A method of measuring skin friction of a gas flow

Abstract

The application relates to a method for measuring airflow wall surface friction, relates to the field of wind tunnels, and aims to solve the problem that shear stress is difficult to be dynamically measured with high precision in wind tunnel experiments, and comprises the following steps: S1, an initialization stage, silicon oil is sprayed or dropped on the model surface measuring point position and spread into an oil film layer, and a multichannel spectral confocal sensor array is arranged at the model surface measuring point position, the focusing points of the channels are arranged in the focal plane according to geometric center symmetry, and the spacing between the focusing points is less than or equal to 2 mm; S2, a test stage, the wind tunnel is started, the wind speed is stabilized to reach the test working condition, spectral data collected by the multichannel spectral confocal sensor are collected, and the collection frequency is greater than or equal to 10 Hz; S3, data processing, the spectral data are converted into oil film thickness values, and the oil film thickness derivative is calculated through a difference method; and S4, according to the oil film thickness derivative, shear stress components are calculated through an oil film equation. The application has a good application prospect in the fields of fluid mechanics testing, precise optical measurement and the like.

Description

Technical Field

[0001] This invention relates to the field of wind tunnels, and more specifically to a method for measuring airflow wall friction. Background Technology

[0002] Accurate measurement of wall friction / shear stress in aerodynamic experiments has always been a challenge, and there is currently no universally applicable measurement method. Commonly used methods include friction balances, Preston tubes, and oil film interferometry, but all have limitations. Friction balances are bulky and can only be installed on planar models; the influence of pressure gradients is difficult to eliminate. Preston tubes rely on the assumption of local flow field similarity and require pre-calibration. They are typically used for friction measurements in high Reynolds number turbulent boundary layers where the incoming flow direction is known; pressure gradients and flow separation significantly affect measurement accuracy. The oil film method derives shear force from the thickness of a thin oil film based on viscous force balance. However, interferometric measurements of oil film thickness have limited range; unless the local oil film thickness is very thin, only the thickness difference can be obtained, not the absolute value of the oil film thickness. Summary of the Invention

[0003] To address the challenge of accurately and dynamically measuring shear stress in wind tunnel experiments, this invention provides a method for measuring airflow wall friction, comprising:

[0004] S1. Initialization stage: Spray or drip silicone oil onto the measuring points on the model surface and spread it into an oil film layer. Arrange a multi-channel spectral confocal sensor array at the measuring points on the model surface. The focal points of each channel are symmetrically arranged in the focal plane according to the geometric center, and the spacing between the focal points is ≤2mm.

[0005] S2. During the test phase, the wind tunnel is started. After the wind speed stabilizes and reaches the test conditions, spectral data is collected through a multi-channel spectral confocal sensor at a frequency ≥10Hz.

[0006] S3. Data processing: Convert the spectral data into oil film thickness values ​​and calculate the derivative of the oil film thickness using the finite difference method;

[0007] S4. Calculate the shear stress components using the oil film equation based on the derivative of the oil film thickness.

[0008] Furthermore, S3 specifically includes:

[0009] S31. Thickness preprocessing: The actual thickness of the oil film is obtained. The formula for converting spectral data into oil film thickness values ​​is as follows:

[0010]

[0011] in, This is the actual thickness of the oil film. These are the spectral peaks corresponding to the upper and lower interfaces of the oil film. The corresponding thickness is given for the spectral confocal sensor. The refractive index of oil;

[0012] S32. The formula for calculating the oil film thickness at the measuring point and its derivative with respect to time and direction using the finite difference method, when employing a three-channel spectral focusing sensor, is as follows:

[0013]

[0014]

[0015]

[0016]

[0017] When using a four-channel spectral focusing sensor, the formula is:

[0018]

[0019]

[0020]

[0021]

[0022] in, This represents the oil film thickness at the measuring point. These are the oil film thickness values ​​measured in each channel. The geometric distance from the focal point to the center of the nominal measuring point. The time sampling interval is... For the current moment, Let be the rate of spatial variation of the oil film thickness in the x-direction. Let be the rate of change of oil film thickness in the y-direction. This is the time derivative.

[0023] Furthermore, the specific formula for calculating the shear stress components using the oil film equation is as follows:

[0024]

[0025] Or written in fractional form:

[0026]

[0027]

[0028] in, For shear stress, The spatial gradient of oil film thickness. For shear stress in the x-direction, The shear stress is in the y-direction. is the kinetic viscosity coefficient of the oil.

