CN116399497A - Train surface shear stress-oriented measuring method and calibration test bed thereof - Google Patents
Train surface shear stress-oriented measuring method and calibration test bed thereof Download PDFInfo
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Abstract
The invention discloses a method for measuring shear stress of a train surface, which comprises the following steps: preparing a film sensor based on the estimated shear modulus required by the surface of the train to be tested; the membrane sensor is arranged on a static calibration test bed, and the surface pixel and the actual displacement are calibrated; sequentially rotating a calibration experiment table to perform load increase and load reduction tests, sequentially photographing, and constructing a relationship between shear strain and shear stress through PIV cross-correlation analysis to obtain the shear modulus of the sensor; setting an auxiliary light source, and capturing the particle position on the film sensor through the PIV camera in a static state; starting a wind tunnel to perform a test, and capturing the positions of particles on the film sensor through the PIV camera; and calculating the particle displacement on the film sensor through the PIV post-processing system, and obtaining the shear stress distribution of the surface through the image processing of the PIV post-processing system. The invention can flexibly, simply and high-resolution test the shear force distribution on the surface of the high-speed train.
Description
Technical Field
The invention relates to the technical field of surface shear stress/friction resistance measurement and research, in particular to a train surface shear stress-oriented measurement method.
Background
Along with the continuous lifting of the high-speed train speed in China, the interaction between the train and the air is aggravated, the ratio of aerodynamic resistance to total resistance of the train is continuously lifted, wherein the friction resistance to total resistance is up to 35%, and the friction resistance is mainly caused by the shearing stress of the wall surface of the train and the air. The high-speed train is an elongated ground-attached running object, so that the front and rear shearing force of the whole train are large in difference, and the friction resistance distribution difference is obvious. However, up to now, there is still no effective test means for the distribution of friction resistance of a high-speed train, so that the related research on friction resistance control cannot be effectively quantified, and further research on optimization design of drag reduction control is limited.
Current general shear stress testing methods include shear stress suspension platforms, cantilever beams, and thermal membrane methods. Among them, the suspension platform and cantilever beam technology generally requires expensive instruments and complex data processing methods, and has high requirements on experimental environment and test parameters. The thermal film method is a relatively simple and easy-to-use technology, and the principle is that the thermal conduction sensed by a thermal film sensor on a wall surface is utilized to measure the surface shear stress, but the sensitivity of the thermal film method is limited by experimental conditions, and the measurement result is easily influenced by the change of the ambient temperature and the flow velocity, so that the data precision is influenced to a certain extent.
The measuring range of the existing test method is difficult to adjust once being determined, so that the precision is reduced and the adaptability is difficult to be presented when the shearing force range is large; meanwhile, the scattered points are distributed in measurement, the resolution ratio is low, and the accuracy of obtaining the shear force distribution rule for measurement is low. Some testing techniques also require complex preparation and adjustment of the test equipment and test subjects, such that the testing process may be somewhat complex and uncertain.
For a high-speed train, the vehicle body is long, the development change of a boundary layer is large, the range of shearing force values is wide, the flexibility requirement on a testing method is high, and the curvature requirement testing technology of the vehicle body appearance has good fit.
It is common practice for a membrane sensor to obtain shear strain by particle displacement, but how to simply and effectively provide a shear force value and effectively calibrate the shear modulus of a prepared sensor membrane is a current difficulty.
Disclosure of Invention
In view of the above-mentioned shortcomings existing at present, the invention provides a method for measuring shear stress of train surface. According to the invention, the sensor flexible films with different shear moduli are flexibly prepared according to actual needs, the precise shear modulus is obtained through simple calibration, and the shearing force distribution of a test surface with high resolution can be combined with the PIV system and wind tunnel test.
