Visual measurement-based underwater cable three-dimensional vortex-induced vibration experimental device and method

Abstract

The invention belongs to the technical field of underwater cable vibration measurement, and discloses an underwater cable three-dimensional vortex-induced vibration experiment device and method based on visual measurement, and the device comprises a wave flow water tank, a wave and current generator, a cable model, a target spot, and a data collection device. The current generator is arranged at one end in the wave flow water tank, the cable model is arranged in the outlet direction of the current generator, the target spot is arranged on the cable model, and the data acquisition device is arranged on the side face of the cable model and used for acquiring vibration data of the cable model. According to the method, the position of the cable model is calibrated by setting the target spot, the coordinate change of the target spot is captured by the visual capture device, visual measurement data can be extracted by using a computer program, and the method is not in contact with the cable model and does not influence a flow field around the cable model. Meanwhile, the visual catcher is immersed in water, light refraction is avoided, measurement errors are reduced, and the result is more accurate.

Description

Technical Field

[0001] The invention belongs to the technical field of underwater cable vibration measurement, and in particular relates to an underwater cable three-dimensional vortex-induced vibration experimental device and method based on visual measurement. Background Art

[0002] The underwater terrain and water flow environment are complex and changeable. Different types of suspended sections often appear under the cable due to scouring caused by water waves. When water and waves pass through the suspended section of the cable, alternating vortices will form behind the cable. As the vortices continue to fall off, the cable will also be affected by the alternating lift in the cross-flow direction. When the vortex shedding frequency, that is, the frequency of the lift on the cable, is close to the first-order natural frequency of the cable, a "locking" phenomenon will occur. When the frequency is locked, the vibration amplitude of the cable in the cross-flow direction increases suddenly, and continuous vibration is likely to cause fatigue damage to the cable.

[0003] Compared with pipeline experiments in real water environments, the study of underwater pipeline vortex-induced vibration by constructing a model experiment in an experimental water tank reduces the experimental cost and cycle, and can also intuitively obtain the dynamic characteristics of the pipeline's vortex-induced vibration. At present, most vibration tests in underwater pipeline vortex-induced vibration model experiments at home and abroad rely on contact devices such as strain gauges and accelerometers. These instruments are close to the surface of the pipeline and will change the flow field environment around the pipeline and the properties of the pipeline itself as the pipeline vibrates, such as density, elastic modulus, Poisson's ratio, wall roughness and other physical parameters that have a key impact on pipeline vibration. In addition, in order to simplify the model, most vortex-induced vibration experimental studies limit the pipeline movement to the vertical range, and often ignore the horizontal movement of the pipeline.

[0004] However, for underwater cables, which are slender and relatively large flexible structures, cables with significant water weight and normal resistance are prone to drooping, and the horizontal and vertical bidirectional coupling strength is large during the vortex-induced vibration process, and have uneven multi-modal dynamic response. At the same time, the cable is a flexible structure composed of multiple layers of materials, and there are many factors affecting its vibration, among which the bending stiffness is a factor that cannot be ignored. This also makes the displacement and strain of the vibration of each layer of the cable inconsistent. Compared with submarine pipelines and risers, the vortex-induced vibration characteristics of the cable are more complicated. The use of contact measurement will greatly affect the vortex emission process around the cable, destroy the original flow field distribution, and also cause the intrinsic parameters of the cable such as the natural frequency, elastic modulus, and bending stiffness to deviate from the experimental design values, thereby affecting the experimental results. Summary of the invention

[0005] The purpose of the present invention is to provide an underwater cable three-dimensional vortex-induced vibration experimental device and method based on visual measurement, so as to solve the problem that the use of contact measurement will greatly affect the vortex emission process around the cable, destroy the original flow field distribution, and also cause the cable's inherent parameters such as natural frequency, elastic modulus, and bending stiffness to deviate from the experimental design values, thereby affecting the experimental results.

[0006] To achieve the above object, the present invention adopts the following technical solutions: An underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement includes a wave and flow flume, a wave and flow maker, a cable model, a target point and a data acquisition device; the flow maker is arranged at one end of the wave and flow flume, the cable model is arranged at the outlet direction of the flow maker, the target point is arranged on the cable model, and the data acquisition device is arranged on the side of the cable model for collecting vibration data of the cable model.

[0007] Furthermore, the output end of the wave and flow maker is connected to a flow guide for evenly distributing the water flow.

