Surface oil film display method suitable for underwater object

Abstract

The invention discloses a surface oil film display method suitable for an underwater object. The method mainly comprises the following steps: firstly, preparing a mixed turbid liquid suitable for an underwater environment; then, in a water-free environment, uniformly brushing the mixed turbid liquid on the surface of an object, so that a layer of surface oil film with uniform thickness is formed on the surface of the object; placing in an underwater test environment, continuously recording an oil flow map of the object surface by using a camera, and obtaining a digital image sequence of the oil flow map of the object surface; processing the preprocessed oil flow map image sequence by using a multi-stage grid iteration cross-correlation method to obtain an instantaneous oil flow vector field sequence; and finally, generating a flow map of the object surface according to the instantaneous oil flow vector field sequence. The method can be suitable for some underwater tests, and the application scene is expanded; the test efficiency is greatly improved, and personal errors caused by excessive manual participation are reduced. In addition, the display and qualitative analysis of the dynamic evolution process of the oil flow map can be realized.

Description

Technical Field

[0001] The invention belongs to a surface oil film flow display method in the field of fluid mechanics experiments, and specifically relates to a surface oil film display method suitable for underwater objects. Background Art

[0002] As a classic, simple and efficient flow display method, surface oil film technology is often used in high and low speed wind tunnel tests. In the process of conducting surface oil film flow display tests, it is first necessary to evenly apply a layer of oil film with a thickness less than the local boundary layer thickness on the flat surface of the model to be tested. After the wind tunnel is turned on, the digital image or video of the oil flow map on the model surface is continuously recorded by imaging equipment. The experiment is stopped when the oil flow map is stable and no longer changes. Then the experimenter analyzes the flow conditions on the model surface based on the recorded oil flow map. The map can qualitatively reflect the local flow characteristics, such as vortex influence area, separation, stable attachment and other phenomena.

[0003] In actual experiments, experimenters often need to record a large number of oil flow map images or videos, and the flow characteristics shown by the oil flow map are mainly based on naked eye judgment, and the streamline map that specifically reflects the flow topology of the model surface is generally drawn manually by the experimenter based on the recorded images or videos. Therefore, it takes a lot of time and manpower to process the surface oil film test results. At the same time, due to the high level of human participation, the test results are highly subjective and may have large human errors. In addition, the oil flow map generally only focuses on and displays the final stable map results, and lacks in presenting its dynamic evolution process. At present, the traditional surface oil film display method is still mainly based on manual processing. There are also some new improved technologies in the industry, such as fluorescent oil flow technology, which has certain digital processing conditions, but it needs to be equipped with additional optoelectronic equipment and introduce optical calibration and other operating procedures, which makes the original advantages of surface oil film technology such as simplicity and strong robustness no longer exist. Summary of the invention

[0004] In view of the problems that the traditional surface oil film flow display method is only applicable to air flow, and the post-processing process of the test results requires much manual participation and is time-consuming and labor-intensive, the present invention proposes a surface oil film display method applicable to underwater objects, that is, a method for generating surface streamline maps of underwater objects. The present invention optimizes the underwater applicability of the specific test settings and implementation process of the traditional surface oil film flow display method, so that it can be applied to underwater scenes. In addition, the present invention also uses a multi-level grid iterative cross-correlation method to achieve full digital post-processing, which can automatically generate streamline topology maps, greatly improve test efficiency, and also make it possible to qualitatively analyze the dynamic evolution process of the oil flow map.

[0005] The technical solution of the present invention is as follows:

[0006] 1. A surface oil film display method suitable for underwater objects

[0007] Step 1: Prepare a mixed suspension suitable for underwater environment;

[0008] Step 2: In a water-free environment, evenly brush the mixed suspension on the surface of the object to form a surface oil film of uniform thickness on the surface of the object;

[0009] Step 3: placing the object coated with the surface oil film in an underwater test environment, and using a camera to continuously record the oil flow map on the surface of the object until the oil flow map is stable, thereby obtaining a digital image sequence of the oil flow map on the surface of the object;

[0010] Step 4: After preprocessing each frame image in the digital image sequence of the oil flow map on the surface of the object, a preprocessed oil flow map image sequence is obtained; and after processing the preprocessed oil flow map image sequence using a multi-level grid iterative cross-correlation method, an instantaneous oil flow vector field sequence is obtained;

[0011] Step 5: Generate a flow map of the object surface based on the instantaneous oil flow vector field sequence.