In order to achieve the above purpose, the invention provides a method for measuring shear stress of train-oriented surface, comprising the following steps:
step 1: preparing a film sensor based on the estimated shear modulus required by the surface of the train to be tested;
step 2: the membrane sensor is arranged on a static calibration test bed, and the surface pixel and the actual displacement are calibrated;
step 3: sequentially rotating a calibration experiment table to perform load increase and load reduction tests, sequentially photographing, and constructing a relationship between shear strain and shear stress through PIV cross-correlation analysis to obtain the shear modulus of the sensor;
step 4: setting an auxiliary light source, and capturing the positions of particles on the film sensor through a PIV (Particle image velocimetry) camera in a static state; starting a wind tunnel to perform a test, and capturing the positions of particles on the film sensor through the PIV camera;
step 5: and calculating the particle displacement on the film sensor through the PIV post-processing system, and obtaining the shear stress distribution of the surface through the image processing of the PIV post-processing system.
According to one aspect of the invention, the estimated shear modulus required for the surface of the train to be tested is determined by the following formulas (1) - (4):
in the above-mentioned (1) - (4),
is friction resistance; />
Is the Reynolds number of the plate, and is based on the distance of the position from the front edge of the plate (the plane of the train target position)>
Can be calculated; />
Is the boundary layer thickness; />
Is air density;
is the incoming flow speed; />
Is shear stress; />
The shear modulus required for the estimated train surface to be tested.
According to one aspect of the invention, the membrane sensor is made of silica gel, and the membrane sensor is made of different shear moduli based on different component ratios and by using cavities with different depths.
According to one aspect of the invention, the calibration test stand comprises an L-shaped support, a semicircular inclinometer is arranged on the L-shaped support, arc grooves are formed in the inclinometer at equal distances relative to the circle center, the inclinometer is movably connected with an L-shaped platform through the circle center and the arc grooves, the L-shaped platform comprises a bottom platform and a support perpendicular to the bottom platform, and a PIV camera perpendicular to the bottom platform is arranged on the support.
According to one aspect of the invention, the bottom platform is a plexiglass plate, a cavity is concavely arranged in the middle of the plexiglass plate, and the black background and the film sensor are sequentially embedded in the cavity from bottom to top.
According to one aspect of the invention, the load enhancement is specifically: the method comprises the steps of placing a weight block on a film sensor, and adjusting the film sensor with the weight block from a horizontal state to a state with a certain included angle with the horizontal direction; the load shedding specifically comprises the following steps: and adjusting the membrane sensor with the weight block to be in a horizontal state from a certain included angle with the horizontal direction.
According to one aspect of the invention, the relationship between the shear strain and the shear stress is determined by the following formulas (5) - (6):
in the above-mentioned (5) - (6),
the weight block is formed by the mass of the weight block; />
The contact area of the weight block and the film sensor;
calibrating the inclination angle of a bottom platform on the test bed; />
Is shear stress; />
Is shear strain; />
Is the displacement of the marker particles; />
Is the thickness of the film sensor.
In accordance with one aspect of the present invention, in step 3, further comprising: and (5) photographing by rotating the initial horizontal position in front of the calibration experiment table.
According to one aspect of the invention, before step 4, further comprising: the membrane sensor and the sensor bottom plate are installed together, a fixed reference virtual particle pattern is arranged beside the membrane sensor, and wind tunnel test is carried out to realize the nuclear reduction of displacement caused by wind tunnel vibration.
Based on the same inventive concept, the invention also discloses a calibration test bed for the measuring method, which comprises an L-shaped support, wherein a semicircular inclinometer is arranged on the L-shaped support, arc grooves are equidistantly arranged on the inclinometer relative to the circle center, the inclinometer is movably connected with an L-shaped platform through the circle center and the arc grooves, the L-shaped platform comprises a bottom platform and a support vertical to the bottom platform, and a PIV camera vertical to the bottom platform is arranged on the support.
