Method for forecasting vortex-induced vibration of armored towing cable

Abstract

The invention provides a method for forecasting vortex-induced vibration of an armored towing cable. The installation and use modes comprise an underwater platform or a water surface platform, the boundary constraint in the storage, release and recovery process of the armored towing cable, the navigational speed and the like. The equivalent bending rigidity, the torsional rigidity, the elastic modulus and the like are obtained through a sample cable test in a laboratory. And performing grid division on each segment according to boundary conditions, and performing step-by-step calculation to obtain the inclination angle and the bending condition of the towrope. In actual use, the real-time forecast of the frequency of the vortex-induced vibration of the armored towing cable can be realized only by inputting the navigational speed and the cable length. The towing cable has the advantages that a traditional two-dimensional columnar rigid structure is abandoned, and the defect that the towing cable flexibility characteristic and the fluid-solid coupling influence are not considered is overcome; a simplified towing cable mechanical parameter model is obtained through laboratory testing; by considering boundary conditions and inclination angles and bending forms at different lengths of the towrope, vortex-induced vibration calculation is more accurate; real-time forecasting, calculation efficiency and instantaneity of the towing cable vortex-induced vibration under different towing conditions can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of sonar, and mainly relates to a method for predicting vortex-induced vibration of armored towed cables. Background Art

[0002] Sonar is a detection, identification, positioning, navigation, and communication system that uses underwater acoustic information. It can be classified into shipborne sonar of surface ships (including hull sonar and towed sonar), submarine-borne sonar, airborne sonar, portable sonar, and shore-based sonar according to the equipped object; it can be divided into passive sonar and active-passive sonar according to the working mode. Towed sonar is further subdivided into towed vehicle sonar (active emission working mode) and towed line array sonar (passive reception working mode) according to the characteristics of the array structure. Towed sonar generally consists of a towed vehicle / line array receiver, a towed cable, a winch device, and electronic equipment. The towed vehicle sonar releases the towed vehicle to a specified depth to emit active sound waves to detect underwater targets, and the towed line array receiver is used for identification. The weight of the towed vehicle ranges from several hundred kilograms to more than ten tons. For towed sonars with extremely large weights, the towed cable is generally an armored towed cable, with a core wire and a sheath inside and armored steel wires wrapped outside to achieve the purpose of withstanding large gravity loads. During towing, periodic vortex shedding occurs after the fluid flows past the towed cable, causing the towed cable to be subjected to periodic changes in the force from the fluid, resulting in "lock-in" resonance and a significant increase in the vibration amplitude of the towed cable. If the towed cable is in resonance for a long time, the service life of the towed cable will be greatly shortened, and the possibility of damage and failure will increase. Once an accident occurs, it may cause equipment damage, economic losses, and even casualties. At present, there are many studies on vortex-induced vibration of relatively rigid columnar structures such as marine risers, and empirical formulas for predicting frequencies have been obtained. However, for towed cables with a certain degree of flexibility, it is mainly based on experimental tests to guide the design and production of towed cables. However, theoretical calculations of vortex-induced vibration are also in progress. The continuous model and the random vibration theory are common methods for vortex-induced vibration, but they can only calculate the maximum response in the lock-in region. At present, finite element is mostly used for calculation, but there are problems such as large computational volume. Especially for towed cables that are not rigid bodies and have deformations, there is no systematic, scientific, reasonable, feasible, and relatively accurate prediction method in engineering.

[0003] At present, the method for predicting vortex-induced vibration of armored towed cables has the following deficiencies and characteristics: (1) Predicting vortex-induced vibration based on two-dimensional columnar rigid structures mainly obtains frequencies and does not fully consider the flexible characteristics of towed cables and the influence of fluid-structure interaction; (2) When calculating by the finite element fluid-structure interaction method, due to the huge scale of the modeling mesh division of towed cables with a large slenderness ratio, the calculation is complex and time-consuming, and it cannot meet the calculation requirements under multi-variable calculation conditions. (3) The vortex-induced vibration of towed cables is affected by factors such as flow velocity and cable length, and therefore real-time prediction cannot be achieved. Therefore, finding a method for quickly predicting the vortex-induced vibration of heavy-duty towed cables is an important issue. Summary of the Invention

[0004] In view of the above problems, the object of the present invention is to propose a method for predicting the vortex-induced vibration of an armored towing cable, obtaining the main mechanical performance parameters of the armored towing cable through the test of a sample cable, and segmenting and meshing the towing cable by simplifying the constraint boundary conditions according to the installation condition of the armored towing cable, so as to realize the prediction of the vortex-induced vibration of the armored towing cable considering the inclination angle and bending state.

