A thin oil film drawing tool, method, frictional resistance measuring method and product

Abstract

The application discloses a thin oil film drawing tool, method, friction resistance measuring method and product, relates to the friction resistance measuring technology, and the drawing tool comprises a thin oil film drawing instrument; the thin oil film drawing instrument comprises a drawing instrument main body, an oil injection port, a trapezoidal limiting device, a pressing handle, a pressing plate and elastic wires; the oil injection port and the trapezoidal limiting device are connected with the drawing instrument main body respectively; the elastic wires are arranged at the bottom of the drawing instrument main body, the elastic wires are connected with the oil injection port through a narrow passage, and the elastic wires are used for drawing a thin oil film; the thin oil film is quantitatively injected with oil drops into the oil injection port according to measurement requirements; the pressing handle and the pressing plate are fixedly connected longitudinally through the drawing instrument main body and are connected with the upper portion of the drawing instrument main body through a spring. After injecting the quantitative oil drops, the oil drops are uniformly distributed on the surface of the elastic wires under the action of the surface tension of the oil drops, at this time, the elastic wires are fully contacted with a surface to be measured, the oil drops will rapidly spread on the surface to be measured, and a thin oil film with uniform thickness is formed.

Description

Technical Field

[0001] This application relates to the field of friction resistance measurement technology, and in particular to a thin oil film drawing tool, method, friction resistance measurement method and product. Background Technology

[0002] Frictional resistance, generated by the relative motion between the surface of a vehicle and the free flow, contributes over 50% of the total resistance in transportation and has become a key factor affecting transportation efficiency and energy consumption. However, because frictional resistance is highly sensitive to changes in the flow field, even small variations in near-wall velocity or pressure can significantly impact its level. Therefore, measuring frictional resistance is particularly challenging, requiring advanced measurement techniques and suitable post-processing methods to improve the accuracy and robustness of frictional resistance measurements.

[0003] Over the past half-century, various methods for measuring frictional resistance have been proposed by the academic community. Based on the type and arrangement of sensors, these methods can be categorized into invasive and non-invasive methods. Invasive methods, such as hot-wire anemometers and Preston tubes, inevitably affect the near-wall flow state, resulting in poor measurement accuracy. Non-invasive methods can be further divided into direct and indirect methods based on their measurement principles. Indirect methods, such as near-wall particle velocimetry and liquid crystal coating methods, involve calibration, simultaneous measurement of multiple physical quantities, or estimation using empirical formulas, thus leading to lower measurement accuracy. Among numerous measurement techniques, oil-film interferometry (OFI), as a non-invasive direct measurement method, combines the advantages of high precision and high accuracy, and is therefore considered a highly promising means of measuring frictional resistance.

[0004] The measurement principle of the thin oil film interferometry method: Under the continuous action of free flow, frictional resistance deforms the thin oil droplets coated on the wall into a thickness along the flow direction. x A uniformly varying wedge shape is formed. Under monochromatic light source illumination, the reflected light from the oil film surface and the wall-reflected light after refraction through the oil film will form constructive or destructive interference on the oil film surface, manifesting as alternating bright and dark fringes along the flow direction. Under the continuous action of frictional resistance, the thickness of the wedge-shaped thin oil film gradually decreases, and the flow-direction width of the interference fringes further widens. Therefore, by measuring the change in fringe width over a certain time period, the time-averaged frictional resistance during that period can be quantitatively calculated. Since the thickness of the thin oil film is typically on the order of micrometers, much smaller than the thickness of the boundary layer viscous sublayer that dominates near-wall frictional resistance, this can be considered a non-invasive measurement method. Furthermore, since the frictional resistance directly causes changes in the interference fringes of the thin oil film, from a measurement principle perspective, this technique belongs to a direct measurement method.

[0005] However, despite its numerous advantages, such as simplicity of implementation, low requirements for environment and measuring equipment, and relatively reliable measurement results, the thin oil film interference friction resistance measurement method has not been widely applied in engineering practice. The main reasons limiting the development and application of this technology are as follows:

[0006] The quality of the oil film application directly affects the measurement results of oil film interferometry. An excessively thick oil film will cause excess oil droplets to spread downstream during the experiment, contaminating the downstream measurement area. An excessively thin oil film, due to its surface tension, will fail to form a wedge shape and produce effective interference fringes under the influence of the incoming flow. However, in previous experiments, the application of thin oil films relied heavily on the experimenter's experience and was simply drawn using existing tools such as combs, rulers, and blades. Therefore, the resulting thin oil films were uneven in thickness and contained local defects, reducing the quality of the interference fringes. The lack of specialized tools for drawing thin oil films has limited the application of this technique. Summary of the Invention

[0007] The purpose of this application is to provide a tool, method, friction resistance measurement method and product for drawing thin oil films, which can draw thin oil films with uniform thickness and improve the quality of interference fringes.

