A method for measuring the fine structure of the turbulent boundary layer flow field on the surface of a wind tunnel model

Abstract

This invention discloses a method for measuring the fine structure of the turbulent boundary layer flow field on the surface of a wind tunnel model. It addresses the inefficiency of existing hot-wire anemometer measurement methods by installing a matching measurement device on or on both sides of the wind tunnel model, which helps the hot-wire anemometer to perform high-precision measurement of the fine structure of the turbulent boundary layer on the complex model surface. At the same time, it significantly improves the level of mobile automation, thereby effectively improving the measurement efficiency.

Description

Technical Field

[0001] This invention relates to the field of wind tunnel testing, and more specifically to a method for measuring the fine structure of the turbulent boundary layer flow field on the surface of a wind tunnel model for flow field measurement. Background Technology

[0002] Turbulent boundary layers are a major concern in fluid mechanics, influencing: 1. Surface friction drag, affecting the aerodynamic characteristics of high-speed vehicles; 2. Surface pulsating pressure, a significant source of noise in aircraft, high-speed trains, and submarines due to vibrations in their hull surfaces; 3. Turbulent separation, leading to stall and instability, and potentially causing aircraft accidents. Therefore, research on turbulent boundary layers has significant academic and engineering value. The fine structure of the turbulent boundary layer flow field is a crucial characteristic determining its evolution and is a focal point in turbulent boundary layer research. This fine structure contains numerous complex unsteady velocity pulsations, making accurate acquisition of these signals key to high-level research. Wind tunnel testing and high-precision numerical simulation are the primary methods for obtaining the fine structure of the turbulent boundary layer flow field, with wind tunnel testing gaining high industry recognition due to its realism and reliability.

[0003] Wind tunnel experiments on turbulent boundary layers primarily utilize methods such as PIV (Particle Image Velocimetry), seven-hole probes, and hot-wire anemometers. PIV measures velocity by using lasers to track the trajectories of tracer particles scattered in the flow field, enabling the study of wide-area flow fields. However, due to the boundary layer's proximity to the wall and its thinness, the tracer particles are difficult to disperse uniformly, and the wall reflects the laser, causing interference. This presents challenges for boundary layer measurements. The seven-hole probe is a high-performance, high-precision flow field measurement instrument. By measuring the pressure values ​​at seven holes and using a calibrated database and local least squares interpolation, it can accurately determine the magnitude and direction of velocity, total and static pressure, Mach number, Reynolds number, and flow field properties such as density and viscosity. However, the seven-hole probe is too coarse to penetrate the thin boundary layer and can only measure time-averaged flow fields, failing to measure the unsteady signals of velocity fluctuations, which are crucial in turbulent boundary layer flows. Therefore, its application is limited. The basic principle of hot-wire anemometers is based on establishing a one-to-one correspondence between the current in the hot wire of the probe and the fluid velocity, thus measuring the flow velocity by measuring the resistance of the current. Due to its small probe, hot-wire anemometers cause minimal interference to the measured flow field, offer high spatial resolution, and have a short response time, effectively capturing unsteady signals of velocity fluctuations. This makes them the ideal means of measuring the fine structure of turbulent boundary layer flow fields. However, the most critical problem with current hot-wire anemometer-based measurement methods is that they can only perform single-point measurements, resulting in low efficiency. However, practical engineering applications urgently require research on wide-area flow fields. Therefore, establishing a highly efficient measurement method based on hot-wire anemometers specifically for the fine structure of turbulent boundary layer flow fields would significantly improve experimental research capabilities and efficiency. Summary of the Invention

[0004] The purpose of this invention is to address the inefficiency of existing hot-wire anemometer measurement methods. By installing a matching measurement device on or on both sides of a wind tunnel model, the hot-wire anemometer can perform high-precision measurements of the fine structure of the turbulent boundary layer on the surface of complex models, while significantly improving the level of mobile automation, thereby effectively improving measurement efficiency.

