A method and system for measuring klebanoff streaks in a boundary layer

Abstract

The application discloses a kind of Klebanoff stripe measurement method and system in boundary layer, belong to aerodynamics experimental measurement technical field.The method is to be measured wall surface by multiple measurement units comprising quasi-wall shear film array, the distribution of streamwise quasi-wall shear force and spanwise quasi-wall shear force is synchronously measured using two groups of orthogonal layout hot film sensing part in each measurement unit, combined with boundary layer thickness, dynamic viscosity and wall surface normal reference height, based on boundary layer velocity distribution model inversion obtains two-dimensional velocity field near wall surface, and obtains wall surface normal vorticity field by space difference, further characterizes the transient spatial structure and dynamic evolution process of Klebanoff stripe.The application is suitable for boundary layer transition measurement and analysis in wind tunnel experiment, blade profile experiment and cascade experiment, and can improve the synchronism, reliability and engineering applicability of near-wall small-scale disturbance structure measurement under complex flow conditions.

Description

Technical Field

[0001] This invention belongs to the field of aerodynamic experimental measurement technology, and relates to wall measurement and characterization technology of near-wall flow structures in boundary layer stability experiments. Specifically, it relates to a method and system for measuring Klebanoff fringes in the boundary layer, which is applicable to the measurement and transient spatial distribution and temporal evolution characterization of dynamic small-scale flow structures such as Klebanoff fringes. Background Technology

[0002] In aerodynamic experiments and boundary layer stability studies, the identification of boundary layer transition processes and their precursor disturbance structures is a fundamental issue in aircraft design, blade aerodynamic performance evaluation, low-drag flow control, and high-load turbine blade experimental research. Klebanoff fringes, as a low-frequency banded flow structure influencing boundary layer transition development, require precise measurement of their characteristic parameters (such as wavelength, amplitude, and propagation velocity) for understanding transition mechanisms and establishing transition prediction models. In particular, effective measurement of the scale, intensity, migration characteristics, and spatiotemporal evolution of Klebanoff fringes in flat-plate wind tunnel experiments, airfoil flow experiments, and engine blade aerodynamic experiments is crucial for revealing near-wall flow mechanisms, verifying numerical calculation results, and establishing transition prediction models.

[0003] In existing aerodynamic experiments, measurement methods related to boundary layer disturbances mainly include hot-wire probes, wall thermal films, pressure measurements, and optical flow field diagnostics. Traditional thermal film sensors are only sensitive to flow along the normal direction of their sensing part, and therefore can usually only measure shear force in a single effective flow direction. When the flow is a complex three-dimensional flow, the wall shear force is a two-dimensional vector with both flow-direction and spanwise components. Traditional unidirectional thermal films cannot simultaneously acquire information on these two directional components. If measurements are performed using rotating sensors, transient synchronous data cannot be obtained, and the operation is cumbersome and spatial resolution is reduced.

[0004] Currently, although some studies have attempted to measure spatial point velocity using cross-wire or triple-wire hot-wire probes, these probes are relatively large, making them difficult to place close to the wall. Furthermore, they are primarily used to measure the three-dimensional velocity vector of spatial points, rather than the two-dimensional shear force vector of the wall. Additionally, these probes generally have characteristics such as extended support structures, limited clearance between the measuring body and the wall, and high requirements for probe attitude control. In aerodynamic experiments in narrow channels, curved walls, or near rotating blade cascades, they often suffer from insufficient installation space, difficulty in near-wall placement, and additional disturbances to the original flow field. Moreover, Klebanoff fringes, as a type of velocity disturbance, affect boundary layer development. Historically, Klebanoff fringes have only been observed using optical methods, which require a wide optical path and are difficult to apply in complex flow channels. They also suffer from low resolution in the small-scale environment of engine blade cascades. At the same time, some existing measurement methods focus more on the outer velocity field or the overall flow pattern, and are still insufficient in revealing the correspondence between the perturbation structure near the wall and the wall response, making it difficult to meet the needs of carrying out refined measurement and analysis of the precursor structure of the boundary layer transition.

[0005] In summary, existing methods for measuring boundary layer Klebanoff fringes under aerodynamic experimental conditions still have shortcomings in terms of applicability to near-wall regions, capability for complex flow channel arrangements, transient synchronous measurement capability, and high spatiotemporal resolution characterization capability. Therefore, developing a technique suitable for aerodynamic experiments, applicable to the near-wall region of the boundary layer, easy to implement in confined spaces and complex flow channel environments, and capable of effectively measuring boundary layer Klebanoff fringes, is an urgent technical problem to be solved. Summary of the Invention

[0006] (a) Purpose of the invention

[0007] This invention addresses the aforementioned deficiencies and shortcomings of existing technologies by providing a method and system for measuring Klebanoff fringes within the boundary layer. This approach constructs a bidirectional quasi-wall shear force measurement method for complex near-wall flow environments, simultaneously acquiring the flow-direction and spanwise shear force components of the boundary layer. By combining boundary layer parameters with the inversion of the two-dimensional velocity field and its normal vorticity distribution near the wall, it achieves effective identification, continuous observation, and quantitative characterization of the transient spatial structure, dynamic evolution process, and impact on boundary layer transition of Klebanoff fringes. This improves the accuracy, synchronicity, and engineering applicability of measurements of related flow structures in complex flow channels and small-scale experimental scenarios, providing reliable technical support for boundary layer stability research, transition mechanism analysis, and aerodynamic experimental evaluation.