[0008] Furthermore, a wave-breaking device is provided at the other end of the wave flow tank to eliminate reflected waves.

[0009] Furthermore, the cable model is fixed to the side of the deflector through a model bracket, and red rectangular targets are evenly arranged on both sides of the cable model horizontally and vertically.

[0010] Furthermore, the data acquisition device is two waterproof high-frame rate cameras, one waterproof high-frame rate camera is facing the middle position below the cable model, and is used to observe the lateral vibration of the cable; the other waterproof high-frame rate camera is at the same horizontal height as the cable model and is arranged facing the cable model.

[0011] Furthermore, a wave height meter and a flow velocity instrument are also provided in the wave flow tank. Both the wave height meter and the flow velocity instrument are immersed in the water of the wave flow tank. The wave height meter is used to measure the height of the wave surface. The flow velocity meter is at the same horizontal height as the cable model and is used to measure the flow velocity of the water in the wave flow tank.

[0012] The experimental method of the underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement includes the following steps: Construct cable models; The target image data collected by the experimental device is used to perform image enhancement; The visually enhanced image is calibrated, and the actual horizontal and vertical displacement distances of the cable model are obtained based on the pixel conversion of the target center displacement.

[0013] Furthermore, a cable model is constructed, including: The cable prototype stiffness is calculated based on the cable prototype, and the test scale is set to obtain the model wall thickness under similar bending stiffness of the cable prototype and the cable model. A cable mold is made based on the model wall thickness, and then a weight is poured into the cable model, and both ends of the cable are sealed to obtain a cable model based on similar bending stiffness. Where: The model wall thickness 𝛿 with similar bending stiffness is: Taking 𝜌s, 𝑔, and 𝐷 as basic quantities, and applying the 𝜋 theorem to perform dimensional analysis, we obtain the similarity criterion π of the cable bending stiffness;

[0014] The bending stiffness ratio is obtained from the bending stiffness similarity criterion

[0015] In the formula, 𝜆 EI is the bending stiffness ratio, 𝜆 ρ is the density scale, where 𝜆 ρ =1, 𝜆 g is the acceleration scale, and the gravity is similar under 𝜆 g =1, 𝜆 l is a length scale; Cable model bending stiffness K EIm Calculated by the following formula:

[0016] In the formula, 𝐸 m is the elastic modulus of the model material; 𝐷 is the outer diameter of the model; 𝛿 is the wall thickness of the model.

[0017] From the formula:

[0018] In the formula, 𝐾 EIp is the cable prototype bending stiffness, 𝐾 EIm is the bending stiffness of the cable model; We can further obtain the model wall thickness 𝛿 under similar bending stiffness: .

[0019] Further, image enhancement: The steps of using the Retinex algorithm to enhance the image are as follows: first, perform Gaussian filtering on the original cable visual image to obtain a blurred image; then, perform cumulative calculations on the blurred images at each scale; finally, perform weighted averaging on the accumulated results according to the weights to obtain an enhanced cable visual image.

[0020] Furthermore, based on the pixel conversion of the target center displacement, the actual horizontal and vertical displacement distances of the cable model are obtained: The function is used to return the pixel values ​​of the target template and the corresponding search area. The moment invariant eigenvalue of each target template is obtained through algebraic operation. The moment invariant eigenvalue is then compared in the corresponding search and identification area to obtain the position of the target point that matches the template. The number of pixels displaced by the center of the target point is converted into the actual length, thereby calculating the actual horizontal and vertical distances of the cable.

[0021] Compared with the prior art, the present invention has the following technical effects: The present invention calibrates the position of the cable model by setting the target point, and uses the visual capturer to capture the coordinate change of the target point, and can use a computer program to extract visual measurement data. This method does not contact the cable model and does not affect the flow field around the cable model. At the same time, the visual capturer is immersed in water, which avoids light refraction, reduces measurement errors, and makes the results more accurate.

[0022] The present invention overcomes the problem that the traditional experimental method only measures the vertical movement of the vortex-induced vibration pipeline. The device can realize the test of the three-dimensional vortex-induced vibration of the seabed, and can measure the vertical and lateral displacement, vibration frequency and other physical quantities of the suspended tunnel truncation model, which is closer to the actual vortex-induced vibration behavior of the cable.

[0023] The present invention manufactures a cable model based on the principle of similar bending stiffness. The cable model can achieve the effect of similar structural elastic modulus by controlling the thickness of the model tube wall to meet the similar bending stiffness (ie, EI similarity).