[0008] To achieve the above objectives, this application provides the following solution:

[0009] In a first aspect, this application provides a thin oil film drawing tool, including: a thin oil film drawing instrument;

[0010] The thin oil film drawing instrument includes: a drawing instrument body, an oil inlet, a trapezoidal limiting device, a pressing handle, a pressing plate, and an elastic wire;

[0011] The oil inlet and the trapezoidal limiting device are respectively connected to the main body of the plotter;

[0012] The elastic wire is located at the bottom of the plotter body and is connected to the plotter body with screws. The tightness of the screws is adjusted to make the elastic wire taut. The elastic wire is connected to the oil injection port through a narrow channel. The elastic wire is used to draw a thin oil film. The thin oil film is drawn by quantitatively injecting oil droplets into the oil injection port according to the measurement requirements.

[0013] The pressing handle and the pressing plate are fixedly connected longitudinally through the plotter body and connected to the upper part of the plotter body by a spring.

[0014] Optionally, it also includes: a pipette;

[0015] The pipette is used to quantitatively extract oil droplets and inject them into the oil inlet of the thin oil film plotter.

[0016] Optionally, the volume of oil droplets extracted by the pipette each time is: ;in, wThis indicates the width of the thin oil film to be drawn, not exceeding 1 mm; h This indicates the thickness of the thin oil film being drawn, not exceeding 0.1 mm; l This indicates the length of the thin oil film being drawn, which is determined based on measurement requirements.

[0017] Optionally, the main body of the plotter is in the shape of an "n";

[0018] The oil inlet and the trapezoidal limiting device are respectively fixed to the outer sides of the two legs of the "n" shape;

[0019] The two legs of the “n” shape are respectively fixed to the two ends of the elastic wire.

[0020] Optionally, the oil inlet is a funnel-shaped oil inlet;

[0021] The elastic yarn is PET elastic yarn.

[0022] Secondly, this application provides a method for drawing a thin oil film using a thin oil film drawing tool according to any one of the above claims, comprising:

[0023] The elastic wire is brought into contact with the area to be measured on the wall, and the position of the trapezoidal limiting device on the area to be measured on the wall is marked to obtain the marked position.

[0024] A quantitative amount of oil droplets is drawn up using a pipette and injected into the oil inlet; a narrow channel at the bottom of the oil inlet guides the oil droplets to be transported to the elastic filament under the action of gravity, and the oil droplets are spread all over the elastic filament under the action of tension;

[0025] Align the trapezoidal limiting device with the marked position, and press down the handle to make the elastic wire covered with oil droplets fully contact the area to be measured on the wall. After the handle automatically rebounds under the action of the spring, remove the thin oil film drawing instrument.

[0026] Thirdly, this application provides a method for measuring frictional resistance, including:

[0027] Acquire several interference fringe images within the measurement time period; the interference fringe images are images of alternating bright and dark interference fringes arranged along the flow direction on the upper surface of the thin oil film after being subjected to the frictional resistance of the free flow; the thin oil film is a thin oil film drawn using any of the thin oil film drawing tools described above.

[0028] Each of the interference fringe images is subjected to quasi-two-dimensional variational mode decomposition along the flow direction, and the first-order mode is retained to obtain several flow-direction denoised interference images;

[0029] Each of the noise-reduced interference images along the spanwise direction is subjected to quasi-two-dimensional variational mode decomposition, retaining the first-order mode, to obtain several high signal-to-noise ratio interference fringe images.

[0030] Based on several high signal-to-noise ratio interference fringe images, the temporal variation of fringe information at a preset spanning position is plotted as a fringe time sequence diagram.

[0031] Based on the fringe time sequence diagram, the temporal changes of the flow direction coordinates of the centers of bright and dark stripes are linearly fitted to obtain the time change rate of the flow direction coordinates of the center points of bright and dark stripes within the measurement time period.

[0032] The time-averaged frictional resistance at the preset spanwise position is calculated based on the time rate of change.