[0008] (II) Technical Solution

[0009] To achieve the objective of this invention and solve its technical problems, the present invention adopts the following technical solution:

[0010] The first objective of this invention is to provide a method for measuring Klebanoff fringes within a boundary layer, used to measure the transient spatial structure and dynamic evolution of Klebanoff fringes within the boundary layer of a test specimen under wind tunnel, airfoil, or cascade test conditions, comprising at least the following steps:

[0011] SS1. Experimental Setup and Coordinate Establishment: Under aerodynamic experimental conditions, a quasi-wall shear thermal film array consisting of multiple measurement units is laid on the test surface of the experimental specimen. Each measurement unit has two sets of orthogonally arranged thermal film sensing parts, and coordinates are established along the flow direction. x and development direction z An unfolded wall coordinate system;

[0012] SS2. Synchronous Acquisition of Bidirectional Quasi-Wall Shear Force: Power is supplied to the thermal film sensing element of each measurement unit and the output signal is acquired synchronously. Based on the orthogonal layout of the two sets of thermal film sensing elements in the same measurement unit, the quasi-wall shear force flowing towards the wall under test is measured. t qx ( x , z and spanwise quasi-wall shear t qz ( x , z The distribution matrix of ).

[0013] SS3. Boundary Layer Parameter Determination: Measure the boundary layer thickness of the wall surface to be measured. d Determine the dynamic viscosity of the fluid under experimental conditions. m and give the reference height of the wall normal. y ;

[0014] SS4. Two-dimensional velocity field inversion: based on flow-direction quasi-wall shear. t qx ( x , z ), spanwise quasi-wall shear t qz ( x , z Using a pre-defined boundary layer velocity distribution model, a two-dimensional velocity field near the wall to be measured is obtained through inversion. The two-dimensional velocity field includes flow velocities. u ( x , z and spanwise velocity w ( x , z )distributed;

[0015] SS5. Determination of normal vorticity of the wall: Based on the inverted two-dimensional velocity field on the tangent plane of the wall to be measured. x - z The distribution on the surface is obtained by spatial difference calculation using data from adjacent measuring points. and Then, the normal vorticity field of the wall can be obtained. w y ( x , z ), and used it as the characterization result of Klebanoff stripes;

[0016] SS6. Measurement Result Output: Combines the wall normal vorticity field obtained at continuous time intervals. w y ( x , z The output Klebanoff fringes are tangent to the test surface. x - z The transient spatial structure and dynamic evolution process on the surface.

[0017] The second objective of this invention is to provide a measurement system for Klebanoff fringes within a boundary layer for implementing the above-described method, comprising at least:

[0018] The quasi-wall shear force hot film array module is applied to the test wall of the experimental specimen in aerodynamic experiments. It consists of multiple measurement units, each of which has two sets of orthogonally arranged hot film sensing parts to sense the quasi-wall shear force in the flow direction and span direction of the test wall.

[0019] The data synchronization acquisition module is communicatively connected to the quasi-wall shear force hot-film array module. It is used to power the hot-film sensing part of each measurement unit and synchronously acquire the output signal to obtain the quasi-wall shear force flowing from the wall under test. t qx ( x , z and spanwise quasi-wall shear t qz ( x , z The distribution matrix of ).

[0020] The two-dimensional velocity field inversion module communicates with the data synchronization acquisition module to obtain the boundary layer thickness of the wall under test. d Dynamic viscosity of fluid under experimental conditions m and wall normal reference height y And based on the quasi-wall shear force of the flow direction t qx ( x , z ), spanwise quasi-wall shear t qz( x , z Boundary layer thickness d Dynamic viscosity m Using a pre-defined boundary layer velocity distribution model, the flow velocity near the test wall is obtained through inversion. u ( x , z and spanwise velocity w ( x , z The two-dimensional velocity field;

[0021] The wall normal vorticity calculation module communicates with the two-dimensional velocity field inversion module. It is used to calculate the vorticity based on the distribution of the two-dimensional velocity field on the tangent plane of the wall under test, by performing spatial difference calculation using data from adjacent measurement points. and Furthermore, the normal vorticity field of the wall is obtained. w y ( x , z );

[0022] The results output module communicates with the wall normal vorticity calculation module to combine the wall normal vorticity field obtained at continuous time points. w y ( x , z The transient spatial structure and dynamic evolution of Klebanoff fringes on the tangent plane of the wall under test are output.

[0023] (III) Technical Effects

[0024] Compared with the prior art, the method and system for measuring Klebanoff fringes in the boundary layer provided by the present invention have the following beneficial and significant technical effects:

[0025] (1) This invention solves the problem that traditional unidirectional thermal films cannot simultaneously obtain two-dimensional wall shear information and rotation measurement cannot maintain transient synchronization by simultaneously acquiring the flow direction quasi-wall shear force and span direction quasi-wall shear force in the same measurement area and establishing a two-dimensional distribution matrix. This significantly improves the integrity, synchronization and reliability of the measurement of complex near-wall disturbance structures in aerodynamic experiments.

[0026] (2) This invention combines the quasi-wall shear force measurement results with the boundary layer thickness, dynamic viscosity and reference height, and inverts the two-dimensional velocity field near the wall based on the boundary layer velocity distribution model, and further calculates the wall normal vorticity field, realizing the extension from point sensing response to surface distribution flow information, which is beneficial to quantitatively characterize the transient spatial structure and dynamic evolution process of Klebanoff stripes.

