosity (WALE) model, and the analytical mesh scale is set to meet the following conditions: the dimensionless mesh size in the wall normal direction does not exceed 1, and the dimensionless mesh sizes in the flow direction and circumferential direction do not exceed 15 and 20 respectively, which meets 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 this time step is less than 1.

4. The multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, In step SS1 above, the boundary conditions for numerical calculation include: the computational domain inlet is 1.5 times the axial chord length from the blade leading edge, and the outlet is located 2 times the axial chord length downstream of the blade trailing edge; the inlet is given a total pressure, total pressure, and airflow angle, and the outlet is given a mean static pressure; the preset inlet Mach number and Reynolds number are achieved by adjusting the inlet total pressure and outlet back pressure; to improve computational convergence, the computational domain space is discretized using bounded central difference, and the time progression uses a second-order backward Euler scheme; the internal iteration has 10 time steps, ensuring that the residual is reduced to 10 after each calculation step. -4 Multiple monitoring points were set up at the front edge of the blade suction, the separation shear layer, and the wake region. After the mean and standard deviation of the monitored variables reached statistical convergence, the flow was continued for 10-15 flow cycles to obtain the flow field statistical parameters under this condition.

5. The multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, In step SS2 above, the boundary layer numerical probes are arranged as follows: a set of numerical probes is arranged at least every 1% of the axial chord length from the leading edge to the trailing edge. Simultaneously, the numerical probe density is appropriately increased in the transition region of the separated shear layer, and at a fixed flow direction position, the number of numerical probes is 0.

02. δ Numerical probes are arranged at intervals along the local normal, and the location of the outermost numerical probes must ensure that they completely cover the boundary layer region while not exceeding the computational domain.

6. The multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, In step SS2 above, the average distribution of velocity and pressure on the blade surface is obtained by averaging the statistically converged three-dimensional flow field data. Based on the near-wall velocity or pressure distribution, the boundary layer development state and boundary layer thickness under the current operating conditions are initially assessed, and then the normal arrangement range of the boundary layer numerical probes is determined. When extracting the time-averaged flow field parameters at each numerical probe location, 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.

7. The multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, In step SS3 above, the local flow angle α is determined based on the blade surface velocity vector, and the circumferential velocity in the Cartesian coordinate system is... V y and axial velocity V z Projected onto local flow direction s and wall normal n In the newly constructed orthogonal coordinate system, the flow velocity components are obtained. V s and normal velocity component V n The algorithm for performing coordinate transformation and velocity projection in the boundary layer region is as follows: Among them, the flow angle α Spatial orientation determination based on local numerical probe sequences.

8. The multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, In step SS3 above, based on the flow velocity component V s Determine the boundary layer velocity profile distribution at various locations on the blade surface, using different wall normal distances. V s The maximum value is the mainstream speed At 99% of mainstream speed The normal distance from the wall is used as the boundary layer thickness. δ Boundary layer displacement thickness Momentum thickness θ and shape factor H 12 The algorithm formula is as follows: V s For the flow velocity component, As the mainstream speed, As the mainstream density, δ For boundary layer thickness, n These are the coordinates of the wall normal.

9. The multi-parameter diagnostic method for the surface separation flow transition characteristics of a compression system under low Reynolds number as described in claim 1, characterized in that, In step SS5 above, the monitoring points in the near-wall area are arranged at a distance from the blade surface no more than 5 times the unit distance of the wall surface. y + The monitoring points are arranged at 1% of the axial chord length along the flow direction, and the spacing is reduced to 0.5% of the axial chord length in areas where transitions may occur. The monitoring time is no 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 the pulsation parameters at different flow directions are extracted, and the time-averaged pulsation parameters are logarithmically transformed using a mathematical transformation method based on the natural logarithm.

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