[0033] Optionally, the formula for calculating the time-averaged frictional resistance at the preset spanwise position based on the time change rate is as follows:

[0034] ;

[0035] in, n and μ These are the refractive index and dynamic viscosity coefficient of the thin oil film, respectively. θ and λ These represent the wavelength and angle of incidence of the incident monochromatic light, respectively. k l ( z 0 )and k d ( z 0 ( ) represent the time rate of change of the flow direction coordinates of the center points of a set of bright and dark fringes, respectively.

[0036] Optionally, each of the interference fringe images is subjected to quasi-two-dimensional variational mode decomposition along the flow direction, retaining the first-order mode, to obtain several flow-direction denoised interference images, specifically including:

[0037] Each interference fringe image is decomposed along the flow direction into K modes with different scales and a residual according to the decomposition order K, as shown in the formula: ;in, Indicates the first i First mode, I e ( x , z ) represents the residual;

[0038] The first-order modes, arranged in descending order of the flow direction feature scale, are retained to obtain several flow direction noise-reduced interferometric images.

[0039] Fourthly, this application provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the friction resistance measurement method described in any one of the above descriptions.

[0040] According to the specific embodiments provided in this application, the following technical effects are disclosed:

[0041] This application provides a thin oil film drawing tool, method, friction resistance measurement method, and product. The drawing tool includes a thin oil film drawing instrument; the thin oil film drawing instrument includes a drawing instrument body, an oil inlet, a trapezoidal limiting device, a pressing handle, a pressing plate, and an elastic wire; the oil inlet and the trapezoidal limiting device are respectively connected to the drawing instrument body; the elastic wire is disposed at the bottom of the drawing instrument body and is connected to the drawing instrument body with screws, and the tightness of the screws is adjusted to make the elastic wire taut; the elastic wire is connected to the oil inlet through a narrow channel, and the elastic wire is used to draw a thin oil film; the thin oil film is drawn by quantitatively injecting oil droplets into the oil inlet according to the measurement requirements; the pressing handle and the pressing plate are longitudinally fixedly connected through the drawing instrument body and are connected to the upper part of the drawing instrument body through a spring. This application has a pressing handle that can move up and down and an elastic wire at the bottom. After a quantitative amount of oil droplets are injected, they are evenly distributed on the surface of the elastic wire under the action of the surface tension of the oil droplets. At this time, the elastic wire is in full contact with the surface to be measured, and the oil droplets will quickly spread on the surface to be measured to form a thin oil film of uniform thickness. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0043] Figure 1 This is a schematic diagram of the structure of a thin oil film drawing tool provided in an embodiment of this application.

[0044] Figure 2 This is a schematic flowchart of a friction resistance measurement method provided in an embodiment of this application.

[0045] Figure 3 This is a schematic diagram illustrating the basic principle of measuring frictional resistance using an oil film interference method provided in an embodiment of this application.

[0046] Figure 4 This is a schematic diagram of alternating bright and dark interference fringes arranged along the flow direction, provided as an embodiment of this application.

[0047] Figure 5 This is a schematic diagram illustrating the evolution of interference fringes in a thin oil film over time, captured by multiple CCD cameras, as provided in one embodiment of this application.

[0048] Figure 6 This is a schematic diagram illustrating the process of denoising an interference fringe image according to an embodiment of this application.

[0049] Figure 7 This is a schematic diagram of the arrangement for measuring frictional resistance according to an embodiment of this application.

[0050] Figure 8 This is a schematic diagram showing the measurement results of friction resistance coefficients with and without noise reduction, according to an embodiment of this application.

[0051] Figure 9 This is a schematic diagram showing the results of measuring the frictional resistance at different locations downstream of a row of blade-shaped eddy current generator using the oil film interference method provided in an embodiment of this application.

[0052] Figure 10 This is a schematic diagram of the structure of a computer device provided in an embodiment of this application. Detailed Implementation

[0053] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0054] This application proposes a method for estimating oil film volume based on measurement requirements, and uses a thin oil film plotter designed in this application to plot standard thin oil films with consistent specifications. Regarding the denoising of the obtained interferometric image, this application proposes a quasi-two-dimensional variational mode decomposition method to sequentially decompose the original interferometric image along the flow direction and normal direction into modes with different characteristic scales, automatically filtering out small-scale noise and extracting interference fringe features. By improving oil film quality through the thin oil film plotting device and method involved in this application, and enhancing the quality of the interferometric image by applying the automatic denoising method proposed in this application, this application demonstrates a significant improvement in measurement accuracy and measurement spatial resolution in exemplary application scenarios.