The invention has the beneficial effects that:
according to the invention, through the designed static calibration experiment table, the film type sensor and the bottom platform rotate together, the shearing force is provided along the parallel component form of the film type sensor based on the gravity of the transparent weight block, the process is easy to operate and implement, and the obtained shearing force value is accurate and reliable. In addition, a practical and reliable implementation method is provided for the calibrated film sensor to carry out test under the wind tunnel condition, and particle displacement interference caused by wind tunnel vibration is effectively solved by referencing the virtual particle arrangement; the arrangement of the auxiliary light source enhances the accuracy of particle identification of the PIV at low speed, and provides a new reference for future measurement application of the surface shear stress sensor.
Drawings
FIG. 1 is a schematic view of a bottom platform horizontal state of a calibration experiment table according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a calibration experiment table according to an embodiment of the present invention, wherein the bottom platform forms an angle with the horizontal state;
FIG. 3 is a graph showing the relationship between the shear force and the shear strain of the shear modulus of the sensor obtained in the load increase and load decrease test according to example 1 of the present invention;
FIG. 4 is a graph of the position of particles captured by a PIV camera on a membrane sensor in a stationary state according to example 1 of the present invention;
FIG. 5 is a graph of the position of particles captured by a PIV camera on a membrane sensor under wind tunnel test as described in example 1 of the present invention;
FIG. 6 is a graph showing the distribution of shear stress measured under a flow-off from a flow-off in example 1 of the present invention.
Description of the reference numerals
1. An L-shaped bracket; 2. an inclinometer; 3. an arc-shaped groove; 4. a bottom platform; 5. a bracket; 6. a PIV camera; 7. a membrane sensor; 8. and a weight block.
Detailed Description
In order that the invention may be more readily understood, the invention will be further described with reference to the following examples. It should be understood that these examples are intended to illustrate the invention and not to limit the scope of the invention, and that the described embodiments are merely some, but not all, of the embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention. Unless defined otherwise, the terms of art used hereinafter are consistent with the meanings understood by those skilled in the art; unless otherwise indicated, all the materials and reagents referred to herein are commercially available or may be prepared by well-known methods.
Current general shear stress testing methods include shear stress suspension platforms, cantilever beams, and thermal membrane methods. Among them, the suspension platform and cantilever beam technology generally requires expensive instruments and complex data processing methods, and has high requirements on experimental environment and test parameters. The thermal film method is a relatively simple and easy-to-use technology, and the principle is that the thermal conduction sensed by a thermal film sensor on a wall surface is utilized to measure the surface shear stress, but the sensitivity of the thermal film method is limited by experimental conditions, and the measurement result is easily influenced by the change of the ambient temperature and the flow velocity, so that the data precision is influenced to a certain extent.
The measuring range of the existing test method is difficult to adjust once being determined, so that the precision is reduced and the adaptability is difficult to be presented when the shearing force range is large; meanwhile, the scattered points are distributed in measurement, the resolution ratio is low, and the accuracy of obtaining the shear force distribution rule for measurement is low. Some testing techniques also require complex preparation and adjustment of the test equipment and test subjects, such that the testing process may be somewhat complex and uncertain.
For a high-speed train, the vehicle body is long, the development change of a boundary layer is large, the range of shearing force values is wide, the flexibility requirement on a testing method is high, and the curvature requirement testing technology of the vehicle body appearance has good fit.
It is common practice for a membrane sensor to obtain shear strain by particle displacement, but how to simply and effectively provide a shear force value and effectively calibrate the shear modulus of a prepared sensor membrane is a current difficulty.
In order to solve the problems, the invention provides a method for measuring the shear stress of a train surface, which comprises the following steps:
step 1: preparing a film sensor based on the estimated shear modulus required by the surface of the train to be tested;
step 2: the membrane sensor is arranged on a static calibration test bed, and the surface pixel and the actual displacement are calibrated;
step 3: sequentially rotating a calibration experiment table to perform load increase and load reduction tests, sequentially photographing, and constructing a relationship between shear strain and shear stress through PIV cross-correlation analysis to obtain the shear modulus of the sensor;
step 4: setting an auxiliary light source, and capturing the particle position on the film sensor through the PIV camera in a static state; starting a wind tunnel to perform a test, and capturing the positions of particles on the film sensor through the PIV camera;
step 5: and calculating the particle displacement on the film sensor through the PIV post-processing system, and obtaining the shear stress distribution of the surface through the image processing of the PIV post-processing system.