[0027] (3) This invention facilitates the two-dimensional array arrangement of the thermal film sensing parts on a plane or curved surface, enabling a more comprehensive response to flow changes. Utilizing mature boundary layer theory, point measurements are extended to field information. The quasi-wall shear thermal film array can achieve high-resolution measurements with minimal inversion calculations, facilitating transient calculation and monitoring. This overcomes the limitations of optical testing techniques in complex flow channels, enabling measurements under harsh conditions of high temperature and high speed. Attached Figure Description

[0028] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 The diagram shows the implementation flowchart of the method for measuring Klebanoff fringes in the boundary layer provided in this embodiment of the invention.

[0030] Figure 2 The diagram shows the arrangement of the quasi-wall shear thermal film array on the test wall of the experimental specimen, where: (a) is a schematic diagram used in the plate experiment, and (b) is a schematic diagram used in the cascade experiment.

[0031] Figure 3 The diagram shows a quasi-wall shear thermal film array, where: (a) is a planar schematic diagram; (b) is a magnified schematic diagram of a local area A.

[0032] Figure 4 The diagram shows the location of the measuring points of each measuring unit in the quasi-wall shear thermal film array, where (a) is the overall schematic diagram and (b) is a magnified schematic diagram of a local area A.

[0033] Figure 5 The Klebanoff fringe distribution is based on the characterization of wall normal vorticity. In the figure: the horizontal axis X represents the normalized position along the flow direction of the test wall of the test specimen, and the vertical axis Z represents the normalized position along the spanwise direction of the test wall of the test specimen; the solid lines represent the positive wall normal vorticity contour lines, the dashed lines represent the negative wall normal vorticity contour lines, and the blank areas represent areas where no measurement points were set or where effective reconstruction was not performed.

[0034] Explanation of reference numerals in the attached figures: 1-Insulating substrate, 2-Coated lead wire, 3-Thermal film sensing part, x-Flow direction, z-Spread direction, 4~8-Site measurement point position. Detailed Implementation

[0035] This invention aims to provide a method and system for measuring Klebanoff fringes within a boundary layer. 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 and intended to explain the invention, not to limit it. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0036] Example 1: Measurement method of Klebanoff fringes in the boundary layer

[0037] like Figure 1 As illustrated, the method for measuring Klebanoff fringes within the boundary layer provided in this embodiment of the invention is used to measure the transient spatial structure and dynamic evolution of Klebanoff fringes within the boundary layer of the test specimen under wind tunnel, airfoil, or cascade test conditions. Compared with traditional methods relying on single-point velocity measurement or cross-sectional flow field observation, this embodiment combines wall array sensing, bidirectional shear force synchronous acquisition, velocity field inversion, and vorticity reconstruction to transform the originally difficult-to-observe fringe structure into a physical quantity that can be continuously characterized and tracked on the wall tangent plane, thus balancing measurement resolution, measurement continuity, and engineering deployment convenience. Specifically, the measurement method mainly includes the following steps:

[0038] SS1. Experimental Setup and Coordinate Establishment:

[0039] Under aerodynamic experimental conditions, a quasi-wall shear thermal film array consisting of multiple measurement units is laid on the test wall of the experimental specimen. Each measurement unit has two sets of orthogonally arranged thermal film sensing parts, and a flow-direction sensing element is established. x and development direction z An unfolded wall coordinate system.

[0040] Preferably, the quasi-wall shear force hot-film array includes multiple measurement units formed on the same insulating substrate surface and arranged in an array. Each measurement unit contains four electrically independent hot-film sensing parts. Every two hot-film sensing parts with collinear midlines in the longitudinal direction form a group. The two groups of hot-film sensing parts are orthogonally arranged to form a quasi-wall shear force measurement base facing two orthogonal directions in the same local area. After being applied to the test wall surface of the test specimen, the midline in the longitudinal direction is along the spanwise direction. z A set of oriented thermal film sensing elements is used to measure the flow-direction quasi-wall shear force. t qx ( x, z The centerline along the length direction is along the flow direction. x Another set of oriented thermal film sensing elements is used to measure spanwise quasi-wall shear force. t qz ( x , z ).

[0041] Preferably, when the test surface of the experimental specimen is an airfoil surface or a curved surface of a cascade, the flow direction is first established based on the local tangent plane geometry of the test surface. x With the direction of development z The wall coordinate system is then used to map the actual positions of each measurement unit on the curved wall surface to the wall coordinate system. Subsequent steps SS2-SS6 construct a two-dimensional shear force distribution matrix, a two-dimensional velocity field, and a wall normal vorticity field based on the mapped coordinate positions, thereby ensuring the spatial consistency of the measurement results under curved surface conditions. Specifically, the orthogonally arranged hot-film sensing element can be laid on the plate to be measured, such as... Figure 2 As shown in Figure (a), the orthogonal arrangement of the thermal film sensing element can be applied to the blade under test, as shown in Figure (a). Figure 2 As shown in Figure (b). See also the quasi-wall shear thermal film array. Figure 3 As shown, each measurement unit in the array mainly consists of three parts: an insulating substrate 1, four orthogonally arranged thermal film sensing parts 3, and coated leads 2.