[0005] The object of the present invention is achieved by the following technical solutions. A method for predicting the vortex-induced vibration of an armored towing cable includes the following steps:

[0006] Step 1: Obtain the installation and use mode of the towed sonar, and segment by simplifying the constraint boundary conditions according to the installation condition of the armored towing cable; the installation and use mode includes an underwater platform or a water surface platform, the boundary constraints during the storage, release and recovery process of the armored towing cable, the ship speed, etc.

[0007] Step 2: Obtain the basic parameters of the armored towing cable, including: outer diameter, density, release length, internal composition and component characteristics.

[0008] Step 3: Test and obtain the equivalent mechanical properties of the armored towing cable; obtain the equivalent bending stiffness, torsional stiffness, elastic modulus, etc. through the test of the sample cable in the laboratory.

[0009] Step 4: Test the drag coefficient at different inclination angles;

[0010] Step 5: According to the boundary conditions, perform mesh division on the segmented towing cable and calculate the cable shape step by step;

[0011] Step 6: Calculate the vortex-induced vibration frequency according to the cable array shape; calculate step by step to obtain the inclination angle and bending condition of the towing cable, and give a more accurate vortex-induced vibration frequency.

[0012] Step 7: Repeat Steps 1 to 6 to obtain the vortex-induced vibration frequency of the entire towing cable with different cable lengths C1 and different towing speeds V1. During prediction, only the ship speed and cable length need to be input to realize the real-time prediction of the vortex-induced vibration frequency of the armored towing cable.

[0013] Further, in Step 1, the towed sonar is composed of a sonar tow body, an armored towing cable, a steering wheel, and a winch and is installed on the water surface ship platform. When the ship platform sails, the towing speed is denoted as V1, the flow velocity range is denoted as Vmin to Vmax, the winch releases the armored towing cable, the cable length is denoted as C1, the cable length range is denoted as C1min to C1max, the sonar tow body is released into the water at the stern of the ship and reaches the specified depth, the depth is denoted as D1, and the towing cable is inclined at a certain angle under the impact of the fluid and generates vortex-induced vibration; identify and determine that the number of boundary constraint points of the armored towing cable is n, and divide the towing cable into several segments according to the boundary constraints of the armored towing cable, which are respectively denoted as L1, L2, L3...

[0014] Further, in the second step, when the cable is placed horizontally naturally, the outer diameter of the armored towing cable is measured no less than three times, and the average value is recorded as the standard outer diameter D1 of the armored towing cable; the cable density is defined as the weight per unit length of the cable, denoted as M1, with the unit of kg / m.

[0015] Further, in the third step, a test method for obtaining the equivalent mechanical properties of the armored towing cable in the laboratory is adopted, including the following steps:

[0016] 1) Cut a cable sample segment no less than 2 meters long, fix both ends with clamps, and conduct a tensile test within the elastic deformation range. Apply a tensile force along the length direction of the cable at one end of the cable. Calculate according to the formula E1 = F / A, where E1 is the longitudinal elastic modulus, F is the applied tensile force, and A is the cross-sectional area. Obtain A according to the circular area formula A = PI*(D1 / 2) 2 Obtained;

[0017] 2) Cut a cable sample segment no less than 2 meters long, fix both ends with clamps, and conduct a torsion test within the elastic deformation range. Apply a torque parallel to the cross-section at one end. Calculate according to the formula K = M / A, where K is the torsional stiffness, M is the applied torsional moment, and A is the angle of torsion of one cross-section relative to the other cross-section;

[0018] 3) Cut a cable sample segment no less than 1 meter long, fix both ends with clamps, and conduct a bending test within the elastic deformation range. Apply a bending moment parallel to the cross-section at one end of the cable. Calculate according to the formula E2 = F / I, where E2 is the elastic modulus during bending and I is the moment of inertia of the cross-section. Obtain I from I = PI*(D1) 4 / 64.

[0019] Further, in the fourth step, the following steps are included: Take a cable sample segment no less than 2 meters long, fix both ends with clamps, place it vertically at different inclination angles, denoted as Aq, and let water flow through the cable at different flow rates V1 within the range of Vmin to Vmax. Obtain the set {Fw} of the water flow impact force Fw obtained by the clamp through a mechanical sensor; Obtain the set {r} of the fluid damping coefficient r through the formula Fw = r×V1×V1. It is a second-order matrix related to Aq and V1.