In some embodiments of the present application, the estimated shear modulus required for the surface of the train to be tested is determined by formulas (1) - (4):
under the turbulent boundary layer, the following expressions (1) - (3) are adopted:
in the above-mentioned (1) - (4),
is friction resistance; />
Is the Reynolds number of the plate, and is based on the distance of the position from the front edge of the plate (the plane of the train target position)>
Can be calculated; />
Is the boundary layer thickness; />
Is air density;
is the incoming flow speed; />
Is shear stress; />
The shear modulus required for the estimated train surface to be tested. From the above formulas (1) - (3), wall shear stress of different flow direction positions can be calculated>
The method comprises the steps of carrying out a first treatment on the surface of the To ensure the geometry and mechanical properties of the membrane sensor, the deformation of the membrane should not exceed 2% of the thickness of the membrane at maximum load, whereby a maximum of 0.02 shear strain can be estimated, the shear stress divided by the maximum shear strain to obtain the approximate shear modulus required for testing>
That is, the above equation (4) can give an estimated shear modulus required for the surface of the train to be tested.
In some embodiments of the present application, the membrane sensor is a silica gel material, and the membrane sensor is a membrane sensor that produces different shear moduli based on different component ratios and using different depth cavities.
In some embodiments of the present application, the membrane sensor has 3 raw materials, specifically: room temperature vulcanized silica gel RTV 3428A/B part A, RTV 3428A/B part B and AK100 silica gel diluent provided by Barnes corporation. RTV 3428A/B part A and part B are translucent addition cure silicon, both are liquid, and the film can be cured only by mixing RTV 3428A/B part A and RTV 3428A/B part B together. RTV 3428A/B is easy to store because part A and part B are separate in the case of hardening. The cured silicon is translucent so that the resulting film is almost transparent and can easily capture the marked particles. The AK100 silica gel diluent is used for diluting the film, and can reduce the rigidity of the film.
In some embodiments of the present application, the membrane sensor is dispersed with a marker particle, specifically Dantec HGS-10, which is a hollow glass sphere particle of 10um diameter. These particles are white and have a high reflectivity to light, especially when located in a black background. At a steady brightness given by ambient light, the marked particles can be captured by a digital camera and analyzed using an ight 4G.
In some embodiments of the present application, the membrane sensor is a silica gel material, and the membrane sensor is a membrane sensor that produces different shear moduli based on different component ratios and using different depth cavities.
In some embodiments of the present application, the preparation process of the film sensor is specifically: mixing RTV 3428A/B part A and RTV 3428A/B part B according to a volume of 10:1 and a certain amount of diluent to obtain a mixture; the above mixture is poured horizontally into a cavity (cavity) on a plexiglass plate for curing (this process usually takes several hours) to obtain a sensor matrix; after the mixture was poured horizontally into a cavity (cavity) on a plexiglass plate, the marking particles were mixed with AK100 silica gel diluent and sprayed onto the sensor substrate, allowed to stand for a certain time so that the marking particles were immersed into the sensor substrate, and baked to obtain a film sensor.
In order to manufacture a thin film sensor with a low shear stress, a relatively large amount of diluent should be added to the mixture.
It should be noted that the amount of AK100 silica gel diluent used was determined by the test and ranged from 50% to 200% of the total amount of the RTV 3428A/B part A and RTV 3428A/B part B mixture.