[0024] The image data is processed by image enhancement. The present invention processes visual data based on Retinex theory and improves the visual effect of the image by simulating the human visual perception mechanism. The algorithm can effectively improve the brightness, contrast and color of the image, making the image clearer and more vivid. This means that in the experiment, the brightness, contrast and color of the image can be effectively improved, making the image clearer and more vivid, thereby realizing effective monitoring of cable vortex-induced vibration displacement. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the test flow chart of the present invention Figure 2 Schematic diagram of the test device of the present invention Figure 3 This is a partial diagram of the cable model and measuring device of the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described below in conjunction with the accompanying drawings: Example 1, please refer to Figure 2The present invention discloses an underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement, comprising a wave and flow flume, a wave and flow maker 1, a cable model 3, a target 4 and a data acquisition device; the flow maker 1 is arranged at one end in the wave and flow flume, the cable model 3 is arranged at the outlet direction of the flow maker 1, the target 4 is arranged on the cable model 3, and the data acquisition device is arranged on the side of the cable model 3 for collecting vibration data of the cable model 3.

[0027] Embodiment 2, the present invention discloses an underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement, specifically: Depend on Figure 2 As shown, a three-dimensional vortex-induced vibration experimental device for underwater cables based on visual measurement includes a wave and flow flume, a wave and flow maker 1, a guide 2, a cable model 3, a target 4, a model bracket 5, waterproof high-frame rate cameras 6 and 8, a wave height meter 7, a current meter 9, and a wave breaking device 10.

[0028] The wave flow water trough at least includes a rectangular section, and the wave flow water trough of the rectangular section is slender, that is, the ratio of length to width is large, and the wave maker 1 and the wave breaking device 10 are respectively arranged at both ends of the wave flow water trough of the rectangular section, and the wave maker 1 is connected to the guide 2, and the wave maker 1 is used to create waves in the wave flow water trough, the guide 2 is used to evenly distribute water flow, and the wave breaking device 10 is used to eliminate reflected waves of waves.

[0029] like Figure 3 As shown, the cable model based on bending stiffness made in Case 1 is installed on the model bracket 5, and the connection between the two is consolidation. Red rectangular target points 4 are evenly arranged on both sides of the cable model horizontally and vertically.

[0030] like Figure 2 and Figure 3 As shown, the wave height meter 7 and the current meter 9 are both immersed in the water of the wave and flow tank. The waves and currents generated by the wave and flow maker 1 can act on the cable model 3 during the movement. The wave height meter 7 is used to measure the height of the wave surface. The current meter 9 is at the same horizontal height as the cable model 3 and is used to measure the flow velocity of the water in the wave and flow tank.

[0031] The waterproof high frame rate cameras 6 and 8 are monocular cameras, both immersed in water. The waterproof high frame rate camera 6 is directly opposite to the middle position below the cable model 3, and is used to observe the lateral vibration of the cable. The waterproof high frame rate camera 8 is at the same level as the cable model 3 and is arranged directly opposite to the cable model 3.

[0032] The test starts the wave maker 1, turns on the wave height meter 7 and the current meter 9 to monitor the state of water particles around the cable, and turns on the waterproof high frame rate cameras 6 and 8 to monitor the visual information of the cable.

[0033] By setting the target point to calibrate the position of the cable model, the coordinate change of the target point is captured by the visual capturer, and the visual measurement data can be extracted using a computer program. This method does not contact the cable model and does not affect the flow field around the cable model. At the same time, the visual capturer is immersed in water, which avoids light refraction, reduces measurement errors, and makes the results more accurate.

[0034] Example 3: Making a cable model The cable is a composite structure composed of main conductor, conductor shield, XLPE insulation, inner sheath, optical fiber unit, armor layer and other materials. Each layer of material has a certain influence on the mechanical properties of the cable. For the cable model test, the complexity of the cable structure also makes it difficult to design the cable model, especially to replicate the bending stiffness of the cable. The components that maintain the bending and tensile resistance of the cable are the inner conductor and the outer armor layer. Therefore, the cable model takes the inner conductor and the armor layer as the main considerations. At the same time, in order to ensure that the model test and the prototype are sufficiently similar, the model test should meet the similarity of hydraulic conditions and structural dynamics, that is, hydroelasticity. Structural dynamic similarity means that the pipeline structure follows the elastic similarity relationship (usually the material density ratio 𝜆ρ=1 and the pipeline elastic modulus ratio 𝜆E equal to the geometric ratio 𝜆l). Elastic similarity can ensure that the vibration regularity of the cable prototype and the model is similar. Since it is difficult to find materials that meet the elastic similarity of the cable structure, the cable model can achieve the effect of similar structural elastic modulus by controlling the thickness of the model pipe wall to meet the similarity of bending stiffness (that is, EI similarity).