[0012] In the step 1, the mixed suspension includes a solid indicator and a carrier oil.

[0013] The particle size of the solid indicator is 10 to 60 μm.

[0014] In step 2, the method for inspecting the surface oil film of the object is as follows:

[0015] Place the object coated with oil film in an underwater test environment and let it stand for several minutes. If the bubbles on the surface of the oil film break away from the oil film and leave holes on the surface of the oil film, the surface oil film is unqualified. Re-apply the mixed suspension until a surface oil film of uniform thickness is obtained.

[0016] In step 4, the preprocessing includes cropping and grayscale enhancement.

[0017] In step 4, the pre-processed oil flow atlas image sequence is processed using a multi-level grid iterative cross-correlation method to obtain an instantaneous oil flow vector field sequence, including:

[0018] For the preprocessed oil flow atlas images of two adjacent frames, a multi-level grid iterative cross-correlation method is used to perform a cross-correlation calculation on the preprocessed oil flow atlas images of two adjacent frames to obtain the instantaneous oil flow vector field corresponding to the current two adjacent frames; after traversing and processing the preprocessed oil flow atlas images of each two adjacent frames, the instantaneous oil flow vector fields corresponding to all two adjacent frames are obtained, thereby obtaining an instantaneous oil flow vector field sequence.

[0019] The step 5 is specifically as follows:

[0020] The cumulative sum of each instantaneous oil flow vector field in the instantaneous oil flow vector field sequence is calculated to obtain the cumulative sum vector field reflecting the final flow topological structure of the object surface.

[0021] The step 5 is specifically as follows:

[0022] The cumulative sum of the first to kj instantaneous oil flow vector fields in the instantaneous oil flow vector field sequence is calculated respectively to obtain J cumulative sum vector fields reflecting the flow topological structure of the object surface, k is the block coefficient, n is the number of instantaneous oil flow vector fields, n=Jk, j=1,...,J.

[0023] 2. A storage medium

[0024] The medium stores a computer program, and when the computer program is executed by a processor, the method for displaying a surface oil film applicable to an underwater object is implemented.

[0025] The computer program is an instruction corresponding to the method for displaying the surface oil film of an underwater object.

[0026] The beneficial effects of the present invention are as follows:

[0027] 1. The present invention optimizes the underwater applicability of the test setting and specific implementation process of the traditional surface oil film flow display method, making it applicable to certain underwater tests and expanding the application scenarios of the technology;

[0028] 2. The present invention uses a multi-layer grid iterative cross-correlation algorithm to achieve full digital post-processing of surface oil film flow display test results, greatly improving test efficiency and reducing human errors caused by excessive manual participation. In addition, it can also realize the display and qualitative analysis of the dynamic evolution process of the oil flow map. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The present invention is a flow chart of the method.

[0030] Figure 2 It is a uniform oil film on the surface of the elliptical hydrofoil model.

[0031] Figure 3 There are bubbles bursting on the surface of the oil film.

[0032] Figure 4 Schematic diagram of the grid for iterative cross-correlation calculation on multiple grids.

[0033] Figure 5 Schematic diagram of the block stacking of the instantaneous oil flow vector field.

[0034] Figure 6 This is a comparison of the oil flow map on the surface of an elliptical hydrofoil before and after digital image processing.

[0035] Figure 7 is the instantaneous oil flow vector field on the elliptical hydrofoil surface.

[0036] Figure 8 It is the stable oil flow map and streamline map (i.e. overall stacking) on ​​the surface of the elliptical hydrofoil at different elliptical hydrofoil attack angles.

[0037] Fig. 9 It is the streamline map of the elliptical hydrofoil surface at different times (i.e., block stacking). DETAILED DESCRIPTION

[0038] In order to enable those skilled in the art to better understand the present invention, and thus to more clearly define the scope of protection claimed in the present invention, the present invention is described in detail with respect to certain specific embodiments and drawings of the present invention. It should be noted that the following are only certain specific implementation methods of the present invention, which are only part of the embodiments of the present invention, wherein the specific and direct description of the relevant structures is only for the convenience of understanding the present invention, and each specific feature does not naturally and directly limit the scope of implementation of the present invention. The conventional selections and substitutions made by those skilled in the art under the guidance of the present invention, as well as the reasonable arrangement and combination of several technical features under the guidance of the present invention, should all be deemed to be within the scope of protection claimed in the present invention.