[0055] The purpose of this application is to provide a standardized method and tool for drawing thin oil films in the oil film interferometry friction resistance measurement method; and to propose an active noise reduction method for extracting interference fringe features from the captured oil film interferometry image to improve the friction resistance measurement accuracy of the oil film interferometry method.

[0056] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0057] In one exemplary embodiment, such as Figure 1 As shown, a thin oil film drawing tool is provided, including a pipette and a thin oil film drawing instrument.

[0058] The thin oil film drawing instrument includes: the drawing instrument body, the oil inlet, the trapezoidal limiting device, the pressing handle, the pressing plate, and the elastic wire.

[0059] The oil inlet and the trapezoidal limiting device are respectively connected to the main body of the plotter. The oil inlet is a funnel-shaped oil inlet.

[0060] The elastic wire is tied to the bottom of the plotter body and connected to the plotter body with screws. The tightness of the screws is adjusted to keep the elastic wire taut. The elastic wire is connected to the oil inlet through a narrow channel. The elastic wire is used to draw a thin oil film. The thin oil film is drawn by quantitatively injecting oil droplets into the oil inlet according to the measurement requirements. The elastic wire is a PET elastic wire. By changing the length of the polyester PET elastic wire, the length of the drawn thin oil film can be changed according to the experimental requirements.

[0061] The pressing handle and the pressing plate are fixedly connected longitudinally through the plotter body and connected to the upper part of the plotter body by a spring.

[0062] The pipette is used to quantitatively extract oil droplets and inject them into the oil inlet of the thin oil film plotter.

[0063] The width of the thin oil film drawn w Generally not exceeding 1 mm, thickness h Generally not exceeding 0.1 mm, length l The required volume of oil droplet to be extracted by the pipette can be determined based on the measurement requirements. Therefore, the volume of oil droplet to be extracted by the pipette is usually determined according to the following formula. V .

[0064] .

[0065] The main body of the plotter is shaped like an "n";

[0066] The oil inlet and the trapezoidal limiting device are respectively fixed to the outer sides of the two legs of the "n" shape;

[0067] The two legs of the “n” shape are respectively fixed to the two ends of the elastic wire.

[0068] There are two main steps before drawing a thin oil film. First, the PET elastic filament is brought into contact with the area to be tested on the wall surface, and the position of the trapezoidal limiting device is marked on the area with a marker. Second, a quantitative amount of oil droplets is drawn up with a pipette and injected into the oil inlet. A narrow channel at the bottom of the oil inlet guides the oil droplets to be transported to the PET elastic filament under gravity. At this time, the oil droplets quickly and evenly cover the PET elastic filament under the action of tension. When drawing the thin oil film, the trapezoidal limiting device is aligned with the marked position on the area to be tested on the wall surface, thereby fixing the thin oil film drawing instrument directly above the surface to be coated with the thin oil film. The pressing handle is pressed down to make the oil-covered PET elastic filament fully contact the surface to be tested for 5 seconds, so that the oil on the surface of the elastic filament is transferred to the wall surface and evenly distributed under the action of surface tension. Then, the pressing plate and the pressing handle automatically rebound under the action of the spring and the thin oil film drawing instrument is removed. At this time, a thin oil film that meets the requirements of this embodiment is drawn.

[0069] This application first uses the invented thin oil film plotter to quantitatively plot a standard thin oil film conforming to the requirements of this application on the surface to be tested. For multiple frames of original interference images captured at different times in the experiment, the quasi-two-dimensional variational mode decomposition method is applied to automatically decompose them sequentially along the flow direction and normal direction, filtering out noise and non-uniform background light, and preserving and enhancing the interference fringe features. Finally, based on the distribution of light intensity of the fringe image at each spanwise position along time, the rate of change of the fringe flow width at that location is calculated, and then the time-averaged frictional resistance at that location is calculated. By applying the professional thin oil film plotting tool proposed in this application, as well as the automatic interference image noise reduction technology, the threshold for implementing the oil film interference frictional resistance measurement method is lowered, and the measurement accuracy and spatial resolution of frictional resistance are improved.

[0070] In one exemplary embodiment, a method for drawing a thin oil film using the thin oil film drawing tool described above is provided, comprising the following steps:

[0071] The elastic wire is brought into contact with the area to be measured on the wall, and the position of the trapezoidal limiting device on the area to be measured on the wall is marked to obtain the marked position.