[0005] The technical solution adopted in this invention is a method for measuring the fine structure of the turbulent boundary layer flow field on the surface of a wind tunnel model, comprising:

[0006] The installation method of the matching bridge-type measuring device is determined based on the scale of the wind tunnel model, specifically including two methods: installation on the model surface or installation on both sides of the model. If the model scale is large, the surface installation method must be used, and mounting holes for the measuring device track must be machined on the model surface; if the model scale is small, the side-mounting method must be used, and profile frames must be installed on both sides of the model to support the measuring device.

[0007] Assemble a matching bridge-type measurement device. This device controls the movement of the hot-wire anemometer probe, enabling the measurement of the fine structure of the turbulent boundary layer flow field using a hot-wire anemometer. The device mainly consists of five parts: a base track, a bridge-type mechanical structure, a hot-wire clamp, a three-degree-of-freedom control system, and supporting operating software. This device can manipulate the hot-wire anemometer probe to perform fine-structure measurements of the turbulent boundary layer, i.e., the pulsating velocity components. Remote monitoring is achieved using the accompanying host computer software, allowing for high-precision individual or coordinated movement in the X, Y, and Z directions. The bridge-type measurement device includes multiple matching Z-axis rectifying bases of varying lengths. When assembling this bridge mechanism, the required length must be selected based on the model and measurement needs. If specific length requirements exist, new Z-axis rectifying bases can be fabricated.

[0008] Install the bridge-type measuring device and build the wind tunnel test platform. Install the assembled bridge-type measuring device and test model together on the wind tunnel test platform. Install the hot-wire anemometer probe on the bridge-type mechanism fixture and conduct joint test blowing and debugging.

[0009] Fine-scale structural measurements of the turbulent boundary layer flow field in the model were conducted. By rationally designing the path of the bridge-type measurement device, precise measurements of the turbulent boundary layer flow field in the model were achieved. The movement path of the bridge-type measurement device was controlled by inputting commands from the host computer. Once the probe, controlled by the bridge-type measurement device, reached each measurement point, it stopped moving for a certain period, at which point the hot-wire anemometer sampling function automatically activated to complete the sampling of pulsating velocity quantities.

[0010] Data processing was performed to obtain the fine-structure fluctuating velocity characteristics of the turbulent boundary layer flow field. These characteristics include the turbulence intensity and self-power spectrum at each point of the fluctuating velocity, the cross-power spectrum and cross-correlation coefficient at multiple points, and the wavenumber-frequency spectrum within the measurement range. Specifically:

[0011] Let the pulsation velocity signal measured by the hot wire probe be... ,in:

[0012] Methods for calculating turbulence intensity at various points

[0013] The mean value of the signals at each measuring point can be used to measure the magnitude of the velocity at the measured point, and its value is defined as:

[0014]

[0015] Standard deviation is a variable used to measure the magnitude of deviation of a pulsating velocity sample from the mean, and is defined as:

[0016]

[0017] The turbulence intensity at each point measured by the hot wire probe is:

[0018]

[0019] Methods for calculating cross-correlation function and cross-power spectral density at various points, and autocorrelation function and auto-power spectral density at a single point.

[0020] Suppose there are two points, whose spatial locations are respectively , The difference between the positions of two points is given by the difference between the positions of these two points at different times t and t. The time-space correlation function between them can be expressed as:

[0021]

[0022] Here, "<>" represents the ensemble mean, which can also be represented by "". The ensemble mean is the statistical expectation of the physical quantity. In a stationary process, the ensemble mean of a random quantity is equal to the time average of the random process; this property is known as the ergodicity of the random process. Therefore, using time-domain signals... x(t) For example, that is:

[0023]

[0024] right Performing a Fourier transform in the time domain yields the cross spectrum of the velocity fluctuations at the two points:

[0025]

[0026] When the two points coincide The cross spectrum is then transformed into the autospectrum of the velocity fluctuation at that point, i.e., . Integrating the autospectrum in the angular frequency domain yields the mean square value of the velocity fluctuation:

[0027]

[0028] Space wavenumber-frequency spectrum calculation method

[0029] Compared to cross-spectrum and auto-spectrum, wavenumber-frequency spectrum is a representation of the spectrum in both time and space. The generalized Fourier transform is specifically represented as follows:

[0030]

[0031] Among them n represent r dimensionality k wavenumber vector ( k 1 , k 2 , k 3 ),in, k 1 For flow direction, k 2 For the direction of development,k 3 In the height direction and with k 1 , k 3 The directions are orthogonal (i.e., normal).