[0035] Taking 𝜌s, 𝑔, and 𝐷 as basic quantities and applying the 𝜋 theorem for dimensional analysis, we can obtain the similarity coefficient π of the cable bending stiffness.

[0036]

[0037] The bending stiffness ratio is obtained from the bending stiffness similarity criterion

[0038] In the formula, 𝜆 EI is the bending stiffness ratio, 𝜆 ρ is the density scale, where 𝜆 ρ =1, 𝜆 g is the acceleration scale, and the gravity is similar under 𝜆 g =1, 𝜆l is the length scale.

[0039] Cable model bending stiffness K EIm Calculated by the following formula:

[0040] In the formula, 𝐸 mis the elastic modulus of the model material; 𝐷 is the outer diameter of the model; 𝛿 is the wall thickness of the model.

[0041] From the formula we can get:

[0042] In the formula, 𝐾 EIp is the cable prototype bending stiffness, 𝐾 EIm is the bending stiffness of the cable model We can further obtain the model wall thickness 𝛿 under similar bending stiffness:

[0043] Based on the specific parameters of the cable prototype that have been determined, we first conducted a detailed calculation and analysis, which was mainly based on the stiffness characteristics of the cable prototype. On this basis, we carefully set the scale required for the test, which is crucial for the precise construction of the subsequent model. Next, we began to select the most suitable cable model material based on the set scale and the stiffness requirements of the prototype. This selection process fully considers many factors such as the physical properties of the material, processing feasibility, and cost-effectiveness to ensure that the final model can accurately reflect the mechanical properties of the prototype.

[0044] After selecting the material, we can use the mathematical formula derived earlier to accurately calculate the wall thickness 𝛿 required for the model while maintaining similar bending stiffness. This calculation process strictly follows the similarity principle to ensure that the bending behavior of the model under stress is consistent with the prototype.

[0045] Subsequently, in order to simulate the weight distribution and settlement characteristics of the cable in actual use, we poured a specific proportion of sand weight into the cable model. During the pouring process, we used the method of manual vibration to ensure that the sand weight could be evenly and densely filled into the model, thereby effectively avoiding the problem of voids or uneven density caused by incomplete filling. Through this step, we successfully achieved the equivalence of the model and the prototype in the density ratio 𝜆ρ, that is, 𝜆ρ=1, laying a solid foundation for subsequent model tests.

[0046] Finally, in order to ensure the integrity and sealing of the model during the test, we carefully sealed both ends of the cable model. This treatment not only effectively prevented the leakage of the internal material of the model, but also ensured that the model could maintain a stable structural form when subjected to force, thereby accurately simulating the mechanical response of the cable prototype in a real environment.

[0047] In summary, by strictly controlling the thickness of the model tube wall to meet the similar bending stiffness (i.e., similar EI), we successfully constructed a cable model that is highly similar to the prototype in terms of structural elastic modulus. This model not only provides a reliable physical basis for subsequent experimental research, but also provides strong support for our in-depth understanding of the mechanical properties of cables and their application in practical engineering.

[0048] Embodiment 4, the cable visual signal processing is mainly divided into two parts: visual image enhancement and vibration image conversion into displacement signal.

[0049] (1) Visual image enhancement Two high-speed cameras take 100 frames of pictures per second respectively, and enhance the visual pictures based on the Retinex theory. Around the 1970s, Land et al. found that under different lighting conditions, the human eye can produce almost consistent color perception. This property is called color constancy. They speculated that this constancy is the result of the joint processing of the retina (Retina) and the cerebral cortex (Cortex), so they combined the two English words into "Retinex" and proposed the Retinex theory to simulate the mechanism of the human visual system (HVS).

[0050] In the present invention, the Retinex theory adopts the path model as follows: For each color perception system, if the intensity of the light it receives is represented by light emittance I (also called brightness), the reflectivity of the object surface for the corresponding wavelength of light is recorded as R, and the illuminance is recorded as B, then the color intensity perceived by each channel can be considered to be obtained by the following simplified model:

[0051] Where x is the coordinate point.