[0042] More preferably, the quasi-wall shear thermal film array can be deployed in a zoned manner along the flow direction. Smaller measurement point spacing is set in areas where rapid amplification of Klebanoff fringes or boundary layer transition is expected, while larger measurement point spacing is set in the upstream stable region and the downstream transitioned region. In steps SS5 and SS6, high-resolution normal vorticity reconstruction is performed on key areas based on the zoned deployment results, while also considering the coverage of the entire measurement area and data acquisition efficiency. Specifically, the determination of the expected transition sensitive area can be estimated based on empirical correlations, and the spanwise measurement point spacing Δ within this area is determined. z Δ should be satisfied z ≤ l z,est / 4, of which l z,est The predicted spanwise characteristic wavelength of the fringes (typically 1 to 2 times the boundary layer thickness) d This ensures that the spanwise spatial sampling meets the Nyquist criterion; while in the upstream stable region and the downstream transitioned turbulent region, the spacing between measuring points can be appropriately increased to 3 to 5 times the spacing between compact regions, so as to reasonably control the total number of sensor channels and the scale of the data acquisition system, and achieve the optimal balance between measurement accuracy and engineering feasibility.

[0043] It should be noted that the key to this step lies in integrating the bidirectional quasi-wall shear force measurement function into the same measurement unit on the same insulating substrate. This is achieved through two sets of thermally independent sensing elements with mutually perpendicular and electrically independent observation sensitive directions, enabling the synchronous acquisition of flow direction and spanwise information under co-location conditions. Ideally, each thermally independent sensing element primarily responds to near-wall flow disturbances in its own observation sensitive direction. This avoids the temporal mismatch and spatial deviation introduced by acquiring bidirectional information through rotating probes, multiple deployments, or asynchronous acquisition methods, thus providing a unified and reliable spatial reference basis for subsequent two-dimensional velocity field inversion and wall normal vorticity reconstruction.

[0044] SS2. Simultaneous acquisition of bidirectional quasi-wall shear force:

[0045] Power is supplied to the thermal film sensing element of each measurement unit and the output signal is acquired synchronously. Based on the orthogonal layout of the two sets of thermal film sensing elements in the same measurement unit, the quasi-wall shear force in the flow direction of the wall under test is measured. t qx ( x , z and spanwise quasi-wall shear t qz ( x , z The distribution matrix of ).

[0046] Preferably, the thermal film sensing element of each measurement unit is powered and multi-channel synchronously acquired. The output signals of each measuring point obtained at the same time are converted into the flow direction and spanwise quasi-wall shear force of the corresponding measuring point, and then calculated along the flow direction coordinates. x with span coordinates z Constructing flow-direction quasi-wall shear t qx ( x , z and spanwise quasi-wall shear t qz ( x , z A two-dimensional distribution matrix is ​​used to maintain the consistency of the time base and the one-to-one correspondence of spatial locations during transient reconstruction.

[0047] In addition, before formal data acquisition, a thermal film signal conversion step is included: under the condition of known wall shear force, the flow direction measurement channel and the spanwise measurement channel in each measurement unit are calibrated to establish the conversion relationship between the thermal film output signal and the corresponding quasi-wall shear force. In subsequent experimental measurements, the real-time output of each thermal film sensing part is converted into the flow direction quasi-wall shear force and the spanwise quasi-wall shear force according to the corresponding conversion relationship, so as to improve the quantitative accuracy of the two-dimensional shear field reconstruction and the consistency between different measurement units.

[0048] It should be noted that the key point of this step is to transform the bidirectional sensing capability at the sensor level into bidirectional shear field data that can be directly used in inversion calculations. Compared with existing single-point, unidirectional, or asynchronous acquisition methods, this invention obtains the flow-direction and spanwise quasi-wall shear force distributions within the same region at the same time through multi-channel synchronous acquisition, thereby avoiding transient distortions introduced by timing misalignment, scanning delay, or asynchronous stitching. Especially for flow structures like Klebanoff fringes, which have obvious unsteady characteristics, spanwise undulations, and local enhancement and attenuation processes, synchronous acquisition can ensure that the phase relationship and spatial correlation between different positions of the fringes are not destroyed. On the other hand, through multi-channel synchronous acquisition and pre-conversion of the hot film signal, the hot film output electrical signal can be stably converted into quasi-wall shear force data, reducing dynamic distortions caused by asynchronous acquisition, channel drift, and measurement point discrepancies, and improving the quantification, repeatability, and comparability of experimental results in Klebanoff fringe transient measurements.

[0049] SS3. Boundary layer parameter determination:

[0050] Measuring the boundary layer thickness of the wall surface to be measured d Determine the dynamic viscosity of the fluid under experimental conditions. m and give the reference height of the wall normal. y As a preferred option, dynamic viscosity... m The reference height for the normal direction of the wall is determined based on the fluid temperature under experimental conditions. y Selected within the normal boundary layer of the wall to be measured and satisfying 0 < y ≤ d By increasing the boundary layer thickness d Dynamic viscosity m and reference height y Together, they serve as parameters for velocity field inversion, establishing a quantitative mapping relationship between the measured quasi-wall shear force distribution and the two-dimensional velocity field near the measured wall, and ensuring that the two-dimensional velocity field obtained from the quasi-wall shear force inversion can still reflect the main perturbation characteristics of Klebanoff fringes within the boundary layer. By arranging Pitot tubes or hot-wire probes in the uniform inflow region upstream of the measured wall, the time-averaged flow velocity profile is measured point by point along the wall normal direction. The boundary layer thickness is defined as the distance along the wall normal where the flow time-averaged velocity reaches 99% of the local outflow velocity. d .

