A simulation device and test method for downwelling scour

Abstract

The present invention discloses a simulation device and test method for downwelling scour, which relates to the fields of water conservancy engineering, marine engineering and civil engineering, and includes an elliptical water pipe and a half glass tube arranged outside the elliptical water pipe, the half glass tube is used to simulate a pile, an observation component is arranged inside the half glass tube, a buoyancy component is arranged above the left side of the observation component, a sand taking component is arranged below the left side of the observation component, and a circulation component is arranged above the sand taking component and the observation component. The present invention adopts the above-mentioned simulation device and test method for downwelling scour, and adopts the ratio n of the short axis to the long axis of the elliptical water pipe as the amplification factor to control the downwelling flow, realizes local scour simulation under the action of large flow rate with a small flow, and expands the flow rate range of the test; the half glass tube with a scale is convenient for the camera to shoot the scour pit inside the glass tube directly, without geometric correction of the image, improving the accuracy and efficiency of computer pattern recognition, and improving the accuracy of scour pit depth observation data.

Description

Technical Field

[0001] The invention relates to the technical fields of water conservancy engineering, marine engineering and civil engineering, and in particular to a simulation device and a test method for downwelling scouring. Background Art

[0002] Under the action of extreme hydrodynamics, local scouring around water-related structures such as cross-sea bridges, offshore wind power plants, and docks in water conservancy and marine engineering projects endangers the safety of the project, and in severe cases, causes the project to fail. According to statistics, about 60% of bridge damage incidents in the world are caused by local scouring under extreme dynamics. Local scouring research has always been a hot topic in the fields of water conservancy, marine engineering, and civil engineering. Due to the complexity of extreme hydrodynamic conditions and sediment movement, the accurate prediction of the maximum local scouring depth of water-related structures has always been a difficult problem.

[0003] The physical model of local scour around the cylinder needs to consider the influence of the side wall on the flow field and scour, which requires a larger model test site, resulting in the phenomenon that the required flow rate is too large and the test flow rate is too small, which makes it difficult to meet the requirements of local scour simulation around the cylinder under extreme flow rates. When the extreme flow rate is greater than the sediment starting flow rate, local scour of the cylinder and natural scour exist at the same time, resulting in unnecessary waste of test materials and reduced test accuracy of the maximum scour depth.

[0004] In view of the shortcomings of previous research tests, such as low flow velocity, waste of test materials and low prediction accuracy, the present invention provides a device and test method that can accurately simulate the scouring depth of downwelling, and proposes a formula for calculating the downwelling velocity distribution and flow in an elliptical water pipe, providing a technical basis for the scouring and protection of engineering foundations such as bridges and offshore wind power. Summary of the invention

[0005] The purpose of the present invention is to provide a simulation device and test method for downwelling scour, so as to solve the problems existing in existing research and tests, such as low flow velocity, waste of test materials and low prediction accuracy.

[0006] To achieve the above-mentioned purpose, the present invention provides a simulation device and test method for downwelling scouring, comprising an elliptical water pipe and a half glass pipe arranged outside the elliptical water pipe, the half glass pipe is used to simulate a pile, an observation component is arranged inside the half glass pipe, a buoyancy component is arranged above the left side of the observation component, a sand taking component is arranged below the left side of the observation component, and a circulation component is arranged above the sand taking component and the observation component;

[0007] The sand collecting component includes a sand collecting funnel, and the inlet diameter l of the sand collecting funnel satisfies the following relationship:

[0008]

[0009] Where: hc is the overflow water depth on the buoyancy baffle, H is the vertical distance from the water surface downstream of the buoyancy baffle to the water inlet, and H S is the thickness of the topdressing sand, Δh is the water level difference between upstream and downstream, ω min is the minimum settling velocity of non-uniform sand, g represents the gravitational acceleration of free fall, and α is taken as ≥1.5.