[0072] A quantitative amount of oil droplets is drawn up using a pipette and injected into the oil inlet; a narrow channel at the bottom of the oil inlet guides the oil droplets to be transported to the elastic filament under the action of gravity, and the oil droplets are spread all over the elastic filament under the action of tension.

[0073] Align the trapezoidal limiting device with the marked position, and press down the handle to make the elastic wire covered with oil droplets fully contact the area to be measured on the wall. After the handle automatically rebounds under the action of the spring, remove the thin oil film drawing instrument.

[0074] In one exemplary embodiment, such as Figure 2 As shown, a method for measuring frictional resistance is provided, including:

[0075] S1. Acquire several interference fringe images within the measurement time period; the interference fringe image is an image of alternating bright and dark interference fringes arranged along the flow direction on the upper surface of the thin oil film after being subjected to the frictional resistance of the free flow; the thin oil film is a thin oil film drawn using the thin oil film drawing tool described above.

[0076] After the thin oil film is drawn, the wind tunnel can be turned on to generate free flow and friction measurements can be taken. After the thin oil film experiences the frictional resistance of the free flow, its upper surface becomes as follows: Figure 3 The uniform wedge shape shown. Figure 3 As shown, when a wedge-shaped thin oil film is irradiated with monochromatic light, the reflected light from the oil film surface interferes with the reflected light from the wall surface. When the optical path difference is an integer multiple of the wavelength of the monochromatic light, constructive interference occurs, resulting in a bright fringe at that location, which is also the center of the bright fringe. When the optical path difference is an odd multiple of half the wavelength of the monochromatic light, destructive interference occurs, resulting in a dark fringe at that location, which is also the center of the dark fringe. Ultimately, under monochromatic light irradiation, the wedge-shaped thin oil film exhibits the following appearance: Figure 4 The diagram shows alternating bright and dark interference fringes arranged along the flow direction. The changes in these interference fringes over a time series are recorded using a camera to calculate the time-averaged frictional resistance. Considering the angle between the camera's phase plane and the surface being measured, a checkerboard calibration paper is attached to the surface before measurement and the data is recorded by the camera. Spatial distortion is then compensated for and the true physical dimensions of the image are calibrated using quadratic polynomial fitting and bilinear interpolation.

[0077] Specifically: Due to perspective distortion caused by the angle between the camera's phase plane and the measured surface (the area to be measured on the wall), a square black-and-white checkerboard calibration paper of known size is first attached to the measured surface and recorded by the camera before measurement. The distortion coefficient is calculated by identifying the checkerboard corner points and fitting a quadratic polynomial, thus finding the geometric transformation relationship between the images before and after distortion and correcting the image. Since the position of pixels in the corrected image may correspond to non-integer coordinates in the original image, bilinear interpolation is used to estimate the pixel values ​​at integer coordinates, thereby maintaining the smoothness of the corrected image. Finally, the spatial resolution of the corrected image is calculated based on the physical dimensions of the checkerboard to determine the true physical dimensions of the thin oil film interference fringes at each moment, which is used in subsequent friction calculations. Therefore, after the wind speed stabilizes, multiple CCD cameras can be used to capture the evolution of the thin oil film interference fringes over time at a frequency set by the synchronizer (e.g., ...). Figure 5 As shown in the figure, the temporal fringe information is recorded and stored in the computer. The process of denoising an interference fringe image is as follows: Figure 6 As shown.

[0078] S2. Perform quasi-two-dimensional variational mode decomposition on each of the interference fringe images along the flow direction, retain the first-order mode, and obtain several flow-direction denoised interference images.

[0079] First, place a certain t Interference fringe image at time 0 I ( x , z ; t 0) The quasi-two-dimensional variational mode decomposition method is applied to perform the first decomposition along the flow direction. The decomposition principle of this method is to decompose the original interferometric image according to the decomposition order along a certain principal decomposition direction. K Decomposed into K Modes with different scales I K ( x , z and a residual I e ( x , z As shown in the following formula:

[0080] ;

[0081] in, Indicates the first i 1st mode.

[0082] In this embodiment, the decomposition order is set to 4, and after decomposition, 4 modes are obtained in descending order of flow direction feature scale. Among them, the 1st mode has the largest flow direction scale. ( x , z The striped features are preserved, while higher-order modes contain small-scale burr noise caused by dust and are therefore discarded. Among these, the first-order mode... ( x , z (superscript) x This indicates decomposition along the flow direction, and the subscript 1 indicates the first-order mode of the flow direction decomposition.