[0051] SS4. Two-dimensional velocity field inversion:

[0052] Based on flow-direction quasi-wall shear t qx ( x , z ), spanwise quasi-wall shear t qz ( x ,z Using a pre-defined boundary layer velocity distribution model, a two-dimensional velocity field near the wall under test is obtained through inversion. This two-dimensional velocity field includes flow velocities. u ( x , z and spanwise velocity w ( x , z ) distribution. Preferably, the preset boundary layer velocity distribution model is: , ,in u ( x , z ( ) represents the velocity distribution in the flow direction. w ( x , z ( ) represents the spanwise velocity distribution. t qx ( x , z (This refers to the shear force flowing towards the quasi-wall.) t qz ( x , z (This refers to the shear force on the quasi-wall in the span direction.) d Boundary layer thickness, m For dynamic viscosity, y The reference height is the normal height of the wall, and the two-dimensional velocity field distribution near the wall to be measured is obtained accordingly.

[0053] It should be noted that this step utilizes a boundary layer velocity distribution model to transform two-dimensional velocity information, which is difficult to obtain directly in large near-wall regions, into field results that can be obtained from two-way quasi-wall shear inversion. This processing method preserves the coupled characterization capability of flow-direction and spanwise perturbations, extending the measurement results from the wall response to the interior of the boundary layer. This provides a continuous velocity information basis for the spatial morphology identification, intensity variation analysis, and subsequent vorticity reconstruction of Klebanoff fringes.

[0054] SS5. Determination of normal vorticity of the wall:

[0055] Based on the inverted two-dimensional velocity field, the tangent plane of the wall surface under test. x - z The distribution on the surface is obtained by spatial difference calculation using data from adjacent measuring points. and Then, the normal vorticity field of the wall can be obtained. w y ( x , z This is used as a characterization result for Klebanoff fringes. Preferably, the wall normal vorticity field... w y ( x ,z Cut plane according to the wall surface to be measured x - z The curl of the inner two-dimensional velocity field is obtained by definition and satisfies... ,in and The calculation is performed using the central difference between adjacent measuring points, the lateral difference of boundary measuring points, or interpolation difference, obtained from continuous sampling times. w y ( x , z Temporal reconstruction was performed to obtain the migration, growth, and decay processes of the high and low value regions of wall normal vorticity in the flow and span directions, so as to characterize the transient spatial structure and dynamic evolution of Klebanoff fringes.

[0056] Furthermore, when using the central difference method for spatial difference calculation: let the flow velocities corresponding to the symmetrical measuring points on both sides of the location to be determined be respectively u 1 and u 2, and the spanwise distance between the two and the position to be determined is equal and denoted as Δ. z The spanwise velocities corresponding to the symmetrical measuring points on both sides of the location to be determined are respectively w 1 and w 2, and the flow direction distances between the two locations and the location to be determined are equal and denoted as Δ. x Then there is =( u 2- u 1) / (2Δ z ), =( w 2- w 1) / (2Δ x Furthermore, the wall normal vorticity value at that location is obtained. .

[0057] It should be noted that this step further transforms the two-dimensional velocity field into a wall-normal vorticity field, changing the characterization of Klebanoff fringes from velocity perturbations to curl information, which is more conducive to identifying the fringe intensity and its migration patterns. After obtaining the local velocity gradient through differential differentiation, the enhancement, attenuation, spanwise variation, and local merging characteristics of the fringes can be reconstructed within a unified coordinate framework, thereby improving the resolvability and spatiotemporal evolution characterization of boundary layer transition precursor structures.

[0058] SS6. Measurement Result Output:

[0059] Combined with the wall normal vorticity field obtained at continuous time intervals w y ( x , z The output Klebanoff fringes are tangent to the test surface. x -z The transient spatial structure and dynamic evolution process on the surface.

[0060] As a preferred option, the wall normal vorticity field obtained at consecutive time points is preferred. w y ( x , z The intensity distribution, gradient changes, or sign alternation characteristics along the flow direction are used to identify the initial development region, significant amplification region, and transition completion region of Klebanoff fringes within the boundary layer of the tested wall. When the wall normal vorticity shows a continuous increase along the flow direction accompanied by spanwise undulation and densification, the corresponding region is identified as a sensitive region dominated by Klebanoff fringes, used to output boundary layer evolution characteristic results in aerodynamic experiments. Furthermore, the wall normal vorticity field can be further analyzed... w y ( x , z The spanwise wavelength, fringe amplitude, propagation velocity, and spatial growth rate of Klebanoff fringes are extracted using a two-dimensional velocity field. The spanwise wavelength is determined by the spanwise spacing between adjacent fringe peaks or valleys at the same flow direction location. The fringe amplitude is determined by the vorticity difference. The propagation velocity is determined by the ratio of the displacement of the characteristic fringe position in the flow direction to the time interval at consecutive moments. The spatial growth rate is determined by the relationship between the fringe disturbance intensity at different flow direction locations, so as to achieve quantitative measurement of Klebanoff fringe parameters.

[0061] More preferably, this step can construct the wall normal vorticity field obtained at continuous time intervals into a three-dimensional spatiotemporal data volume, and identify and track Klebanoff fringes according to preset threshold segmentation, fringe centerline extraction and morphological tracking rules. The fringe centerline is determined by the spatial connection result of the local extreme value zone of the wall normal vorticity at the same time. The morphological tracking rules are used to establish the correspondence of the same fringe structure between adjacent time intervals, thereby outputting the generation position, extension direction, spanwise change, merging and splitting and attenuation process of the fringes, realizing the automated measurement of the entire evolution process of Klebanoff fringes.

[0062] It should be noted that this step does not merely output measurement patterns at a single moment, but rather constructs the spatiotemporal evolution results of fringes based on the wall normal vorticity field at continuous moments, thus achieving an improvement from static distribution observation to dynamic process identification. By introducing region identification, parameter extraction, and trajectory tracking, the development stages, propagation characteristics, and morphological change laws of the fringes can be further obtained, providing directly usable data for boundary layer stability analysis, transition determination, and model verification in aerodynamic experiments.