[0020] Further, in the fifth step, the following steps are included: According to the equivalent mechanical parameters and damping coefficients obtained in the third and fourth steps, conduct grid division on L1, ……, L3 in terms of length, and the number of divisions is denoted as NL1, NL2…… respectively; Establish the mechanical equilibrium relationship of the cable gravity, cable tension Fd, and fluid impact force Fw for each grid segment of the grid division, obtain the inclination angle denoted as Ac, and perform step-by-step calculations to obtain the overall cable shape. The set of angles between each grid segment and the vertical direction is denoted as {Ac3-1, ……, Ac3-NL3}, where the inclination angle refers to the angle between the grid segment and the vertical direction.

[0021] Further, in the sixth step, the following steps are included: for different grid segments located in water with different angles with the vertical direction, the equivalent flow velocities causing vortex-induced vibration in the lateral direction of the towing cable are different. Calculate the fluid component vectors affecting the lateral vortex-induced vibration, and then calculate the vortex shedding frequencies at various lengths of the entire towing cable according to the empirical vortex shedding formula f = St×v / D.

[0022] Due to the adoption of the above technical solutions, the present invention has the following advantages: (1) Abandon the traditional simplification to a two-dimensional columnar rigid structure, avoiding the disadvantages of not considering the flexible characteristics of the towing cable and the influence of fluid-structure interaction; (2) Obtain a simplified mechanical parameter model of the towing cable through laboratory tests; (3) According to the towing characteristics of the armored towing cable, considering the boundary conditions, the inclination angles and bending forms at different lengths of the towing cable, the calculation of vortex-induced vibration is more accurate; (4) It can realize the real-time prediction of the vortex-induced vibration of the towing cable under different towing conditions, with high calculation efficiency and timeliness. Therefore, it can be widely used in the field of sonar technology and can also be used in other fields. Brief Description of the Drawings

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art or ordinary technicians, without creative efforts, other drawings can be obtained based on these drawings.

[0024] Figure 1 Schematic diagram of the use of an armored towing cable and a sonar.

[0025] Figure 2 Schematic diagram of the segmented armored towing cable. Detailed Embodiments

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0027] As shown in the figure, the present invention proposes a method for predicting the vortex-induced vibration of an armored towing cable, including the following steps:

[0028] Step 1, obtain the installation and use methods of the towed sonar. Such as Figure 1As shown in the figure, the towed sonar consists of a sonar tow body 1, an armored tow cable 2, a steering wheel 3, and a winch 4, and is installed on a surface ship platform 5. When the ship platform is sailing, the towing speed (i.e., the ship's speed through the water) is denoted as V1, and the flow velocity range is denoted as Vmin to Vmax. The winch releases the tow cable, and the cable length is denoted as C1, and the cable length range is denoted as C1min to C1max. The sonar tow body is released into the water at the stern of the ship and reaches the specified depth, denoted as D1. The tow cable is subjected to fluid impact and presents a certain inclination angle, and vortex-induced vibration is generated. The number of boundary constraint points of the armored tow cable is identified and determined to be n. In this example, n = 4. The winch is denoted as R1, the steering wheel is denoted as R2, the water surface line is denoted as R3, and the sonar tow body is denoted as R4. According to the boundary constraints of the armored tow cable, the tow cable is divided into several segments, denoted as L1, L2, L3... As Figure 2 shown. The present invention proposes a simplified and general installation and use method for towed sonars, improving usability.

[0029] Step 2: Obtain the basic parameters of the armored tow cable. The armored tow cable generally consists of an outer layer of armored steel wires, a middle layer of elastic sheath, an inner layer of copper wires, an optical fiber core wire, etc. The present invention proposes a method for measuring the outer diameter of the armored tow cable, that is, measuring the outer diameter of the armored tow cable in a natural flat state not less than 3 times and taking the average value as the standard outer diameter D1 of the armored tow cable. The cable density is defined as the weight per unit length of the cable, denoted as M1, and the unit is kg / m.

[0030] Step 3: Test and obtain the equivalent mechanical properties of the armored tow cable. The present invention proposes a test method for obtaining the equivalent mechanical properties of the armored tow cable in the laboratory. This method simplifies the complex tow cable into a parameter input in a physical sense. The specific steps are as follows: 1) Cut a tow cable sample section not less than 2 meters, fix both ends with special fixtures, and conduct a tensile test within the elastic deformation range. Apply a tensile force along the length direction of the tow cable at one end of the tow cable. Calculate according to the formula E1 = F / A, where E1 is the longitudinal elastic modulus, F is the applied tensile force, and A is the cross-sectional area. Obtain it according to the circular area formula A = PI*(D1 / 2) 2 2) Cut a tow cable sample section not less than 2 meters, fix both ends with special fixtures, and conduct a torsion test within the elastic deformation range. Apply a torque parallel to the cross-section at one end. Calculate according to the formula K = M / A, where K is the torsional stiffness, M is the applied torsional moment, and A is the angle of torsion of one end relative to the cross-section of the other end. 3) Cut a tow cable sample section not less than 1 meter, fix both ends with special fixtures, and conduct a bending test within the elastic deformation range. Apply a bending moment parallel to the cross-section at one end of the tow cable. Calculate according to the formula E2 = F / I, where E2 is the elastic modulus during bending and I is the moment of inertia of the cross-section. Obtain I from I = PI*(D1) 4 / 64.