[0039] The present invention proposes a surface oil film display method suitable for underwater objects, such as Figure 1 As shown, the method comprises the following steps:

[0040] Step 1: Prepare a mixed suspension suitable for underwater environment; the mixed suspension includes a solid indicator and a carrier oil. The solid indicator is a powder raw material that is insoluble in water and oil. The particle size of the solid indicator is 10 to 60 μm, so as to ensure that the movement of particles in the oil flow can be more accurately identified in the cross-correlation calculation. The carrier oil is generally a transparent organic oil with stable physical and chemical properties, certain viscosity, non-toxicity and insolubility in water. Mix the solid indicator and the carrier oil in a volume ratio of 1:5, filter out the large particles that have condensed into a mass, and obtain a fine mixed suspension oil.

[0041] Step 2: In a water-free environment, brush the mixed suspension evenly on the flat and smooth surface of the object, allowing the oil to fully spread on the surface of the model under the action of fluidity and its own surface tension, and remove excess oil to form a surface oil film of uniform thickness on the surface of the object;

[0042] In step 2, the method for checking the surface oil film of the object is as follows:

[0043] If the oil is applied too quickly and the oil is not fully spread on the surface of the model, some bubble structures will form on the surface of the oil film. When the model is placed in water, these bubbles will quickly burst or float up and separate from the oil film, leaving holes on the surface of the oil film. Figure 3 As shown, this will cause problems such as loss of local oil flow map information, which directly affects the accuracy of the test results. Therefore, it is necessary to place the object coated with oil film in an underwater test environment and let it stand for several minutes. If the bubbles on the surface of the oil film break away from the oil film and leave holes on the surface of the oil film, the surface oil film is unqualified, and the mixed suspension is re-coated until a surface oil film with uniform thickness is obtained, so as to further carry out the flow display test.

[0044] Step 3: Place the object coated with the surface oil film in an underwater test environment, and use a high-speed camera to continuously record the oil flow map on the surface of the object until the oil flow map is stable. At this time, the test is ended, and the oil flow map recording is stopped to obtain a digital image sequence of the oil flow map on the surface of the object;

[0045] Step 4: After preprocessing each frame image in the digital image sequence of the oil flow map on the surface of the object, a preprocessed oil flow map image sequence is obtained; then the preprocessed oil flow map image sequence is processed using a multi-level grid iterative cross-correlation method to obtain an instantaneous oil flow vector field sequence; preprocessing includes image processing methods such as cropping and grayscale enhancement, which are used to improve the visibility, contrast and detail expression of the image, so that the image is clearer and more accurate in subsequent analysis and processing.

[0046] Among them, after processing the pre-processed oil flow atlas image sequence using the multi-level grid iterative cross-correlation method, the instantaneous oil flow vector field sequence is obtained, including:

[0047] For the preprocessed oil flow atlas images of two adjacent frames, a cross-correlation calculation is performed using the multi-level grid iterative cross-correlation method to obtain the instantaneous oil flow vector field corresponding to the current two adjacent frames; after traversing and processing the preprocessed oil flow atlas images of each two adjacent frames, the instantaneous oil flow vector field corresponding to all two adjacent frames is obtained. The instantaneous oil flow vector field contains the displacement and direction of the oil, reflecting the movement of the oil on the model surface at different times, thereby obtaining the instantaneous oil flow vector field sequence.

[0048] like Figure 4 As shown in the figure, the main idea of ​​the multi-level grid iterative cross-correlation method is to perform cross-correlation calculations on the image layer by layer from larger grids to smaller grids, and introduce the calculation results of the previous layer when calculating the next layer of grids, so as to improve the calculation efficiency, refine the calculation results, and have good robustness. According to the approximate pixel displacement of the oil flow between two adjacent frames of digital images, a suitable calculation grid size is set, usually with 2 to 4 levels of nested grids. The figure shows a schematic diagram of the multi-level grid iterative cross-correlation calculation grid.

[0049] Step 5: Generate a flow map of the object surface based on the instantaneous oil flow vector field sequence.

[0050] Since the surface oil film method requires a relatively long time to form a stable oil flow map on the model surface, a single instantaneous oil flow vector field cannot comprehensively and accurately evaluate the flow structure of the model surface, and generally requires the stacking of several instantaneous oil flow vector fields. The present invention proposes two instantaneous oil flow vector field stacking methods, namely, overall stacking and block stacking.