[0032] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0033] This method overcomes the problems of complexity, inefficiency, and high time consumption in the use of traditional hot-wire anemometers. By using a specialized mechanical structure, it significantly improves the measurement efficiency of the turbulent boundary layer flow field on the model surface in the wind tunnel. With the help of supporting data processing methods, it can obtain flow field velocity characteristics, including cross spectrum and wavenumber-frequency spectrum, and establish a mathematical description of the spatial distribution structure of the measured flow field, thereby realizing the fine structure measurement of the turbulent boundary layer flow field. Attached Figure Description

[0034] The present invention will be described by way of example and with reference to the accompanying drawings, wherein:

[0035] Figure 1 This is a schematic diagram of the measuring device;

[0036] Figure 2 The wavenumber-frequency spectrum measurement results of the turbulent boundary layer velocity field on the surface of an airfoil model obtained using the measurement method of this invention are as follows: 1 is the model, 2 is the base track, 3 is the X-axis drive motor, 4 is the Z-axis drive motor, 5 is the Z-axis base, 6 is the hot wire clamp, 7 is the Z-axis guide rail, 8 is the probe, 9 is the support rod, 10 is the Y-axis guide rail, 11 is the Y-axis base, and 12 is the Y-axis drive motor. Detailed Implementation

[0037] All features disclosed in this specification, or all steps in all disclosed methods or processes, may be combined in any way, except for mutually exclusive features and / or steps.

[0038] Any feature disclosed in this specification (including any appended claims, abstract, and drawings) may be replaced by other equivalent or similar features for a similar purpose, unless specifically stated otherwise. That is, unless specifically stated otherwise, each feature is merely one example of a series of equivalent or similar features.

[0039] like Figure 1 As shown, to implement the measuring device required in this embodiment, the structure of the measuring device includes:

[0040] The base track 2 consists of two parallel tracks. The base guide rail is an integrated combination of guide rail, lead screw, and motor, driven by the X-axis drive motor 3, enabling movement in the X direction. The two parallel guide rails have no fixed connection device, and their installation position can be adjusted according to actual needs or model size. When model 1 is large, it can be directly fixed to the surface of model 1; when model 1 is small, they can be distributed on both sides of the model, with the spacing adjusted according to the model boundary layer measurement requirements. The length of the base guide rails can also be changed as needed.

[0041] The bridge-type mechanical structure consists of two Y-axis guide rails 10 and one Z-axis guide rail 7. An angle-controllable connecting device is installed on the Z-axis guide rail 7, allowing the mounting of the probe 8. During operation, the Y-axis guide rail 10 is driven by a Y-axis drive motor 12, causing the Z-axis base 5 to move in the Y-axis direction. The Z-axis drive motor 4 of the Z-axis guide rail 7 moves the hot-wire probe holder and probe 8 in the Z-axis direction. The hot-wire probe holder can be manually adjusted within a 0-45° angle range to enable measurement of the flow field boundary layer at various points on the curved surface. The Y-axis guide rails 10 and Z-axis guide rail 7 are interchangeable, and both can be replaced with integrated guide rails of different sizes according to measurement requirements. The control system comprehensively controls the base rail 2, the Y-axis base 11, and the Z-axis base 5 to achieve measurement of the flow field boundary layer at any point on the model surface.

[0042] The integrated guide rail, consisting of a motor, rectifier base, lead screw, and guide rail, is a single unit that enables modularization and miniaturization of the guide rail, thereby reducing its impact on the flow field and facilitating installation. The motor is an AC synchronous servo motor with natural cooling, a brake alarm function, an integrated multi-turn absolute encoder, and is driven by a matching driver.

[0043] The hot-wire clamp 6 is used to fix the hot-wire anemometer probe support rod 9. The distal end of the support rod 9 is connected to the probe 8 and is used to measure the structure within the turbulent boundary layer; the proximal end of the support rod 9 is fixed by the clamp, and the measurement position is controlled by this invention. The hot-wire clamp has a self-adjusting angle disk structure, which can control the rod angle according to the needs of boundary layer measurement and the hot-wire probe structure.