[0010] Preferably, the observation assembly includes a camera arranged on the outside of the half glass tube and an observation box arranged on the inside of the half glass tube, a sand layer is laid in the observation box, and a water layer is arranged above the sand layer.

[0011] Preferably, the buoyancy assembly includes a buoyancy baffle arranged on the left side of the observation box, and a buoyancy ball 1 and a buoyancy ball 2 are symmetrically arranged above the buoyancy baffle.

[0012] Preferably, a sand taking pipeline is provided below the sand taking funnel, an upper valve is provided at the upper opening of the sand taking pipeline, and a lower valve is provided at the lower opening of the sand taking pipeline;

[0013] One side of the sand taking pipeline is connected to a water filling pipe, and the water filling pipe is connected to a tap water pipe. The other side of the sand taking pipeline is connected to an exhaust pipe, and the height of the exhaust outlet of the exhaust pipe is higher than the height of the observation box.

[0014] Preferably, the circulation component comprises a circulation water pipe, a variable frequency pump and a flow meter are arranged in the circulation water pipe, and the flow meter is connected to the variable frequency pump;

[0015] The circulating water pipe is arranged as an L-shaped structure, the vertical outlet end of the circulating water pipe is arranged as a water inlet, the horizontal outlet end of the circulating water pipe is connected to the elliptical water pipe through an elliptical conversion head, the outlet cross-section of one end of the elliptical conversion head connected to the circulating water pipe is arranged as a circle, and the outlet cross-section of one end of the elliptical conversion head connected to the elliptical water pipe is arranged as an ellipse.

[0016] Preferably, the variable frequency pump, the flow meter and the camera are all electrically connected to a control system.

[0017] Preferably, the half glass tube is provided with a scale, and the half glass tube is fixedly arranged on the side wall of the observation box.

[0018] Preferably, the buoyancy F generated by the first and second floats v Slightly larger than half of the underwater weight G of the buoyancy baffle. The specific formula is as follows:

[0019] 2F v ≥G=(ρ 1 -ρ 2 )whδ

[0020] Where: 1 is the density of the buoyancy baffle, ρ is the water density, w is the width of the buoyancy baffle, h is the height of the buoyancy baffle, and δ is the thickness of the buoyancy baffle.

[0021] A test method for a downwelling scour simulation device comprises the following steps:

[0022] Step 1: Lay the test sand in the observation box so that the thickness of the sand layer is H. s Equal to the estimated maximum local scour depth d s 1.2 times; the initial height of the buoyancy baffle is higher than the thickness of the sand layer, open the upper valve, and slowly inject water into the sand taking assembly from the water filling pipe. When it exceeds the top of the sand layer, the buoyancy baffle rises to the set height h under the action of buoyancy so that:

[0023]

[0024] Where: H is the vertical distance from the water surface downstream of the buoyancy baffle to the water inlet, Q is the flow rate of the downflow in the elliptical water pipe, g is the gravity acceleration of free fall, and w is the width of the buoyancy baffle;

[0025] Step 2: Calculate the flow rate Q of the descending flow in the elliptical water pipe by the following formula, compare it with the flow meter reading, and adjust the variable frequency pump through the computer to make the two approximately equal;

[0026]

[0027] Where: β is the proportionality coefficient, U is the average flow velocity, n is the flow rate or flow velocity amplification factor, Γ is the gamma function, D is the diameter of half of the glass tube, and H is the vertical distance from the water surface downstream of the buoyancy baffle to the water inlet;

[0028] Step 3: Use a camera to shoot the thickness change of the sand layer outside the half glass tube, display the depth of the scour pit in real time based on image recognition technology, and calculate the relative change value δ of the scour depth. When δ is less than 10 -2 The test ends when the scour pit depth is recorded during the test. The scour pit depth finally displayed and recorded is the maximum scour depth d caused by the downwelling. S ;

[0029] Step 4: During the downflow scouring test, the upper valve is kept open, and when the test is finished, it is closed, and the lower valve is opened to take out the model sand in the sand taking assembly;

[0030] Step 5: Close the lower valve and fill the sand extraction pipeline with water to discharge the gas;

[0031] Step 6: When the sand extraction pipe is full of water, open the upper valve and repeat the above steps.