[0031] Step 4: Resistance coefficient test at different inclination angles. Take a tow cable sample section not less than 2 meters, fix both ends with clamps, place it vertically at different inclination angles, and record the inclination angle as Aq, generally in the range of 90° - 45°. Let different flow velocities V1 in the range of Vmin to Vmax flow through the tow cable, and obtain the set {Fw} of the water flow impact force Fw obtained by the clamp through a mechanical sensor. Through the formula Fw = r×V1×V1, obtain the set {r} of the fluid damping coefficient r, which is a second-order matrix related to Aq and V1. The present invention proposes a method for measuring the fluid resistance coefficient at different inclination angles and flow velocities under laboratory conditions, and evaluates from two parameter variables.

[0032] Step 5: Perform mesh division on the tow cable segments and calculate the tow cable shape step by step. According to the equivalent mechanical parameters and damping coefficients obtained in Steps 3 and 4, perform mesh division on L1, ……, L3 in terms of length. The mesh length is not less than 20 times the outer diameter of the tow cable, and the number of divisions is recorded as NL1, NL2, …… respectively. The tow cables L1 in the water surface air and L2 above the water surface are not in the water and there is no vortex-induced vibration, so they can be not analyzed. Taking L3 as an example, the mesh division is shown in the figure. The segmented meshes are respectively recorded as L3-1, ……, L3-NL3. Each segment is affected by the tow cable tension and fluid impact, resulting in inclination. The tow cable tension is recorded as Fd = the gravity of the towing body in the water, and the water flow impact is recorded as Fw = f(D1, V1, r). Fw is a function of the outer diameter of the tow cable, the flow velocity, and the fluid damping coefficient, and can be calculated by the classical fluid mechanics Morison formula. Establish the mechanical equilibrium relationship of the tow cable gravity, the tow cable tension Fd, and the fluid impact force Fw for each mesh segment, and obtain the inclination angle (the angle between the mesh segment and the vertical direction) recorded as Ac, and perform step-by-step calculation to obtain the entire tow cable shape. The set of the angles between each mesh segment and the vertical direction is recorded as {Ac3-1, ……, Ac3-NL3}.

[0033] Step 6: Calculate the vortex-induced vibration frequency according to the cable array shape. For different mesh segments of L3, the angles with the vertical direction are different, that is, {Ac3-1, Ac3-NL3}, then the equivalent flow velocities causing vortex-induced vibration in the transverse direction of the tow cable are different, and after calculation, they are respectively {V1×sin(Ac3-1), ……, V1×sin(Ac3-NL3)}. According to the empirical vortex shedding formula f = St×v / D, where St is the Strouhal number, taking 0.2 for a cylinder, v is the flow velocity, and D is the diameter. Then, the vortex shedding frequencies at different cable lengths of the entire tow cable can be calculated to obtain the vortex-induced vibration shedding frequencies at different cable length positions.

[0034] Step 7: Repeat Steps 1 to 6, and the vortex-induced vibration frequencies of the entire tow cable at different cable lengths C1 and different towing speeds V1 can be obtained. During prediction, selection can be made according to different towing conditions (C1 and V1) of the actual ship.

[0035] The present invention is also applicable to armored towing cables or general towing cables of towed arrays with manned or unmanned surface ships and submarines as platforms.

[0036] As shown above, the above embodiments are only used to illustrate the present invention, and the steps and expressions therein can be changed. Any equivalent transformation and improvement based on the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A method for predicting vortex-induced vibration of an armored tow cable, characterized in that: The steps include: Step 1: Obtain the installation and use methods of the towed sonar, and simplify the constraint boundary condition segmentation according to the installation conditions of the armored tow cable; Step 2: Obtain basic parameters of the armored tow cable; Step 3: Test and obtain the equivalent mechanical performance of the armored tow cable; Step 4: Drag coefficient test at different tilt angles; Step 5: According to the boundary conditions, the towline is divided into segments and the towline shape is calculated step by step; Step 6: Calculate the vortex-induced vibration frequency according to the cable array shape; Step seven, repeat steps one to six to obtain the vortex-induced vibration frequency of the entire tow cable at different cable lengths C1 and different towing speeds V1. When forecasting, it can be selected according to different towing conditions of the actual ship.