[0029] The beneficial effects of this invention are:

[0030] A method for measuring oil film thickness using spectral confocal measurement was proposed, which overcomes the limitation of interferometric measurement, which can generally only obtain the thickness difference but cannot accurately obtain the absolute value of the thickness, and greatly expands the application scope of oil film friction measurement technology. Attached Figure Description

[0031] Figure 1 This is a flowchart of a method for measuring airflow wall friction;

[0032] Figure 2 This is a schematic diagram of the optical system of a multi-channel spectral confocal sensor.

[0033] Figure 3 This is a sensor layout diagram for a non-transparent material model.

[0034] Figure 4 This is a diagram showing the arrangement of sensors on the flow field side of a transparent material model.

[0035] Figure 5 This is a diagram showing the arrangement of sensors on the side of a transparent material model.

[0036] Figure 6 This is a schematic diagram showing the position of the focal point of a four-channel spectral focusing sensor.

[0037] Figure 7 This is a schematic diagram showing the location of the focal point of a three-channel spectral focusing sensor.

[0038] In the figure, 1-first fiber optic end, 2-second fiber optic end, 3-third fiber optic end, 4-fourth fiber optic end, 5-transmitter head cross-section, 6-white light source, 7-spectrometer, 8-transmitter head, 9-focusing cross-section, 11-airflow, 12-spectral confocal sensor, 13-oil film, 14-non-transparent medium, 10-fiber optic coupler, 15-transparent medium. Detailed Implementation

[0039] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0040] Example 1, combined with Figure 1 This embodiment describes a method for measuring airflow wall friction, comprising:

[0041] S1. Initialization stage: Spray or drip silicone oil onto the measuring points on the model surface and spread it into an oil film layer. Arrange a multi-channel spectral confocal sensor array at the measuring points on the model surface. The focal points of each channel are symmetrically arranged in the focal plane according to the geometric center, and the spacing between the focal points is ≤2mm.

[0042] S2. During the test phase, the wind tunnel is started. After the wind speed stabilizes and reaches the test conditions, spectral data is collected through a multi-channel spectral confocal sensor at a frequency ≥10Hz.

[0043] S3. Data processing: Convert the spectral data into oil film thickness values ​​and calculate the derivative of the oil film thickness using the finite difference method;

[0044] S4. Calculate the shear stress components using the oil film equation based on the derivative of the oil film thickness.

[0045] Specifically, a multi-channel spectral confocal sensor consists of multiple independent channels arranged side-by-side or relative to each other. This arrangement ensures that the sensor's focal point is positioned within a customized location in the region adjacent to the shear stress measurement point on the focal plane, typically within 2mm intervals, with 0.5mm-1.0mm recommended. When using three or more channels, an equilateral triangle or square arrangement is recommended. Figure 2 As shown, and These represent the minimum and maximum values ​​of the sensor's operating wavelength range, respectively. Their corresponding focal positions define the sensor's axial measurement range. A square four-channel arrangement is used, with the white light source coupled to the fiber optic spectrometer of each channel. The fiber optic transmitter terminals are integrated into a common transmitter head, with the fiber optic terminals evenly spaced relative to each other's centers, and a fixed small displacement d relative to the optical axis. The focal point offset D from the optical axis satisfies:

[0046]

[0047] in This is the focal length of the front lens group. This refers to the focal length of the rear lens group. The oil film thickness measurement accuracy of each channel of the spectral confocal sensor is better than 0.1. When using a non-transparent substrate, the measurement range is greater than the oil film thickness, typically greater than 100. When using a transparent medium substrate, the total range must be greater than the sum of the medium thickness and the oil film thickness, and to ensure accuracy, it is usually no more than 5mm.

[0048] The data acquisition system used in this invention has the ability to acquire spectral data simultaneously from multiple channels, with an acquisition frequency higher than 10 Hz.

[0049] The oil film raw material used in this invention should have the physical properties of being transparent, not easily volatilized in an airflow environment, having high viscosity, and being non-polluting. It is recommended to use silicone oil with a viscosity coefficient of 200 cSt or higher.