In some embodiments of the application, a static calibration test stand as shown in fig. 1 comprises an L-shaped support 1, a semicircular inclinometer 2 is arranged on the L-shaped support 1, arc grooves 3 are formed in the inclinometer 2 at equal distances relative to the circle center, the inclinometer 2 is movably connected with an L-shaped platform through the circle center and the arc grooves 3, the L-shaped platform comprises a bottom platform 4 and a support 5 perpendicular to the bottom platform, and a PIV camera 6 perpendicular to the bottom platform 4 is arranged on the support 5. The bottom platform 4 is an organic glass plate, a cavity is concavely arranged in the middle of the organic glass plate, a black background and a film type sensor 7 are sequentially embedded into the cavity from bottom to top, and a weight block 8 is placed on the film type sensor 7.
Since the membrane sensor is an optical measurement module, the weight is made of a transparent material.
Exemplary, the weights are 0.06345 kg and 0.1067 kg thick glass.
In some embodiments of the present application, the L-shaped bracket is made of an aluminum profile; the inclinometer resolution is 0.5 degrees.
In some embodiments of the present application, the load test is from the state shown in fig. 1 to the state shown in fig. 2; the load shedding state is from the state shown in fig. 2 to the state shown in fig. 1. The load increasing specifically comprises the following steps: the method comprises the steps of placing a weight block on a film sensor, and adjusting the film sensor with the weight block from a horizontal state to a state with a certain included angle with the horizontal direction; the load shedding specifically comprises the following steps: and adjusting the membrane sensor with the weight block to be in a horizontal state from a certain included angle with the horizontal direction.
It should be noted that, the angle of the film sensor provided with the weight block is adjusted, so that the gravity of the transparent weight block generates a tangential component along the sensor, the tangential component is used as a shear force standard quantity of the sensor, and the displacement of the mark point on the sensor film under the action of different shear forces is captured and obtained through the PIV camera, so that the shear modulus of the film sensor is calculated and obtained, and the film sensor can be used for the next wind tunnel test.
The PIV cross-correlation analysis is to capture the position of the marked particle on the film sensor by using a PIV camera in the horizontal state of the bottom platform on the calibration test bed, rotate the bottom platform to a certain angle, and displace the marked particle on the film sensor by a certain length due to the tangential acting force of the transparent weight block along the film sensor, and then capture the position of the marked particle on the film sensor by using the PIV camera, compare the position of the marked particle with the position of the marked particle in the horizontal initial state, and analyze the displacement direction and distance of each marked particle in two pictures, namely the PIV cross-correlation analysis. The PIV cross-correlation analysis can obtain displacement information of the marked particles, and further obtain the response quantity of the film sensor to shearing force (provided by weight components).
The test of increasing load and reducing load by rotating the calibration test bed in turn is to perform static calibration, thereby constructing shear strain #
) With shear stress ()>
) The shear modulus of the sensor is obtained. The equations used in the static calibration include the equation (5) and the equation (6), and are specifically as follows:
in the formula (5), the amino acid sequence of the compound,
the weight block is formed by the mass of the weight block; />
The contact area of the weight block and the film sensor; />
Calibrating the inclination angle of a bottom platform on the test bed; />
Is shear stress;
in the formula (6), the amino acid sequence of the compound,
is the displacement of the marker particles; />
Is the thickness of the film sensor; />
Is of radian and is->
For shear strain, which depends on the displacement of the marker particles on the membrane sensor>
And the thickness of the membrane sensor +.>
；
Constructing a relationship between the shear stress and the shear strain (displacement) according to formulas (5) - (6), so as to obtain the shear modulus of the sensor; wherein the shear strain (displacement) is obtained by PIV cross-correlation analysis.
In some embodiments of the present application, the PIV camera is Nikon 50mm Nikon AF NIKKOR, which is used to capture marker particle displacement on film-based sensors. The pixels of the digital camera are 2352 multiplied by 1768. Each pixel was 42.52um after camera calibration. The lens is mounted on the TSI company's control device. Synchronizer model 610036 connects controlling means through the signal line, when "candid photograph" signal that software weight 4G sent arrives, control the camera and shoot. The synchronizer is also a TSI company and is connected to the control device by a cable. A computer provided with the weight 4G software is connected to the synchronizer through a cable to perform image acquisition and data analysis processing.