[0051] Overall stacking: Calculate the cumulative sum of each instantaneous oil flow vector field in the instantaneous oil flow vector field sequence to obtain the cumulative sum vector field I that reflects the final flow topological structure of the object surface f , the formula is as follows:

[0052]

[0053] Among them, I i is the i-th instantaneous oil flow vector field, and n is the number of instantaneous oil flow vector fields.

[0054] Block stacking: Figure 5 As shown in the figure, the cumulative sum of the first to kj instantaneous oil flow vector fields in the instantaneous oil flow vector field sequence is calculated respectively to obtain J cumulative sum vector fields reflecting the flow topological structure of the object surface. k is the block coefficient, which is preferably divisible by the total number of oil flow vector fields n. The value of k is set according to specific needs. Generally, the flow map obtained by the first stacking is roughly stable and orderly. n is the number of instantaneous oil flow vector fields, n = Jk, j = 1,..., J. The J cumulative sum vector fields reflecting the flow topological structure of the object surface can be used to further qualitatively analyze the dynamic trend and characteristics of the model surface flow map evolving over time.

[0055] Taking the surface flow of elliptical hydrofoil as an example, an underwater surface oil film flow display experiment was carried out.

[0056] First, titanium dioxide with a particle size of 10 μm was selected as a solid indicator, and dimethyl silicone oil with a viscosity of 500 cs was selected as a carrier oil. The two were mixed evenly in a volume ratio of 1:5 and filtered. The filtrate was evenly applied on the smooth upper surface of the elliptical hydrofoil. Figure 2 As shown in the figure. The experimental water speed was 2m / s, the attack angles of the elliptical hydrofoil were set to 5°, 10° and 15° respectively, the resolution of the high-speed camera used to record the oil flow map was 1024×1024 pixels, the shooting frequency was 1kHz, the shooting time was 5s, and 5000 surface oil flow maps were obtained for each working condition. During the experiment, no bubbles broke or fell off on the surface of the oil film.

[0057] Next, all digital images of oil flow maps are optimized by pre-processing. First, by cropping, the oil film display area on the model surface is made to fill the entire image as much as possible, which improves the efficiency of subsequent processing and calculation processes, and reduces the influence of the non-oil film display area in the subsequent cross-correlation calculation. Next, the cropped oil flow image is subjected to grayscale enhancement transformation (here, a linear transformation of contrast stretching is used), which improves the visual effect and analyzability of the image and highlights the image details. Figure 6 (a) and Figure 6 (b) are the digital images of oil flow maps on the object surface before and after preprocessing.

[0058] Then, multi-level grid iterative cross-correlation calculations are performed on adjacent oil flow maps in sequence. In this embodiment, the combinations of calculation pairs are [1+2], [2+3], …, [4999+5000], where the cross-correlation calculations between each pair of images are used to analyze the motion characteristics of the oil flow on the model surface at a certain moment. Three levels of cross-correlation calculation grids are used here, which are 64×64 pixels, 32×23 pixels, and 16×16 pixels. The overlap rate of each level of grid is 50%, and the calculations are made in sequence according to the size of the grid. In the current embodiment, it only takes 7 minutes to complete the cross-correlation calculation of one working condition, and 4999 surface oil flow vector fields can be obtained, which greatly improves the test efficiency. The instantaneous oil flow vector field is as follows: Figure 7 As shown, the specific surface flow characteristics cannot be observed at this time and further post-processing is required.

[0059] Finally, two stacking methods are used to post-process the oil flow vector field. Overall stacking: 4999 instantaneous oil flow vector fields are accumulated and summed to obtain the stable oil flow vector field on the surface of the elliptical hydrofoil, and a streamline map that can intuitively reflect the local flow topology is drawn based on the vector field, such as Figure 8 As shown, Figure 8 (a) Figure 8 (b) and Figure 8 (c) are the stable oil flow maps and streamline maps on the elliptical hydrofoil surface at elliptical hydrofoil attack angles of 5°, 10° and 15° respectively. It can be seen that under various working conditions, the final model surface streamline map is very consistent with the corresponding oil flow map, which shows that the streamline map obtained by cross-correlation calculation and stacking post-processing of the surface oil film flow display results can accurately describe the flow state and flow structure of the model surface, proving the practicality and accuracy of the proposed post-processing method.