2. The method for predicting vortex-induced vibration of an armored tow cable according to claim 1, characterized in that: In the step 1, the towed sonar is composed of a sonar tow body, an armored tow cable, a steering wheel, and a retractable winch and is installed on a surface ship platform. When the ship platform is sailing, the towing speed is recorded as V1, and the flow velocity range is recorded as Vmin~Vmax. The retractable winch releases the armored tow cable, and the cable length is recorded as C1, and the cable length range is recorded as C1min~C1max. The sonar tow body is released into the water at the stern and reaches a specified depth, and the depth is recorded as D1. The tow cable presents a certain inclination angle due to the impact of the fluid and generates vortex-induced vibration; the number of boundary constraint points of the armored tow cable is identified and determined as n, and the tow cable is divided into several sections according to the boundary constraints of the armored tow cable, which are recorded as L1, L2, L3...

3. The method for predicting vortex-induced vibration of an armored tow cable according to claim 2, characterized in that: In the step 2, the outer diameter of the armored tow cable is measured in a naturally flat state for no less than 3 times, and the average value is recorded as the standard outer diameter D1 of the armored tow cable; the density of the tow cable is defined as the weight per unit length of the cable, recorded as M1, and the unit is kg / m.

4. The method for predicting vortex-induced vibration of an armored tow cable according to claim 3, characterized in that: In step 3, a test method for obtaining the equivalent mechanical performance of the armored towed cable in a laboratory is used, including the following steps: 1) Cut a cable sample section of no less than 2 meters, fix both ends with a clamp, and perform a tensile test within the elastic deformation range. Apply tension along the length of the cable at one end of the cable, and calculate according to the formula E1 = F / A, where E1 is the longitudinal elastic modulus, F is the loading tension, and A is the cross-sectional area. According to the circle area formula A = PI*(D1 / 2) 2 get; 2) Cut a tow cable sample section of not less than 2 meters, fix both ends with a clamp, and perform a torsion test within the elastic deformation range. Apply a torque parallel to the cross section at one end, and calculate it by the formula K=M / A, where K is the torsional stiffness, M is the applied torsional moment, and A is the angle of torsion of one end relative to the cross section of the other end; 3) Cut a cable sample of not less than 1 meter, fix both ends with a clamp, and perform a bending test within the elastic deformation range. Apply a bending moment parallel to the cross section at one end of the cable. Calculate according to the formula E2 = F / I, where E2 is the elastic modulus during bending, I is the moment of inertia of the section, and I = PI*(D1) 4 / 64.

5. The method for predicting vortex-induced vibration of an armored tow cable according to claim 4, characterized in that: The step four includes the following steps: taking a streamer sample section of not less than 2 meters, fixing both ends with clamps, placing it vertically at different inclination angles, where the inclination angle is denoted as Aq, flowing through the streamer at different flow velocities V1 in the range of Vmin to Vmax, and obtaining a set {Fw} of water flow impact forces Fw obtained by the clamp through a mechanical sensor; obtaining a set {r} of fluid damping coefficients r through the formula Fw=r×V1×V1, which is a second-order matrix related to Aq and V1.

6. The method for predicting vortex-induced vibration of an armored tow cable according to claim 5, characterized in that: The step five includes the following steps: according to the equivalent mechanical parameters and damping coefficients obtained in the steps three and four, L1, ..., L3 are gridded in length, and the number of divisions is recorded as NL1, NL2, ...; for each grid segment, a mechanical equilibrium relationship among the tow cable gravity, the tow cable tension Fd, and the fluid impact force Fw is constructed, and the inclination angle is obtained and recorded as Ac, and the entire tow cable shape is obtained by step-by-step calculation, and the set of angles between each grid segment and the vertical direction is recorded as {Ac3-1, ..., Ac3-NL3}, wherein the inclination angle refers to the angle between the grid segment and the vertical direction.

7. The method for predicting vortex-induced vibration of an armored tow cable according to claim 6, characterized in that: The step six includes the following steps: for different grid segments in the water, the angles between them and the vertical direction are different, so the equivalent flow velocity that causes vortex-induced vibration in the lateral direction of the towline is different, and the fluid component vector that affects the lateral vortex-induced vibration is calculated, and then the vortex release frequency at each length of the entire towline is calculated by the empirical vortex release formula f=St×v / D.

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