In some embodiments of the present application, the PIV aftertreatment system is ight 4G. The combination of the Instrument 4G, which can effectively analyze and display global images on vector and scalar fields, with a digital camera provides a high quality assistant for analyzing hydrodynamic mechanisms. Therefore, the Instrument 4G can effectively measure the displacement of the label particles embedded on the membrane-based sensor: since the membrane sensor is elastic, the movement of the label particles embedded thereon can be captured by a digital camera and measured by the ight 4G. After the image is captured, it is important to enhance the objective characteristics of the image so that the software processing result is more accurate. In this experiment, the displacement of the marker particles is the basic information in the image. Although some particles in the image are apparent and some are too dark to be identified, the pixel intensity values in the region of interest may be increased. For a 12-bit image, a total of 4096 intensity levels are available for selection. Setting the image to a pseudo-color makes the mark easier to observe is an effective method. Since the black background is good at absorbing light, other parts reflect light, making the black area white. To solve this problem, it is necessary to reduce the global pixel intensity, which can eliminate most of the reflected light in the dark areas and keep the particles low. In summary, the image can be improved by adding the image to itself, which means that the pixel intensity is doubled, making the contrast of the label particles to the background more pronounced. In the Insight 4G, the spatial dimension of the image captured by the PIV camera can be calibrated and detected according to the pixel parameters, and since the aspect ratio can be determined according to the number of pixels, only one corresponding actual parameter is required to be input to obtain the spatial actual dimension of the image, and the remaining parameters are automatically calculated. More importantly, after the dimensional information is determined, the resolution of the film sensor is also determined. Basically, the minimum displacement of the particles should be at least 1 pixel to ensure that the displacement can be identified.
It should be noted that the flight 4G provides multiple capture types, single, sequential and sequential. In this experiment, a single sum sequence can be used to capture the pictures before and after film transformation. In analyzing the displacement of particles on the sensor, image processing using ight 4G is required to make the particles apparent.
In some embodiments of the present application, the bottom platform is an organic glass plate, a cavity is concavely arranged in the middle of the organic glass plate, and the black background and the film sensor are sequentially embedded into the cavity from bottom to top. The black background is to highlight the marker particles, and the PIV camera is to recognize the marker particles more easily.
In some embodiments of the present application, in step 3, further includes: and (5) photographing by rotating the initial horizontal position in front of the calibration experiment table.
In some embodiments of the present application, before step 4, further comprising: the membrane sensor and the sensor bottom plate are installed together, a fixed reference virtual particle pattern is arranged beside the membrane sensor, and wind tunnel test is carried out to realize the nuclear reduction of displacement caused by wind tunnel vibration.
It should be noted that the plexiglass plate acts as a support for the membrane sensor and the cavity must be sanded to ensure that its bottom is smooth and to avoid additional friction leading to inaccurate results.
The cavity is exemplified by a dimension of 100 x 70mm and a depth of 1mm.
Further details will be provided below in connection with specific examples.
Example 1
A method for measuring shear stress of train surface comprises the following steps:
step 1: preparing a film sensor based on the estimated shear modulus required by the surface of the train to be tested;
estimating that the shear modulus required by the surface in the train wind tunnel test is about 1100 Pa according to the relation between the friction resistance in the turbulent boundary layer and the wall shear stress; a cavity (the size is 100 multiplied by 70mm, and the depth is 1 mm) of the bottom platform with corresponding depth is manufactured according to the shear modulus; pouring the mixture (RTV 3428A/B part A, RTV 3428A/B part B and AK100 silica gel diluent) into the cavity of the bottom platform; the above mixture is poured horizontally into a cavity (cavity) on a plexiglass plate for curing (this process usually takes several hours) to obtain a sensor matrix; after the mixture was poured horizontally into a cavity (cavity) on a plexiglass plate, the marking particles were mixed with AK100 silica gel diluent and sprayed onto the sensor substrate, allowed to stand for a certain time so that the marking particles were immersed into the sensor substrate, and baked to obtain a film sensor. Wherein RTV 3428A/B part A, RTV 3428A/B part B in the mixture is provided by Barnes, inc. and is mixed in a volume of 10:1; AK100 silica gel diluents specifically AK100 silica gel diluents are 180% of the total amount of RTV 3428A/B part a and RTV 3428A/B part B mixtures, depending on the shear modulus desired.