[0032] Therefore, the present invention adopts a downflow scouring simulation device and test method of the above structure, which has the following beneficial effects:

[0033] (1) The present invention uses the ratio n of the short axis to the long axis of the elliptical water pipe as an amplification factor to control the downflow flow rate, realizes local scouring simulation under the action of large flow rate with a smaller flow rate, expands the flow rate range of the test, and reduces energy consumption.

[0034] (2) The present invention provides a half glass tube with a scale, which is convenient for the camera to shoot the scour pit inside the glass tube. There is no need to perform geometric correction on the image, which improves the accuracy and efficiency of computer pattern recognition, thereby significantly improving the accuracy of scour pit depth observation data.

[0035] (3) The downflow scouring test method of the present invention establishes the downflow velocity distribution and flow calculation formula in the elliptical water pipe, quantitatively reflects the relationship between the downflow and the incoming flow and the size of the cylinder, and can accurately calculate the flow required for downflow scouring.

[0036] (4) The present invention ensures that the model sand settles in the funnel through the calculation formula of the inlet diameter of the sand collecting funnel, and the floating baffle can control the static water depth, thereby reducing the test cost.

[0037] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a front cross-sectional view of an embodiment of a downwelling scour simulation device and a test method of the present invention;

[0039] Figure 2 It is a rear cross-sectional view of an embodiment of a downwelling scour simulation device and a test method of the present invention;

[0040] Figure 3 A top view of an observation box of a downwelling scour simulation device and a test method of the present invention;

[0041] Figure 4 It is a schematic diagram of the scale of elliptical water pipe and cylinder;

[0042] Figure 5 Decomposition of the force and radial force of the incoming flow on the cylinder;

[0043] Figure 6 It is a schematic diagram of the relationship between the downward force of the elliptical water pipe and the cylinder;

[0044] Figure 7 is the geometric relationship between the downflow and the incoming flow;

[0045] Figure numerals: 1. elliptical water pipe; 2. half glass tube; 3. observation component; 31. camera; 32. observation box; 33. sand layer; 34. water layer; 4. buoyancy component; 41. buoyancy baffle; 42. float one; 43. float two; 5. sand taking component; 51. sand taking funnel; 52. sand taking pipeline; 53. upper valve; 54. lower valve; 55. water filling pipe; 56. exhaust pipe; 6. circulation component; 61. circulating water pipe; 62. frequency conversion pump; 63. flow meter; 64. elliptical conversion head; 7. control system. DETAILED DESCRIPTION

[0046] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0047] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0048] Example

[0049] See also Figure 1-7 The present invention provides a simulation device and test method for downwelling scouring, comprising an elliptical water pipe 1 and a half glass pipe 2 arranged outside the elliptical water pipe 1, wherein the half glass pipe 2 is used to simulate a circular pile column and is convenient for video observation. An observation component 3 is arranged inside the half glass pipe 2, a buoyancy component 4 is arranged above the left side of the observation component 3, a sand collection component 5 is arranged below the left side of the observation component 3, and a circulation component 6 is arranged above the sand collection component 5 and the observation component 3.

[0050] The scale of the elliptical water pipe 1 is designed according to the principle that the downflow is the largest on the frontal surface of the cylinder center and gradually decreases on both sides. Assuming that the major axis 2a is the same as the cylinder diameter D, the ratio of the minor axis to the major axis is:

[0051] n=b / a (n≤1) (1)

[0052] Where: b is the minor semi-axis. When n is equal to 1, the ellipse becomes a circular tube.