[0083] S3. Perform quasi-two-dimensional variational mode decomposition on each of the noise-reduced interference images along the spanwise direction, retain the first-order mode, and obtain several high signal-to-noise ratio interference fringe images.

[0084] This embodiment focuses on the first-order mode of the first decomposition. ( x , z A second decomposition is performed along the spanwise direction using the quasi-two-dimensional variational mode decomposition method, again setting the decomposition order to 4. The resulting decomposition yields 4 modes arranged in descending order of spanwise characteristic scales. Among these, the 1st-order mode has the largest spanwise scale. ( x , z The non-uniform background light caused by different angles of monochromatic incident light, second-order mode. ( x ,z The first and second decompositions represent the interference fringes after noise reduction. Other higher-order modes are localized small-scale noise caused by bubbles in the oil film. Optionally, higher decomposition orders can be set for both the first and second decompositions, but the verification results are consistent with this preferred example, i.e., the first-order mode after flow-direction decomposition... ( x , z The first-order mode exhibits striped characteristics and is decomposed into convection directions. ( x , z The second-order modes are then subjected to spanwise decomposition. ( x , z The image shows the interference fringes after noise reduction. In summary, the quasi-two-dimensional variational mode decomposition method was applied to analyze each moment recorded by each CCD camera. t Interference fringe image I ( x , z , t Following the flow direction and spanwise decomposition in this embodiment, the following can be obtained: Figure 4 The high signal-to-noise ratio interferometric fringe images at each time step after noise reduction are shown. ( x , z , t ).

[0085] S4. Based on several high signal-to-noise ratio interference fringe images, the temporal changes of fringe information at a preset spanning position are plotted as a fringe time sequence diagram.

[0086] S5. Based on the stripe time sequence diagram, linearly fit the time sequence changes of the flow direction coordinate positions of the centers of the bright and dark stripes respectively to obtain the time change rate of the flow direction coordinates of a set of bright and dark stripe center points within the measurement time period.

[0087] S6. Calculate the time-averaged frictional resistance at the preset spanwise position based on the time change rate.

[0088] Obtaining high signal-to-noise ratio interference fringe images in time sequence ( x , z , t After that, a certain spanwise position can be determined. z 0 (e.g.) Figure 6 (c) Temporal stripe information (shown by the red one-dimensional dashed line area in the second left figure) ( x , z 0, t ) drawn as Figure 6 (d) Stripe time sequence diagram I ( x ,t ; z 0), and linearly fitted the temporal variations of the flow direction coordinates of the centers of bright and dark fringes, respectively. x ( t , z 0) To reduce random errors introduced by single-frame interferometric images. Based on the time change rate of the flow direction coordinates of the center points of a set of bright and dark fringes within the measurement time period. k l ( z 0) and k d ( z 0) (respectively) Figure 6 (d) The slope of the blue and pink dashed lines can be used to calculate the average frictional resistance over that time period using the following formula. :

[0089] ;

[0090] in, n and μ These are the refractive index and dynamic viscosity coefficient of the thin oil film, respectively. θ and λ These represent the wavelength and incident angle of the incident monochromatic light, respectively. This calculation is performed at each spanwise position to obtain the frictional resistance information within the measurement area of ​​the thin oil film. The incident monochromatic light is composed of... Figure 7 The high-pressure sodium lamp emits light that is used to illuminate a thin oil film to produce alternating bright and dark interference fringes.