Step 2: the membrane sensor is arranged on a static calibration test bed, and the surface pixel and the actual displacement are calibrated; wherein each pixel is 42.52um after camera calibration.
The static calibration test bed comprises an L-shaped support, a semicircular inclinometer is arranged on the L-shaped support, arc grooves are formed in the inclinometer at equal distance relative to the circle center, the inclinometer is movably connected with an L-shaped platform through the circle center and the arc grooves, the L-shaped platform comprises a bottom platform and a support of a vertical bottom platform, and a PIV camera of the vertical bottom platform is arranged on the support; the bottom platform is an organic glass plate, a cavity is concavely arranged in the middle of the organic glass plate, and the black background and the film type sensor are sequentially embedded into the cavity from bottom to top.
Step 3: photographing at the initial horizontal position, sequentially rotating a calibration experiment table to perform load increase and load reduction tests, sequentially photographing, and constructing a relationship between shear strain and shear stress through PIV cross-correlation analysis to obtain the shear modulus of the sensor;
the load increasing specifically comprises the following steps: the method comprises the steps of placing a weight block on a film sensor, and adjusting the film sensor with the weight block from a horizontal state to a state with a certain included angle with the horizontal direction; the load shedding specifically comprises the following steps: and adjusting the membrane sensor with the weight block to be in a horizontal state from a certain included angle with the horizontal direction.
The relationship between the shear force and the shear strain obtained by the load increase and decrease test is shown in FIG. 3, and the obtained shear modulus is 1174Pa.
Step 4: the method comprises the steps of installing a membrane sensor and a sensor bottom plate together, arranging a fixed reference virtual particle pattern beside the membrane sensor, and performing wind tunnel test to realize the nuclear reduction of displacement caused by wind tunnel vibration; setting an auxiliary light source, and capturing the position of particles on the film sensor through a PIV (Particle image velocimetry) camera in a static state, wherein the position is shown in fig. 4; starting a wind tunnel to perform a test, and capturing the positions of particles on the film sensor through the PIV camera, wherein the position is shown in FIG. 5;
step 5: particle displacement on the membrane sensor is calculated by the PIV post-processing system, and shear stress distribution of the surface is obtained by image processing of the PIV post-processing system, as shown in fig. 6.
The implementation advantage of this application: according to the invention, the sensor flexible films with different shear moduli are flexibly prepared according to actual needs, the precise shear modulus is obtained through simple calibration, and the shearing force distribution of a test surface with high resolution can be combined with the PIV system and wind tunnel test. According to the invention, through the designed static calibration experiment table, the film type sensor and the bottom platform rotate together, the shearing force is provided along the parallel component form of the film type sensor based on the gravity of the transparent weight block, the process is easy to operate and implement, and the obtained shearing force value is accurate and reliable. In addition, a practical and reliable implementation method is provided for the calibrated film sensor to carry out test under the wind tunnel condition, and particle displacement interference caused by wind tunnel vibration is effectively solved by referencing the virtual particle arrangement; the arrangement of the auxiliary light source enhances the accuracy of particle identification of the PIV at low speed, and provides a new reference for future measurement application of the surface shear stress sensor.