[0063] More specifically, in this embodiment, after applying a quasi-wall shear thermal film array to the tested area, as follows: Figure 4As shown, at position 4, t qx ( x 1, z 1) Obtained at position 5 t qz ( x 1, z 1) Obtained at position 6 t qx ( x 2, z 2), obtained at position 7 t qz ( x 2, z 2), and then through calculation, it can be seen that... t qx ( x 1, z 1) t qx ( x 2, z 2) t qz ( x 1, z 1) t qz ( x 2, z 2) Obtain u 1, u 2, w 1, w 2. Positions between 4 and 8 z The directional spacing between positions 6 and 8 z Equal directional spacing, denoted as Δ z Positions between 5 and 8 x The directional spacing between positions 7 and 8 x Equal directional spacing, denoted as Δ x The eight locations can be obtained by using the central difference method. , Finally, the vorticity at position 8 was obtained.

[0064] As can be seen from the above local calculation process, this invention, by simultaneously acquiring flow-direction and spanwise quasi-wall shear force information at adjacent measuring points and combining a unified velocity inversion model with local spatial difference relationships, can reconstruct the normal vorticity distribution near the wall without intruding into the boundary layer. Compared to schemes that require the arrangement of hot wires, PIV observation windows, or complex optical measurement systems in the boundary layer, this embodiment has lower requirements for experimental space conditions, causes less disturbance to the main flow field, and is more suitable for array deployment under confined flow channels, cascade passages, or curved wall conditions.

[0065] Figure 5 The results of the wall normal vorticity distribution obtained under aerodynamic experimental conditions are shown. In the figure, dashed lines represent negative vorticity, solid lines represent positive vorticity, and Klebanoff fringes are represented by vorticity. This method enables the measurement of Klebanoff fringe development. The X-axis is the horizontal axis, and the Z-axis is the vertical axis. All coordinates have been normalized. The Z-axis range of the measurement area is -0.06 to 0.06. The blank areas in the figure represent unmeasured areas. Figure 5 The wall normal vorticity represents the transient moment. It can be seen that the fringe structure is mainly concentrated in the near-central spanwise region. In the upstream region, the overall amplitude is relatively small. As it develops downstream along the flow direction, the high-value and low-value vorticity regions gradually migrate, stretch, split, and recombine, reflecting the amplification, evolution, and interaction characteristics of Klebanoff fringes during their development. Furthermore, from... Figure 5 Furthermore, a dramatic change in vorticity can be observed after the 60% mark in the X-axis direction, indicating the end of the transition and the formation of the fully turbulent region. Within the range of approximately 80% to 100% in the X-axis direction, the vorticity structure becomes further refined and fragmented, indicating a reduction in the near-wall disturbance scale and an enhancement in local mixing. This verifies that the method of this invention can effectively measure and characterize the entire process of Klebanoff fringes from generation and amplification to fragmentation and evolution.

[0066] Example 2: Measurement System for Klebanoff Stripes in the Boundary Layer

[0067] Based on Embodiment 1 above, Embodiment 2 further provides a measurement system for Klebanoff fringes within the boundary layer to implement the above method. This measurement system is used to measure and output the transient spatial structure and dynamic evolution process of Klebanoff fringes within the boundary layer of the test specimen under wind tunnel, airfoil, or cascade test conditions. It mainly includes the following modules:

[0068] A quasi-wall shear force hot-film array module, applied to the test wall of an experimental specimen in an aerodynamic experiment, consists of multiple measurement units. Each measurement unit has two sets of orthogonally arranged hot-film sensing elements for sensing the quasi-wall shear force in the flow direction and spanwise direction of the test wall. Preferably, each measurement unit in the quasi-wall shear force hot-film array module is formed on the same insulating substrate surface. The two sets of hot-film sensing elements in each measurement unit are perpendicular to each other in the observation sensitive direction and electrically independent of each other, so as to simultaneously obtain the near-wall flow response facing two orthogonal directions in the same local area. When the test wall is an airfoil surface or a cascade curved surface, the hot-film array module is also used to establish a local tangential plane coordinate relationship, so that the actual placement position of each measurement unit on the curved wall can be mapped to a unified flow direction and spanwise coordinate system.

[0069] The data synchronization acquisition module is communicatively connected to the quasi-wall shear force hot-film array module. It is used to power the hot-film sensing part of each measurement unit and synchronously acquire the output signal to obtain the quasi-wall shear force flowing from the wall under test. t qx ( x , z and spanwise quasi-wall shear t qz ( x , z The distribution matrix of the data synchronous acquisition module includes a multi-channel synchronous sampling unit and a signal conversion unit. The former is used to ensure that the output signals of each measuring point are acquired synchronously under a unified time reference at the same time. The latter is used to convert the thermal film output signal into the flow direction quasi-wall shear force and the span direction quasi-wall shear force according to the pre-established conversion relationship, thereby constructing a two-dimensional shear force distribution matrix that corresponds one-to-one with the wall coordinate position.

[0070] The two-dimensional velocity field inversion module communicates with the data synchronization acquisition module to obtain the boundary layer thickness of the wall under test. d Dynamic viscosity of fluid under experimental conditions m and wall normal reference height y And based on the quasi-wall shear force of the flow direction t qx ( x , z ), spanwise quasi-wall shear t qz ( x , z Boundary layer thickness d Dynamic viscosity m Using a pre-defined boundary layer velocity distribution model, the flow velocity near the test wall is obtained through inversion. u ( x , z and spanwise velocity w ( x , z The two-dimensional velocity field.