[0091] The method of this embodiment is described below with reference to a specific application scenario. The steps for measuring frictional resistance using the oil film drawing technology and noise reduction method of this embodiment are as follows: Figure 5 As shown in the diagram, the measurement setup is as follows: Figure 7 As shown, the frictional resistance coefficients are with and without noise reduction. C f The measurement results are as follows Figure 8 As shown. In this embodiment, the fully developed flat plate turbulent boundary layer in the wind tunnel is used as the control object, and the friction Reynolds number is... Re τ ≈ 4380. Two 50W low-pressure sodium lamps illuminate the test area with monochromatic light at an incident angle of 35° and a wavelength of 589 nm. The surface of the test plate is covered with a Mylar film to produce recognizable interference fringes. Eight thin oil films, each 150 mm long, approximately 1 mm wide, and no more than 0.1 mm thick, are uniformly applied to the test area using the plotting method proposed in this embodiment. The resulting interference fringes are captured by four CCD cameras positioned at the top of the wind tunnel. After the incoming wind speed stabilizes, the four cameras simultaneously record the changes in the thin oil film interference fringes over 20 minutes at a frequency of 3 frames per minute, and apply... Figure 2The automatic noise reduction method proposed in this embodiment extracts interference fringe features and then applies the formula... Frictional resistance is considered in the calculation. During the measurement, the wind speed of the free flow is... U = varies within the range of 10 ~ 20 m / s to measure different Reynolds numbers. Re x The frictional drag coefficient was determined. Seven repeated measurements were performed for each wind speed, and the ensemble average of the repeated measurements was... Figure 8 The uncertainty is represented by circles or squares, while the uncertainty based on the standard deviation of repeated measures is given as an error bar. For comparison, Figure 8 The diagram shows the empirical curve (black curve) of the turbulent frictional resistance coefficient of a flat plate wall and its 3% error band (gray area), the measurement results of the oil film interferometry method without noise reduction (blue hollow circles), and the measurement results of the oil film interferometry method after applying the automatic noise reduction technology proposed in this embodiment (red solid squares). Clearly, the frictional resistance coefficient measured after noise reduction has higher measurement accuracy and lower uncertainty, demonstrating that this embodiment achieves a significant performance improvement in the oil film interferometry method for measuring frictional resistance.

[0092] Figure 9 It also demonstrated the application of the oil film interferometry method to measure the frictional resistance at different locations downstream of a row of blade-shaped eddy current generators. τ w The results. In the measurement, the wind speed of the free flow was... U = 14 m / s. Similarly, this measurement was performed in 7 repeated measurements. The ensemble mean is represented by solid dots, and the uncertainty based on the standard deviation of repeated measurements is given as the error band. The relative flow direction position of the measurement location from the trailing edge of the vortex generator. x Eddy current generator height h Dimensionless transformation. The complex vortex system induced downstream of the vortex generator generates frictional resistance along the spanwise direction. z The non-uniform periodic distribution. Comparing the results measured by the oil film interferometry method before and after noise reduction, it can be seen that by applying the automatic noise reduction technology proposed in this invention to remove small-scale noise in the interferometric image, the uncertainty of measuring frictional resistance and the small-scale noise in the spanwise direction can be effectively reduced, and the spanwise measurement resolution of the oil film interferometry method can be significantly improved, thereby enabling precise resolution of the periodic changes in frictional resistance downstream of the eddy current generator.

[0093] In one exemplary embodiment, a computer device is provided, which may be a server or a terminal, and its internal structure diagram may be as follows. Figure 10As shown, this computer device includes a processor, memory, input / output (I / O) interfaces, and a communication interface. The processor, memory, and I / O interfaces are connected via a system bus, and the communication interface is also connected to the system bus via the I / O interfaces. The processor provides computational and control capabilities. The memory includes non-volatile storage media and internal memory. The non-volatile storage media stores the operating system, computer programs, and databases. The internal memory provides the environment for the operation of the operating system and computer programs stored in the non-volatile storage media. The I / O interfaces are used for exchanging information between the processor and external devices. The communication interface is used for communicating with external terminals via a network connection. When the computer program is executed by the processor, it implements a method for measuring frictional resistance.

[0094] Those skilled in the art will understand that Figure 10 The structure shown is merely a block diagram of a portion of the structure related to the present application and does not constitute a limitation on the computer device to which the present application is applied. Specific computer devices may include more or fewer components than those shown in the figure, or combine certain components, or have different component arrangements.

[0095] In one exemplary embodiment, a computer device is also provided, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0096] In one exemplary embodiment, a computer-readable storage medium is provided storing a computer program that, when executed by a processor, implements the steps in the above-described method embodiments.

[0097] In one exemplary embodiment, a computer program product is provided, including a computer program that, when executed by a processor, implements the steps in the above-described method embodiments.

[0098] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of the relevant data must comply with relevant regulations.

[0099] Those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium. When executed, the computer program can include the processes of the embodiments described above. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM).