[0044] The measuring device performs positioning motion control and velocity motion control in the X, Y, and Z directions, controls the probe to collect data on the boundary layer of the flow field, and then performs analysis and processing.

[0045] Taking the surface velocity field measurement of an airfoil model as an example:

[0046] Because the test model is small, the base guide rails are distributed on both sides of the model surface, and the entire test model is set between the two Y guide rails of the measuring device.

[0047] The control bridge measuring device controls the hot wire anemometer probe to move to each measuring point. After the hot wire anemometer probe stops moving for a certain period of time, the hot wire anemometer sampling function is automatically activated to complete the sampling of the pulsating velocity.

[0048] Data processing was performed to obtain the fine structure and fluctuating velocity characteristics of the turbulent boundary layer flow field:

[0049] The pulse velocity signal measured by the hot wire probe is set as follows: The mean value of the signal at each measuring point is The standard deviation is The turbulence intensity at each point measured by the hot wire probe for:

[0050] ,

[0051] S32: Set the positions of two points in space as follows , The difference between the positions of two points at different times. and Cross spectrum of velocity fluctuations between two points:

[0052] ,

[0053] When two points coincide, that is The cross spectrum is transformed into the self spectrum of the velocity pulsation at that point;

[0054] S33: In terms of time and space Performing a generalized Fourier transform yields the wavenumber-frequency spectrum of the velocity field in the turbulent boundary layer, which is expressed as follows:

[0055] ,

[0056] In the above expression: time-space correlation function ,

[0057] <> represents the ensemble mean, n is the dimension of the point, k is the wavenumber vector, N is the data sample size, and i is the sample index. The velocity signal was measured during sample indexing. for The conjugate of complex numbers, ω is the angular frequency.

[0058] Specifically, the hot-wire anemometer at each point collects data at a frequency of 25.6 kHz for 20 seconds. After completing a measurement at one point, the mechanism automatically moves the hot-wire anemometer probe to the next measurement point according to the designed path after 5 seconds. After another 5 seconds of stabilization, the measurement at that point is then performed, completing data collection at a total of 32 points. From the start of the invention's control to the completion of all point measurements, the entire process takes approximately 17 minutes. Using the calculation method of this invention, the cross-spectrum of any two points can be obtained, and the spatial wavenumber-frequency spectrum can be calculated. Figure 2 This displays the wavenumber-frequency spectrum calculation results at a certain wind speed.

[0059] This invention is not limited to the specific embodiments described above. The invention extends to any new feature or combination disclosed in this specification, as well as any new method or process step or combination disclosed herein.

Claims

1. A method for measuring the fine structure of the turbulent boundary layer flow field on the surface of a wind tunnel model, characterized in that... Includes the following steps: S1: Install a bridge-type measuring device in the measurement area of ​​the test model to control the movement of the hot-wire anemometer probe in the flow field; S2: The bridge-type measuring device controls the hot-wire anemometer probe to move to each measuring point. After the hot-wire anemometer probe stops moving for a certain period of time, the hot-wire anemometer sampling function is automatically turned on to complete the sampling of the pulsating velocity. S3: Conduct data processing to obtain the fine structure and fluctuating velocity characteristics of the turbulent boundary layer flow field, including: S31: Set the pulsation velocity signal measured by the hot wire probe to be... The mean value of the signal at each measuring point is The standard deviation is The turbulence intensity at each point measured by the hot wire probe for: ， S32: Set the positions of two points in space as follows , The difference between the positions of two points at different times. and Cross spectrum of velocity fluctuations between two points: , When two points coincide, that is The cross spectrum is transformed into the self spectrum of the velocity pulsation at that point; S33: In terms of time and space Performing a generalized Fourier transform yields the wavenumber-frequency spectrum of the velocity field in the turbulent boundary layer, which is as follows: , In the above expression: time-space correlation function , <> represents the ensemble mean, n is the dimension of the point, k is the wavenumber vector, N is the data sample size, and i is the sample index. The velocity signal was measured during sample indexing. for The conjugate of complex numbers, ω is the angular frequency.

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