[0071] The wall normal vorticity calculation module communicates with the two-dimensional velocity field inversion module. It is used to calculate the vorticity based on the distribution of the two-dimensional velocity field on the tangent plane of the wall under test, by performing spatial difference calculation using data from adjacent measurement points. and Furthermore, the normal vorticity field of the wall is obtained. w y ( x , zAs a preferred method, the local velocity gradient is calculated using central difference, one-sided difference, or interpolation difference, and the normal vorticity field of the wall is obtained based on the curl relationship of the two-dimensional velocity field in the tangent plane of the wall, so as to uniformly convert the near-wall perturbation characteristics of Klebanoff fringes into vorticity characterization results that can be continuously reconstructed and compared.

[0072] The results output module communicates with the wall normal vorticity calculation module to combine the wall normal vorticity field obtained at continuous time points. w y ( x , z This module outputs the transient spatial structure and dynamic evolution of Klebanoff fringes on the tangent plane of the wall under test. Furthermore, the output module is used to reconstruct the wall normal vorticity field at consecutive time points and output the spatial distribution results of the initial development region, significant amplification region, and transition completion region of the Klebanoff fringes. It can also further output the spanwise wavelength, amplitude, propagation velocity, spatial growth rate, and the evolution trajectory of the fringe centerline to serve boundary layer stability analysis and transition determination in aerodynamic experiments.

[0073] 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 method for measuring Klebanoff fringes within a boundary layer, characterized in that, At least the following steps are included: SS1. Under aerodynamic experimental conditions, a quasi-wall shear thermal film array consisting of multiple measurement units is laid on the test surface of the experimental specimen. Each measurement unit has two sets of orthogonally arranged thermal film sensing parts, and a flow-direction sensing element is established. x and development direction z An unfolded wall coordinate system; SS2. Power is supplied to the thermal film sensing element of each measurement unit and the output signal is acquired synchronously. Based on the orthogonal layout of the two sets of thermal film sensing elements in the same measurement unit, the quasi-wall shear force in the flow direction of the wall under test is measured. τ qx ( x , z and spanwise quasi-wall shear τ qz ( x , z The distribution matrix of ). SS3. Measure the boundary layer thickness of the wall surface to be measured. δ Determine the dynamic viscosity of the fluid under experimental conditions. μ and give the reference height of the wall normal. y ; SS4. Based on flow-direction quasi-wall shear τ qx ( x , z ), spanwise quasi-wall shear τ qz ( x , z Using a pre-defined boundary layer velocity distribution model, a two-dimensional velocity field near the wall under test is obtained through inversion. This two-dimensional velocity field includes flow velocities. u ( x , z and spanwise velocity w ( x , z )distributed; SS5. Based on the inverted two-dimensional velocity field, a tangent plane is drawn on the wall surface to be measured. x - z The distribution on the surface is obtained by spatial difference calculation using data from adjacent measuring points. and Then, the normal vorticity field of the wall can be obtained. w y ( x , z ), and used it as the characterization result of Klebanoff stripes; SS6. Combining the wall normal vorticity field obtained at continuous time intervals w y ( x , z The output Klebanoff fringes are tangent to the test surface. x - z The transient spatial structure and dynamic evolution process on the surface.

2. The method according to claim 1, characterized in that, In step SS1, the quasi-wall shear force thermal film array includes multiple measurement units formed on the same insulating substrate surface and arranged in an array. Each measurement unit contains four electrically independent thermal film sensing parts. Every two thermal film sensing parts with collinear midlines in the longitudinal direction form a group. The two groups of thermal film sensing parts are orthogonally arranged as a whole, and after being applied to the test wall surface of the test piece, the midlines in the longitudinal direction are aligned along the longitudinal direction. z A set of oriented thermal film sensing elements is used to measure the flow-direction quasi-wall shear force. τ qx ( x , z The centerline along the length direction is along the flow direction. x Another set of oriented thermal film sensing elements is used to measure spanwise quasi-wall shear force. τ qz ( x , z ).

3. The method according to claim 1 or 2, characterized in that, In step SS1, when the test surface of the experimental specimen is an airfoil surface or a curved surface of a cascade, the flow direction is first established based on the local tangent plane geometry of the test surface. x With the direction of development z The wall coordinate system is then used to map the actual positions of each measurement unit on the curved wall to the wall coordinate system. In subsequent steps SS2 to SS6, a two-dimensional shear force distribution matrix, a two-dimensional velocity field, and a wall normal vorticity field are constructed based on the mapped coordinate positions.

4. The method according to claim 1, characterized in that, In step SS2, power is supplied to the thermal film sensing part of each measurement unit and multi-channel synchronous acquisition is performed. The output signals of each measuring point obtained at the same time are converted into the flow direction and spanwise quasi-wall shear force of the corresponding measuring point, and the coordinates are along the flow direction. x with span coordinates z Constructing flow-direction quasi-wall shear τ qx ( x , z and spanwise quasi-wall shear τ qz ( x , z The two-dimensional distribution matrix of ).

5. The method according to claim 1 or 4, characterized in that, Step SS2, before formal data acquisition, also includes a thermal film signal conversion step: under the condition of known wall shear force, the flow direction measurement channel and the spanwise measurement channel in each measurement unit are calibrated respectively, and the conversion relationship between the thermal film output signal and the corresponding quasi-wall shear force is established. In subsequent experimental measurements, the real-time output of each thermal film sensing part is converted into the flow direction quasi-wall shear force and the spanwise quasi-wall shear force according to the corresponding conversion relationship.