[0053] The oval water pipe 1 is close to the half glass pipe 2, and the outlet of the oval water pipe 1 is below the water surface, with a height H from the original bed surface. e Push-to-adjust:

[0054]

[0055] Where: H is the depth of incoming water, The value is determined by experiment and is generally The average is 0.12.

[0056] The observation assembly 3 includes a camera 31 arranged outside the half glass tube 2 and an observation box 32 arranged inside the half glass tube 2. A sand layer 33 is laid inside the observation box 32, and a water layer 34 is arranged above the sand layer 33. The camera 31 is placed outside the half glass tube 2 to observe the morphological changes of the scouring pit inside the half glass tube 2 in real time.

[0057] The buoyancy assembly 4 includes a buoyancy baffle 41 disposed on the left side of the observation box 32 , and a float 1 42 and a float 2 43 are symmetrically disposed above the buoyancy baffle 41 .

[0058] The buoyancy F generated by the float 1 42 and the float 2 43 v Slightly greater than half of the underwater weight W of the buoyancy baffle 41, the specific formula is as follows:

[0059] 2F v ≥G=(ρ 1 -ρ 2 )whδ (3)

[0060] Where: 1 is the density of the buoyancy baffle, ρ is the water density, w is the width of the buoyancy baffle, h is the height of the buoyancy baffle, and δ is the thickness of the buoyancy baffle.

[0061] The sand taking component 5 includes a sand taking funnel 51 connected to the lower left part of the observation box 32, a sand taking pipeline 52 is arranged below the sand taking funnel 51, an upper valve 53 is arranged at the upper end opening of the sand taking pipeline 52, and a lower valve 54 is arranged at the lower end opening of the sand taking pipeline 52;

[0062] One side of the sand taking pipeline 52 is connected to a water filling pipe 55, which is connected to a tap water pipe to fill water into the sand taking pipeline 52. The other side of the sand taking pipeline 52 is connected to an exhaust pipe 56, and the height of the exhaust outlet of the exhaust pipe 56 is higher than the height of the observation box 32.

[0063] The inlet diameter l of the sand collecting funnel 51 satisfies the following relationship:

[0064]

[0065] Where: h cis the overflow water depth on the buoyancy baffle, H is the vertical distance from the water surface downstream of the buoyancy baffle to the water inlet, and H S is the thickness of the topdressing sand, Δh is the water level difference between upstream and downstream, ω min is the minimum settling velocity of non-uniform sand, g represents the gravity acceleration of free fall, which is taken as 9.8, and α ≥ 1.5 to ensure that the time required for the horizontal movement of sediment particles is much longer than the time required for sedimentation, so that the sediment settles in the sand collecting funnel.

[0066] The circulation component 6 includes a circulation water pipe 61 , in which a variable frequency pump 62 and a flow meter 63 are arranged. The flow meter 63 is connected to the variable frequency pump 62 . The function of the variable frequency pump 61 is to maintain water circulation.

[0067] The circulating water pipe 61 is set as an L-shaped structure, the vertical outlet end of the circulating water pipe 61 is set as a water inlet, the horizontal outlet end of the circulating water pipe 61 is connected to the elliptical water pipe 1 through an elliptical conversion head 64, the outlet cross-section of one end connecting the elliptical conversion head 64 and the circulating water pipe 61 is set as a circle, and the outlet cross-section of one end connecting the elliptical conversion head 64 and the elliptical water pipe 1 is set as an ellipse.

[0068] The variable frequency pump 62 , the flow meter 63 and the camera 31 are all electrically connected to the control system 7 . The control system 7 controls the circulation component 6 and the observation component 3 .

[0069] A scale is provided on the half glass tube 2, and the half glass tube 2 is fixedly arranged on the side wall of the observation box 32. The half glass tube 2 is arranged to simulate a bridge pier or an offshore wind power pile, and the depth change of a local scour pit can be directly observed.