[0100] The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0101] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0102] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A thin oil film drawing tool, characterized in that, include: Thin oil film plotter; The thin oil film drawing instrument includes: a drawing instrument body, an oil inlet, a trapezoidal limiting device, a pressing handle, a pressing plate, and an elastic wire; The oil inlet and the trapezoidal limiting device are respectively connected to the main body of the plotter; The elastic wire is located at the bottom of the plotter body and is connected to the plotter body with screws. The tightness of the screws is adjusted to make the elastic wire taut. The elastic wire is connected to the oil injection port through a narrow channel. The elastic wire is used to draw a thin oil film. The thin oil film is drawn by quantitatively injecting oil droplets into the oil injection port according to the measurement requirements. The pressing handle and the pressing plate are fixedly connected longitudinally through the plotter body and are connected to the upper part of the plotter body by a spring; The main body of the plotter is shaped like the letter "n". The oil inlet and the trapezoidal limiting device are respectively fixed to the outer sides of the two legs of the "n" shape; The two legs of the "n" shape are respectively fixed to the two ends of the elastic wire.

2. The thin oil film drawing tool according to claim 1, characterized in that, Also includes: pipette; The pipette is used to quantitatively extract oil droplets and inject them into the oil inlet of the thin oil film plotter.

3. The thin oil film drawing tool according to claim 2, characterized in that, The volume of oil droplets extracted by the pipette each time is: ;in, w This indicates the width of the thin oil film to be drawn, not exceeding 1 mm; h This indicates the thickness of the thin oil film being drawn, not exceeding 0.1 mm; l This indicates the length of the thin oil film being drawn, which is determined based on measurement requirements.

4. The thin oil film drawing tool according to claim 1, characterized in that, The oil inlet is a funnel-shaped oil inlet; The elastic yarn is PET elastic yarn.

5. A method for creating a thin oil film using a thin oil film drawing tool according to any one of claims 1-4, characterized in that, include: The elastic wire is brought into contact with the area to be measured on the wall, and the position of the trapezoidal limiting device on the area to be measured on the wall is marked to obtain the marked position. A quantitative amount of oil droplets is drawn up using a pipette and injected into the oil inlet; a narrow channel at the bottom of the oil inlet guides the oil droplets to be transported to the elastic filament under the action of gravity, and the oil droplets are spread all over the elastic filament under the action of tension; Align the trapezoidal limiting device with the marked position, and press down the handle to make the elastic wire covered with oil droplets fully contact the area to be measured on the wall. After the handle automatically rebounds under the action of the spring, remove the thin oil film drawing instrument.

6. A method for measuring frictional resistance, characterized in that, include: Acquire several interference fringe images within the measurement time period; The interference fringe image is an image of alternating bright and dark interference fringes arranged along the flow direction on the upper surface of a thin oil film after being subjected to the frictional resistance of a free flow. The thin oil film is a thin oil film drawn using the thin oil film drawing tool according to any one of claims 1-4; Each of the interference fringe images is subjected to quasi-two-dimensional variational mode decomposition along the flow direction, and the first-order mode is retained to obtain several flow-direction denoised interference images; Each of the noise-reduced interference images along the spanwise direction is subjected to quasi-two-dimensional variational mode decomposition, retaining the first-order mode, to obtain several high signal-to-noise ratio interference fringe images. Based on several high signal-to-noise ratio interference fringe images, the temporal variation of fringe information at a preset spanning position is plotted as a fringe time sequence diagram. Based on the fringe time sequence diagram, the temporal changes of the flow direction coordinates of the centers of bright and dark stripes are linearly fitted to obtain the time change rate of the flow direction coordinates of a set of bright and dark stripe center points within the measurement time period. The time-averaged frictional resistance at the preset spanwise position is calculated based on the time rate of change.

7. The method for measuring frictional resistance according to claim 6, characterized in that, The formula for calculating the time-averaged frictional resistance at the preset spanwise position based on the time rate of change is as follows: ； in, n and μ These are the refractive index and dynamic viscosity coefficient of the thin oil film, respectively. θ and λ These represent the wavelength and angle of incidence of the incident monochromatic light, respectively. k l ( z 0 )and k d ( z 0 ( ) represent the time rate of change of the flow direction coordinates of the center points of a set of bright and dark fringes, respectively.

8. The method for measuring frictional resistance according to claim 6, characterized in that, Each of the aforementioned interference fringe images is subjected to quasi-two-dimensional variational mode decomposition along the flow direction, retaining the first-order mode, to obtain several flow-direction denoised interference images, specifically including: Each of the interference fringe images is analyzed along the flow direction according to the decomposition order. K Decomposed into K Given a mode with different scales and a residual, the formula is: ;in, Indicates the first i First mode, I e ( x , z ) represents the residual; The first-order modes, arranged in descending order of the flow direction feature scale, are retained to obtain several flow direction noise-reduced interferometric images.

9. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by the processor, it implements the friction resistance measurement method according to any one of claims 6-8.

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