[0052] Given a normalized digital image I, any path can be selected from a point y on the image to the target point x. Assume that the corresponding brightness values ​​of two adjacent pixels on the path are I(xk) and I(xk+1), respectively, and the ratio is

[0053] Then along this path, the relative brightness value at x can be obtained as

[0054] Where: n represents the number of pixels in the path; δϵ is the threshold function.

[0055] To reduce the impact of noise, multiple such paths can be calculated, and the final relative brightness value at x can be expressed as

[0056] If the threshold mechanism δϵ is ignored, it can be simplified to

[0057] The steps of using the Retinex algorithm to enhance the image are as follows: first, perform Gaussian filtering on the original cable visual image to obtain a blurred image; then, perform cumulative calculations on the blurred images at each scale; finally, perform weighted averaging on the accumulated results according to the weights to obtain an enhanced cable visual image.

[0058] (2) Vibration image converted into displacement signal The visually enhanced image is calibrated by a computer and related programs. The function is used to return the pixel values ​​of the target template and the corresponding search area. The moment invariant eigenvalue of each target template is obtained through algebraic operations. The moment invariant eigenvalues ​​are then compared in the corresponding search and identification area to obtain the position of the target point that matches the template. The number of pixels displaced by the center of the target point is converted into the actual length, thereby calculating the actual horizontal and vertical distances of the cable.

[0059] The high-speed camera in this experiment takes grayscale images. The probability density distribution function f(i, j) can be used to characterize the grayscale information of each two-dimensional digital image. f(i, j) is the grayscale level of the pixel. For a two-dimensional digital image with M×N pixels, the p+q order geometric moment mpq is defined as:

[0060] If an image is regarded as a thin plate, and f(i,j) is the density of the corresponding coordinate point, then the zero-order moment of the image represents the mass of the thin plate, which is

[0061] The first-order moment of the image can represent the center of gravity of the thin plate, which is

[0062] If the coordinate origin is moved to the center of gravity, the image center moment is obtained, which is independent of the target position and is

[0063] The central moment of the image is translation and rotation invariant. By normalizing it, we can obtain the normalized central moment with constant scale.

[0064] By comparing multiple invariant moment features of the target image, the position of the same image in the new image after transformation can be determined, thereby obtaining the displacement of the cable model in vortex-induced vibration.

[0065] In summary, a cable model is made, and after processing the visual signal, the three-dimensional displacement of the vortex-induced vibration of the cable model under the action of waves and currents is obtained, and then the flow field information recorded by the wave height meter 7 and the flow velocity instrument 9 is analyzed, which is the complete underwater cable three-dimensional vortex-induced vibration experimental device and method based on visual measurement.

[0066] The present invention is deeply rooted in the Retinex theory, the core of which is to simulate the unique perception mechanism of the human visual system, and use this as a basis for fine-tuning visual data. Through a carefully designed algorithm flow, the present invention can not only deeply analyze the light distribution and color composition in the image, but also intelligently adjust key visual elements such as image brightness, contrast and color. Specifically, the algorithm can dynamically balance the light and dark areas in the image, so that the overly dark or overly bright parts can be optimized, presenting a more natural brightness transition; at the same time, it can also enhance the contrast between different colors in the image, making the details more prominent and the colors more full and vivid.

[0067] In actual application, the effects of this series of optimization measures are particularly significant. In multiple strictly controlled experiments, the present invention has demonstrated excellent image processing capabilities, whether it is image brightness improvement, contrast enhancement, or accurate color restoration, all reaching remarkable levels. This not only greatly improves the visual quality of the image, making the image content clearer and more discernible, but also lays a solid foundation for subsequent image analysis and processing.

[0068] What is particularly important is that the successful application of this invention has also extended to a broader field - the effective monitoring of cable vortex-induced vibration displacement. In marine engineering, the vortex-induced vibration of cables caused by the action of water flow is a complex and critical issue. Accurate monitoring of its displacement changes is essential to ensure cable safety and the smooth progress of marine activities. By optimizing image processing technology, the present invention makes the monitoring of cable vortex-induced vibration displacement more accurate and reliable, providing strong technical support for the safe operation and maintenance of marine engineering. Therefore, this invention not only has profound theoretical value, but also shows broad application prospects and huge practical value.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. The three-dimensional vortex-induced vibration experimental device of underwater cables based on visual measurement is characterized by: The invention comprises a wave flow flume, a wave and flow generator (1), a cable model (3), a target point (4) and a data acquisition device; the wave and flow generator (1) is arranged at one end of the wave flow flume, the cable model (3) is arranged in the outlet direction of the wave and flow generator (1), the target point (4) is arranged on the cable model (3), and the data acquisition device is arranged on the side of the cable model (3) for collecting vibration data of the cable model (3).