[0070] The test method involves the flow formula for descending flow in an elliptical water pipe, and the derivation principle is as follows:

[0071] Due to the obstruction of the cylinder to the water flow, part of the positive pressure, that is, the radial component N(θ), is converted into a downward force N d (θ), causing the change of water flow momentum and generating downflow, so the downward force N d The relationship between (θ) and the radial force N(θ) is:

[0072] N d (θ)=βN(θ) (5)

[0073] Where: θ is the radial angle between the incoming flow velocity and the cylinder; β is the proportionality coefficient, that is, the ratio of the downward force to the positive pressure on the frontal surface. According to the test, β∈[1 / 25,2 / 25], the average is 3 / 50.

[0074] Step 1: If Figure 4 As shown in the figure, the force of the unit width flow on the cylindrical frontal surface is decomposed into the positive pressure N(θ):

[0075]

[0076] Where: λ is the friction coefficient, ρ is the water density, H is the incoming water depth, and θ is the radial angle between the direction of the average incoming flow velocity U and the cylinder.

[0077] The relationship between the average flow velocity U and the radial component u(θ) is:

[0078] u(θ)=Ucosθ (7)

[0079] Step 2: Convert the partial positive pressure N(θ) into the downflow force N d (θ), substituting equations (6) and (7) into equation (5), we obtain:

[0080]

[0081] Where: λ is the friction coefficient, ρ is the water density, H is the incoming water depth, θ is the radial angle between the direction of the average incoming velocity U and the cylinder, and β is the proportionality coefficient, that is, the ratio of the downward force to the positive pressure on the headwind surface. According to the test, β∈[1 / 25,2 / 25], the average is 3 / 50.

[0082] Step 3: Establish the pressure P(θ) per unit width and the average flow velocity u of the elliptical water pipe along the long axis d (θ)

[0083]

[0084] Where: λ is the friction coefficient, l(θ) is the x-direction water flow width in the elliptical water pipe, and ρ is the water density.

[0085] From the ellipse equation we get

[0086]

[0087] Where: λ is the friction coefficient, ρ is the water density, n is the amplification factor of flow rate or flow velocity, and D is the diameter of half of the glass tube.

[0088] Step 4: Based on Figure 5 The geometric relationship shown in the figure shows that the cylinder is subjected to a downward force N d (θ) is equal to the pressure P(θ) per unit width of the elliptical water pipe 1 along the long axis. The pressure P(θ) of the elliptical water pipe along the long axis is related to the average flow velocity u d (θ) satisfies the following relationship:

[0089]

[0090] Where: λ is the friction coefficient, ρ is the water density, l(θ) is the x-direction water flow width in the elliptical water pipe.

[0091] From the ellipse equation

[0092]

[0093] get

[0094]

[0095] Furthermore, substituting formula (10) into formula (9) yields:

[0096]

[0097] Where: λ is the friction coefficient, ρ is the water density, n is the amplification factor of flow rate or flow velocity, and D is the diameter of half of the glass tube.

[0098] From equation (8) and equation (11), we can get:

[0099]

[0100] Further, the y-direction distribution of the average flow velocity in the x-direction of the elliptical water pipe can be obtained as:

[0101]

[0102] For the case of θ=0°, we can further obtain:

[0103]

[0104] Step 5: Obtain the downflow flow rate through integration, as follows:

[0105] The flow integral in the elliptical tube is

[0106]

[0107] Further based on Figure 6 Geometric relations

[0108]

[0109]

[0110] Further substituting equation (16) into equation (15), the required flow rate for simulating downwelling scouring can be obtained as:

[0111]

[0112] The downwelling flow calculation formula is then:

[0113]

[0114] Where: β is the proportional coefficient, U is the average flow velocity, n is the flow rate or flow velocity amplification factor, Γ is the gamma function, D is the diameter of half of the glass tube, and H is the vertical distance from the water surface downstream of the buoyancy baffle to the water inlet.