[0060] Taking the working condition of elliptical hydrofoil with an attack angle of 5° as an example, the instantaneous oil flow vector field is further post-processed by block stacking to show the dynamic evolution of the oil flow map on the model surface. Selecting k = 250, 4999 instantaneous oil flow vector fields can obtain about 20 cumulative sum vector fields representing different moments. Fig. 9 (a) Fig. 9 (b) Fig. 9 (c) Fig. 9 (d) Fig. 9 (e) and Fig. 9 (f) are the streamline maps corresponding to time t1, t5, t9, t13, t17 and t20 respectively. It can be seen that over time, the streamline map on the model surface gradually becomes more stable and orderly, and the characteristic flow structure becomes clearer and more visible. Therefore, the dynamic evolution process and characteristics of the flow map on the model surface can be qualitatively displayed by using the block stacking method.

[0061] Finally, it should be noted that the above embodiments and explanations are only used to illustrate the technical solution of the present invention rather than to limit it. Those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope disclosed in the technical solution of the present invention, which should be included in the scope of protection of the claims of the present invention.

Claims

1. A method for displaying oil film on the surface of an underwater object, characterized in that: The following steps are involved: Step 1: Prepare a mixed suspension suitable for underwater environment; Step 2: In a water-free environment, evenly brush the mixed suspension on the surface of the object to form a surface oil film of uniform thickness on the surface of the object; Step 3: placing the object coated with the surface oil film in an underwater test environment, and using a camera to continuously record the oil flow map on the surface of the object until the oil flow map is stable, thereby obtaining a digital image sequence of the oil flow map on the surface of the object; Step 4: After preprocessing each frame image in the digital image sequence of the oil flow map on the surface of the object, a preprocessed oil flow map image sequence is obtained; and after processing the preprocessed oil flow map image sequence using a multi-level grid iterative cross-correlation method, an instantaneous oil flow vector field sequence is obtained; Step 5: Generate a flow map of the object surface based on the instantaneous oil flow vector field sequence.

2. A surface oil film display method suitable for underwater objects according to claim 1, characterized in that: In the step 1, the mixed suspension includes a solid indicator and a carrier oil.

3. The method for displaying the surface oil film of an underwater object according to claim 1, characterized in that: The particle size of the solid indicator is 10 to 60 μm.

4. The method for displaying the surface oil film of an underwater object according to claim 1, characterized in that: In step 2, the method for inspecting the surface oil film of the object is as follows: Place the object coated with oil film in an underwater test environment and let it stand for several minutes. If the bubbles on the surface of the oil film break away from the oil film and leave holes on the surface of the oil film, the surface oil film is unqualified. Re-apply the mixed suspension until a surface oil film of uniform thickness is obtained.

5. The method for displaying the surface oil film of an underwater object according to claim 1, characterized in that: In step 4, the preprocessing includes cropping and grayscale enhancement.

6. The method for displaying the surface oil film of an underwater object according to claim 1, characterized in that: In step 4, the pre-processed oil flow atlas image sequence is processed using a multi-level grid iterative cross-correlation method to obtain an instantaneous oil flow vector field sequence, including: For the preprocessed oil flow atlas images of two adjacent frames, a multi-level grid iterative cross-correlation method is used to perform a cross-correlation calculation on the preprocessed oil flow atlas images of two adjacent frames to obtain the instantaneous oil flow vector field corresponding to the current two adjacent frames; after traversing and processing the preprocessed oil flow atlas images of each two adjacent frames, the instantaneous oil flow vector fields corresponding to all two adjacent frames are obtained, thereby obtaining an instantaneous oil flow vector field sequence.

7. The method for displaying the surface oil film of an underwater object according to claim 1, characterized in that: The step 5 is specifically as follows: The cumulative sum of each instantaneous oil flow vector field in the instantaneous oil flow vector field sequence is calculated to obtain the cumulative sum vector field reflecting the final flow topological structure of the object surface.

8. The method for displaying the surface oil film of an underwater object according to claim 1, characterized in that: The step 5 is specifically as follows: The cumulative sum of the first to kj instantaneous oil flow vector fields in the instantaneous oil flow vector field sequence is calculated respectively to obtain J cumulative sum vector fields reflecting the flow topological structure of the object surface, k is the block coefficient, n is the number of instantaneous oil flow vector fields, n=Jk, j=1,...,J.

9. A storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the method for displaying a surface oil film applicable to an underwater object as claimed in any one of claims 1 to 8 is implemented.

10. A storage medium according to claim 9, characterized in that: The computer program is an instruction corresponding to the method for displaying the surface oil film of an underwater object as described in any one of claims 1 to 8.

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