The foregoing is merely illustrative of the present invention, and the present invention is not limited thereto, and any changes or substitutions easily contemplated by those skilled in the art within the technical scope of the present invention should be included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (10)
1. The method for measuring the shear stress of the train surface is characterized by comprising the following steps of:
step 1: preparing a film sensor based on the estimated shear modulus required by the surface of the train to be tested;
step 2: the membrane sensor is arranged on a static calibration test bed, and the surface pixel and the actual displacement are calibrated;
step 3: sequentially rotating a calibration experiment table to perform load increase and load reduction tests, sequentially photographing, and constructing a relationship between shear strain and shear stress through PIV cross-correlation analysis to obtain the shear modulus of the sensor;
step 4: setting an auxiliary light source, and capturing the particle position on the film sensor through the PIV camera in a static state; starting a wind tunnel to perform a test, and capturing the positions of particles on the film sensor through the PIV camera;
step 5: and calculating the particle displacement on the film sensor through the PIV post-processing system, and obtaining the shear stress distribution of the surface through the image processing of the PIV post-processing system.
2. The method for measuring shear stress on a train surface according to claim 1, wherein the estimated required shear modulus of the train surface to be tested is determined by the following formulas (1) to (4):
in the above-mentioned (1) - (4),
is friction resistance; />
Is the Reynolds number of the plate, according to the distance of the position from the front edge of the plate>
Can be calculated; />
Is the boundary layer thickness; />
Is air density; />
Is the incoming flow speed; />
Is shear stress;
the shear modulus required for the estimated train surface to be tested.
3. The method for measuring the shear stress of the train surface according to claim 1, wherein the membrane sensor is made of silica gel, and the membrane sensor is prepared by using cavities with different depths and based on different composition ratios.
4. The method for measuring the shear stress of the train surface according to claim 1, wherein the calibration test bed comprises an L-shaped support, semicircular inclinometers are arranged on the L-shaped support, arc grooves are formed in the inclinometers at equal distances relative to the circle center, the inclinometers are movably connected with an L-shaped platform through the circle center and the arc grooves, the L-shaped platform comprises a bottom platform and a support perpendicular to the bottom platform, and PIV cameras perpendicular to the bottom platform are arranged on the support.
5. The method for measuring the shear stress of the train surface according to claim 3, wherein the bottom platform is a plexiglass plate, a cavity is concavely arranged in the middle of the plexiglass plate, and a black background and a film sensor are sequentially embedded in the cavity from bottom to top.
6. The method for measuring shear stress on a train surface according to claim 1, wherein the load increasing means comprises: the method comprises the steps of placing a weight block on a film sensor, and adjusting the film sensor with the weight block from a horizontal state to a state with a certain included angle with the horizontal direction; the load shedding specifically comprises the following steps: and adjusting the membrane sensor with the weight block to be in a horizontal state from a certain included angle with the horizontal direction.
7. The method for measuring the shear stress of the train-facing surface according to claim 1, wherein the relationship between the construction shear strain and the shear stress is determined by the following formulas (5) to (6):
in the above-mentioned (5) - (6),
the weight block is formed by the mass of the weight block; />
The contact area of the weight block and the film sensor; />
Calibrating the inclination angle of a bottom platform on the test bed; />
Is shear stress; />
Is shear strain; />
Is the displacement of the marker particles; />
Is the thickness of the film sensor.
8. The method of measuring shear stress on a train-facing surface according to claim 1, further comprising, in step 3: and (5) photographing by rotating the initial horizontal position in front of the calibration experiment table.
9. The method of measuring train-facing surface shear stress of claim 1, further comprising, prior to step 4: the membrane sensor and the sensor bottom plate are installed together, a fixed reference virtual particle pattern is arranged beside the membrane sensor, and wind tunnel test is carried out to realize the nuclear reduction of displacement caused by wind tunnel vibration.
10. A calibration test bed for a measurement method according to any one of claims 1 to 9, wherein the calibration test bed comprises an L-shaped support, a semicircular inclinometer is arranged on the L-shaped support, arc grooves are formed in the inclinometer at equal distances relative to the circle center, the inclinometer is movably connected with an L-shaped platform through the circle center and the arc grooves, the L-shaped platform comprises a bottom platform and a support perpendicular to the bottom platform, and a PIV camera perpendicular to the bottom platform is arranged on the support.
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