[0115] Simulated downwelling velocity V d According to the flow rate Q and the ellipse area A d The ratio is obtained, and the expression is:

[0116]

[0117] In the formula: In the formula: β is the proportional coefficient, U is the average flow velocity, n is the amplification factor of the flow rate or flow velocity, Γ is the gamma function, D is the diameter of half of the glass tube, and H is the vertical distance from the water surface downstream of the buoyancy baffle to the water inlet.

[0118] A test method for a downwelling scour simulation device comprises the following steps:

[0119] Step 1: Lay the test sand in the observation box 32 so that the thickness of the sand layer is H s Equal to the estimated maximum local scour depth d s 1.2 times; the initial height of the buoyancy baffle 41 is slightly higher than the thickness of the sand layer, the upper valve 53 is opened, and water is slowly injected into the sand taking assembly 5 from the water filling pipe 56. When it exceeds the top of the sand layer 33, the buoyancy baffle 41 rises to the set height h under the action of buoyancy so that:

[0120] h+h c =H (20)

[0121]

[0122]

[0123] Where: h c is the overflow water depth on the buoyancy baffle, H is the vertical distance from the water surface downstream of the buoyancy baffle to the water inlet, Q is the flow rate of the downflow in the elliptical water pipe, g is the gravity acceleration of free fall, and w is the width of the buoyancy baffle.

[0124] Step 2: Calculate the flow rate Q of the downflow in the elliptical water pipe 1 by formula (18), compare it with the reading of the flow meter 63, and adjust the variable frequency pump 62 by computer to make the two approximately equal;

[0125] Step 3: Use the camera 31 to shoot the thickness change process of the sand layer 33 outside the half glass tube 2, display the depth of the scouring pit in real time based on the image recognition technology, and calculate the relative change value δ of the scouring depth. When δ is less than 10 -2 The test ends when the scour pit depth is recorded during the test. The scour pit depth finally displayed and recorded is the maximum scour depth d caused by the downwelling.s ;

[0126] Step 4: During the downflow scouring test, the upper valve 53 is kept open, and is closed when the test is finished, and the lower valve 54 is opened to take out the model sand in the sand taking component 5;

[0127] Step 5: Close the lower valve 54 and fill the sand taking pipe 52 with water to discharge the gas;

[0128] Step 6: When the sand taking pipeline 52 is full of water, open the upper valve 53 and repeat the above steps.