[0050] Figure 3 , 4 Section 5 illustrates a typical sensor installation arrangement during testing. When the model surface material is a non-transparent medium such as metal, plastic, or coating, the sensor is installed outside the model, and the oil film thickness is the distance from the focal point of the liquid surface. Distance from the focal point of the solid surface The difference. When the model surface material is a transparent medium such as glass, the sensor is installed inside or outside the model. Because the refractive indices of the medium and oil are similar, there is only weak reflection at the oil-medium interface. The strong reflection detected by the sensor occurs at the gas-oil interface, corresponding to the center wavelength. and the gas-transparent medium interface, corresponding to the center wavelength. The oil film thickness is the sensor-measured thickness value minus the medium thickness, i.e.:

[0051]

[0052] Where H is the thickness of the medium.

[0053] S3 specifically includes:

[0054] S31. Thickness preprocessing: The actual thickness of the oil film is obtained. The formula for converting spectral data into oil film thickness values ​​is as follows:

[0055]

[0056] in, This is the actual thickness of the oil film. These are the spectral peaks corresponding to the upper and lower interfaces of the oil film. The corresponding thickness is given for the spectral confocal sensor. The refractive index of oil;

[0057] S32. The formula for calculating the oil film thickness at the measuring point and its derivative with respect to time and direction using the finite difference method, when employing a three-channel spectral focusing sensor, is as follows:

[0058]

[0059]

[0060]

[0061]

[0062] When using a four-channel spectral focusing sensor, the formula is:

[0063]

[0064]

[0065]

[0066]

[0067] in, This represents the oil film thickness at the measuring point. These are the oil film thickness values ​​measured in each channel. The geometric distance from the focal point to the center of the nominal measuring point. The time sampling interval is... For the current moment, Let be the rate of spatial variation of the oil film thickness in the x-direction. Let be the rate of change of oil film thickness in the y-direction. This is the time derivative.

[0068] Specifically, such as Figure 6 and Figure 7 As shown, when a three-channel arrangement is used, the focal point is located at the three vertices of an equilateral triangle, and the nominal measuring point is located at the center of the triangle.

[0069] A four-channel array is used, with the focal points symmetrically located at the four vertices of a square, and the nominal measurement point located at the center of the square. The four measuring points are labeled as shown in the figure.

[0070] The specific formula for calculating the shear stress components using the oil film equation is as follows:

[0071]

[0072] Or written in fractional form:

[0073]

[0074]

[0075] in, For shear stress, The spatial gradient of oil film thickness. For shear stress in the x-direction, The shear stress is in the y-direction. is the kinetic viscosity coefficient of the oil.

Claims

1. A method of measuring wall friction of a gas flow, characterized by, Comprise: S1. initialization stage, spray or drop silicon oil on the model surface measurement point position, and spread into an oil film layer, arrange a multi-channel spectral confocal sensor array at the model surface measurement point position, the focusing points of each channel are arranged in the focal plane according to the geometric center symmetry, and the focusing point spacing is ≤2mm; S2. Test stage, start the wind tunnel, and after the wind speed is stable to reach the test working condition, collect the spectral data collected by the multi-channel spectral confocal sensor, and the collection frequency is ≥10Hz; S3. Data processing, convert the spectral data into oil film thickness value, and calculate the oil film thickness derivative by difference method; S4. According to the oil film thickness derivative, calculate the shear stress component through the oil film equation; S3 specifically comprises: S31. Thickness pretreatment, obtain the actual thickness of the oil film, and the formula for converting the spectral data into the oil film thickness value is: ； wherein, is the actual thickness of the oil film, is the spectral peak corresponding to the upper and lower interface of the oil film, is the corresponding thickness given by the spectral confocal sensor, is the refractive index of the oil; S32. Calculate the oil film thickness and its derivative with time and direction change at the measurement point by difference method, and when a three-channel spectral focusing sensor is used, the formula is: ； ； ； ； When a four-channel spectral focusing sensor is used, the formula is: ； ； wherein, the value, is the oil film thickness value measured for each channel, is the geometric distance of the focal point to the center of the nominal measuring point, is the time sampling interval, is the current time, is the spatial rate of change of the oil film thickness in the x direction, is the spatial rate of change of the oil film thickness in the y direction, is the time derivative; The specific formula for calculating the shear stress component through the oil film equation is: ； Or written in component form: ； ； wherein is the shear stress, is the spatial gradient of is the shear stress in the x-direction, is the shear stress in the y-direction, is the dynamic viscosity coefficient of the oil.

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