2. The underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 1 is characterized in that: The output end of the wave and flow generator (1) is connected to a flow guide (2) for evenly distributing the water flow.

3. The underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 1 is characterized in that: A wave absorbing device (10) is provided at the other end of the wave flow tank, for eliminating reflected waves.

4. The underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 2 is characterized in that: The cable model (3) is fixed to the side of the deflector (2) through a model bracket (5), and red rectangular target points (4) are evenly arranged on both sides of the cable model (3) horizontally and vertically.

5. The underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 1 is characterized in that: The data acquisition device comprises two waterproof high frame rate cameras (6), one waterproof high frame rate camera (6) is located directly in the middle position below the cable model (3) and is used to observe the lateral vibration of the cable; the other waterproof high frame rate camera (6) is at the same level as the cable model (3) and is arranged directly in front of the cable model (3).

6. The underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 1 is characterized in that: A wave height meter (7) and a flow velocity instrument (9) are also provided in the wave flow water tank. The wave height meter (7) and the flow velocity instrument (9) are both immersed in the water of the wave flow water tank. The wave height meter (7) is used to measure the height of the wave surface. The flow velocity meter (9) is at the same horizontal height as the cable model (3) and is used to measure the flow velocity of the water in the wave flow water tank.

7. The experimental method of the underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement is characterized by: The experimental device according to any one of claims 1 to 6 comprises the following steps: Construct cable models; The target image data collected by the experimental device is used to perform image enhancement; The visually enhanced image is calibrated, and the actual horizontal and vertical displacement distances of the cable model are obtained based on the pixel conversion of the target center displacement.

8. The experimental method of the underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 7 is characterized in that: Build a cable model, including: The cable prototype stiffness is calculated based on the cable prototype, and the test scale is set to obtain the model wall thickness under similar bending stiffness of the cable prototype and the cable model. A cable mold is made based on the model wall thickness, and then a weight is poured into the cable model, and both ends of the cable are sealed to obtain a cable model based on similar bending stiffness. Where: The model wall thickness 𝛿 with similar bending stiffness is: Taking 𝜌s, 𝑔, and 𝐷 as basic quantities, and applying the 𝜋 theorem to perform dimensional analysis, we obtain the similarity criterion π of the cable bending stiffness; The bending stiffness ratio is obtained from the bending stiffness similarity criterion In the formula, 𝜆 EI is the bending stiffness ratio, 𝜆 ρ is the density scale, where 𝜆 ρ =1, 𝜆 g is the acceleration scale, and the gravity is similar under 𝜆 g =1, 𝜆 l is a length scale; Cable model bending stiffness K EIm Calculated by the following formula: In the formula, 𝐸 m is the elastic modulus of the model material; 𝐷 is the outer diameter of the model; 𝛿 is the wall thickness of the model; From the formula: In the formula, 𝐾 EIp is the cable prototype bending stiffness, 𝐾 EIm is the bending stiffness of the cable model; We can further obtain the model wall thickness 𝛿 under similar bending stiffness: 。 9. The experimental method of the underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 7 is characterized in that: Image Enhancement: The steps of using the Retinex algorithm to enhance the image are as follows: first, perform Gaussian filtering on the original cable visual image to obtain a blurred image; then, perform cumulative calculations on the blurred images at each scale; finally, perform weighted averaging on the accumulated results according to the weights to obtain an enhanced cable visual image.

10. The experimental method of the underwater cable three-dimensional vortex-induced vibration experimental device based on visual measurement according to claim 7 is characterized in that: Based on the pixel conversion of the target center displacement, the actual horizontal and vertical displacement distances of the cable model are obtained: The function is used to return the pixel values ​​of the target template and the corresponding search area. The moment invariant eigenvalue of each target template is obtained through algebraic operation. The moment invariant eigenvalue is then compared in the corresponding search and identification area to obtain the position of the target point that matches the template. The number of pixels displaced by the center of the target point is converted into the actual length, thereby calculating the actual horizontal and vertical distances of the cable.

Images