6. The method according to claim 1, characterized in that, In step SS3, the dynamic viscosity μ The reference height for the normal direction of the wall is determined based on the fluid temperature under experimental conditions. y Selected within the normal boundary layer of the wall to be measured and satisfying 0 < y ≤ δ By arranging Pitot tubes or hot wire probes in the uniform inflow zone upstream of the wall to be measured, and measuring the time-averaged flow velocity profile point by point along the wall normal direction, the boundary layer thickness is defined as the distance along the wall normal where the flow time-averaged velocity reaches 99% of the local outflow velocity. δ .

7. The method according to claim 1, characterized in that, In step SS4, the preset boundary layer velocity distribution model is: in, u ( x , z ( ) represents the velocity distribution in the flow direction. w ( x , z ( ) represents the spanwise velocity distribution. τ qx ( x , z (This refers to the shear force flowing towards the quasi-wall.) τ qz ( x , z (This refers to the shear force on the quasi-wall in the span direction.) δ Boundary layer thickness, μ For dynamic viscosity, y The reference height is the normal height of the wall, and the two-dimensional velocity field distribution near the wall to be measured is obtained accordingly.

8. The method according to claim 1, characterized in that, In step SS5, the wall normal vorticity field w y ( x , z Cut plane according to the wall surface to be measured x - z The curl of the inner two-dimensional velocity field is obtained by definition and satisfies... ,in and The calculation is performed using the central difference between adjacent measuring points, the lateral difference of boundary measuring points, or interpolation difference, obtained from continuous sampling times. w y ( x , z Temporal reconstruction was performed to obtain the migration, growth, and decay processes of the high and low value regions of wall normal vorticity in the flow and span directions, thus characterizing the transient spatial structure and dynamic evolution of Klebanoff fringes.

9. The method according to claim 1 or 8, characterized in that, In step SS5, the spatial difference calculation is performed using the central difference method: Let the flow velocities corresponding to the symmetrical measuring points on both sides of the location to be determined be respectively... u 1 and u 2, and the spanwise distance between the two and the position to be determined is equal and denoted as Δ. z The spanwise velocities corresponding to the symmetrical measuring points on both sides of the location to be determined are respectively w 1 and w 2, and the flow direction distances between the two locations and the location to be determined are equal and denoted as Δ. x Then there is =( u 2- u 1) / (2Δ z ), =( w 2- w 1) / (2Δ x Furthermore, the wall normal vorticity value at that location is obtained. .

10. The method according to claim 1, characterized in that, In step SS6, the wall normal vorticity field obtained at consecutive time points is... w y ( x , z The intensity distribution, gradient changes, or sign alternation characteristics along the flow direction are used to identify the initial development zone, significant amplification zone, and transition completion zone of Klebanoff stripes in the boundary layer of the wall under test. When the wall normal vorticity shows continuous enhancement along the flow direction and is accompanied by spanwise undulation and densification, the corresponding area is determined to be a sensitive area dominated by the development of Klebanoff stripes.

11. The method according to claim 1 or 10, characterized in that, In step SS6, the spanwise wavelength, fringe amplitude, propagation velocity, and spatial growth rate of Klebanoff fringes are extracted based on the wall normal vorticity field and the two-dimensional velocity field. The spanwise wavelength is determined based on the spanwise spacing between the peaks or valleys of adjacent fringes at the same flow direction position. The fringe amplitude is determined based on the vorticity difference. The propagation velocity is determined based on the ratio of the displacement of the characteristic position of the fringe in the flow direction to the time interval at consecutive moments. The spatial growth rate is determined based on the relationship between the fringe disturbance intensity at different flow direction positions.

12. A measurement system for Klebanoff fringes within a boundary layer, used to implement the measurement method for Klebanoff fringes within a boundary layer as described in any one of claims 1 to 11, characterized in that, At least including: The quasi-wall shear force hot film array module is applied to the test wall of the test specimen in aerodynamic experiments. It consists of multiple measurement units, each of which has two sets of orthogonally arranged hot film sensing parts. The data synchronization acquisition module is communicatively connected to the quasi-wall shear force hot-film array module. It is used to power the hot-film sensing part of each measurement unit and synchronously acquire the output signal to obtain the quasi-wall shear force flowing from the wall under test. τ qx ( x , z and spanwise quasi-wall shear τ qz ( x , z The distribution matrix of ). The two-dimensional velocity field inversion module communicates with the data synchronization acquisition module to obtain the boundary layer thickness of the wall under test. δ Dynamic viscosity of fluid under experimental conditions μ and wall normal reference height y and based on τ qx ( x , z ), τ qz ( x , z ), δ , μ Using a pre-defined boundary layer velocity distribution model, the flow velocity near the test wall is obtained through inversion. u ( x , z and spanwise velocity w ( x , z The two-dimensional velocity field; The wall normal vorticity calculation module communicates with the two-dimensional velocity field inversion module. It is used to calculate the vorticity based on the distribution of the two-dimensional velocity field on the tangent plane of the wall under test, by performing spatial difference calculation using data from adjacent measurement points. and Furthermore, the normal vorticity field of the wall is obtained. w y ( x , z ); The results output module communicates with the wall normal vorticity calculation module to combine the wall normal vorticity field obtained at continuous time points. w y ( x , z The transient spatial structure and dynamic evolution of Klebanoff fringes on the tangent plane of the wall under test are output.

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