[0129] Therefore, the present invention adopts the above-mentioned downflow scour simulation device and test method, and controls the downflow flow rate by using the ratio n of the short axis to the long axis of the elliptical water pipe as the amplification factor, so as to realize local scour simulation under the action of large flow rate with a smaller flow rate, expand the flow rate range of the test, and reduce energy consumption; by setting a half glass tube with a scale, it is convenient for the camera to shoot the scour pit inside the glass tube, without the need for geometric correction of the image, thereby improving the accuracy and efficiency of computer pattern recognition, thereby significantly improving the accuracy of scour pit depth observation data; through the downflow scour test method, the downflow velocity distribution and flow calculation formula in the elliptical water pipe are established, which quantitatively reflects the relationship between the downflow and the incoming flow and the size of the cylinder, and can accurately calculate the flow required for downflow scour; through the calculation formula of the inlet diameter of the sand taking funnel, it is ensured that the model sand settles in the funnel, and the floating baffle can control the static water depth and reduce the test cost.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A downwelling scouring simulation device, characterized in that: It comprises an elliptical water pipe and a half glass tube arranged outside the elliptical water pipe, the half glass tube is used to simulate a pile, an observation component is arranged inside the half glass tube, a buoyancy component is arranged above the left side of the observation component, a sand taking component is arranged below the left side of the observation component, and a circulation component is arranged above the sand taking component and the observation component; The sand collecting assembly comprises a sand collecting funnel, the inlet diameter of the sand collecting funnel is Satisfies the following relationship: ； Where: is the overflow water depth on the buoyancy baffle, is the depth from the top of the sand layer to the water surface. is the vertical distance from the water surface downstream of the buoyancy baffle to the circular outlet of the circulating water pipe, is the thickness of the sand, is the upstream and downstream water level difference, is the minimum settling velocity of non-uniform sand, represents the gravitational acceleration of free fall, take ; The observation assembly includes a camera arranged outside the half glass tube and an observation box arranged inside the half glass tube, a sand layer is laid in the observation box, and a water layer is arranged above the sand layer; The buoyancy assembly comprises a buoyancy baffle plate arranged on the left side of the observation box, and a buoyancy ball 1 and a buoyancy ball 2 are symmetrically arranged above the buoyancy baffle plate; A sand taking pipeline is arranged below the sand taking funnel, an upper valve is arranged at the upper opening of the sand taking pipeline, and a lower valve is arranged at the lower opening of the sand taking pipeline; One side of the sand taking pipeline is connected to a water filling pipe, the water filling pipe is connected to a tap water pipe, and the other side of the sand taking pipeline is connected to an exhaust pipe, the height of the exhaust pipe outlet is higher than the height of the observation box; The circulation assembly includes a circulation water pipe, in which a variable frequency pump and a flow meter are arranged, and the flow meter is connected to the variable frequency pump; The circulating water pipe is arranged as an L-shaped structure, the vertical outlet end of the circulating water pipe is arranged as a water inlet, the horizontal outlet end of the circulating water pipe is connected to the elliptical water pipe through an elliptical conversion head, the outlet cross-section of one end of the elliptical conversion head connected to the circulating water pipe is arranged as a circle, and the outlet cross-section of one end of the elliptical conversion head connected to the elliptical water pipe is arranged as an ellipse.

2. A downwelling scouring simulation device according to claim 1, characterized in that: The variable frequency pump, the flow meter and the camera are all electrically connected to a control system.

3. A downwelling scouring simulation device according to claim 2, characterized in that: The half glass tube is provided with a scale, and the half glass tube is fixedly arranged on the side wall of the observation box.

4. A test method for a downwelling scour simulation device as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: Step 1: Lay the test sand in the observation box to make the sand layer thickness Equal to the estimated maximum local scour depth The initial height of the buoyancy baffle is higher than the thickness of the sand layer. Open the upper valve and slowly inject water into the sand taking component from the water filling pipe. When it exceeds the top of the sand layer, the buoyancy baffle rises to the set height under the action of buoyancy. So that: ； Where: is the depth from the top of the sand layer to the water surface. is the flow rate of the downward flow in the elliptical water pipe, is the gravitational acceleration of free fall, is the width of the buoyancy baffle; Step 2: Calculate the flow rate of the downward flow in the elliptical water pipe by the following formula: , compare it with the flow meter reading, and adjust the variable frequency pump through the computer to make the two approximately equal; ； Where: is the proportionality coefficient, is the average flow velocity, is the amplification factor of flow rate or velocity, which is the ratio of the short axis to the long axis of the elliptical water pipe. is the gamma function, is the diameter of half of the glass tube, The depth from the top of the sand layer to the water surface; Step 3: Use a camera to shoot the thickness change of the sand layer outside the half glass tube, display the depth of the scour pit in real time based on image recognition technology, and calculate and display the relative change value of the scour depth. ,when Less than The test ends when the scour pit depth is recorded during the test. The scour pit depth finally displayed and recorded is the maximum scour depth caused by the downflow. ; Step 4: During the downflow scouring test, the upper valve is kept open, and when the test is finished, it is closed, and the lower valve is opened to take out the model sand in the sand taking assembly; Step 5: Close the lower valve and fill the sand extraction pipeline with water to discharge the gas; Step 6: When the sand extraction pipe is full of water, open the upper valve and repeat the above steps.

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