CN107480334A - A kind of three joint machine fish hydrodynamics method for numerical simulation - Google Patents

Abstract

The invention discloses a numerical simulation method for hydrodynamics of a three-joint robotic fish. The selected calculation area of the invention is 10 times that of the three-joint bionic robotic fish. The grid division adopts a structure tetrahedron, and the boundary condition adopts a multi-reference model method. Digital simulation simulation analysis. It can be concluded that within the cycle, the pressure is concentrated on the head and both sides at its maximum, and when t=1/4T and t=3/4T, that is, when the tail flicks to the maximum extent, the pressure received is the largest, and the water flow at the tail is also the largest. The force is the largest; according to the simulation result data, the cause of the actual head damage was found, and the specific location of the damage appeared in the front of the mouth, about 1/3 of the front. Three support frames are designed at the front 1/3 of the inner side to improve the phenomenon of head damage.

Description

A numerical simulation method for hydrodynamics of a three-joint robotic fish

technical field

The invention relates to the technical field of bionic robotic fish, in particular to a numerical simulation method for hydrodynamics of a three-joint robotic fish.

Background technique

Computational fluid dynamics (CFD) is to use the existing electronic computer to carry out numerical simulation of the actual model, to analyze the physical phenomena such as fluid flow and heat transfer, to obtain the calculation results, and then to guide the actual model. CFD technology has developed rapidly since the last century. Through the analysis of the flow characteristics in the fluid through the server that can handle a huge amount of data, CFD technology enables people to predict the current situation of the flow field movement in the real model through analysis on the computer, and obtain output data and process the data. Using the theoretical knowledge of fluid mechanics and related CFD simulation software Fluent for analysis can provide deeper theoretical research and faster tools for the swimming research of robotic fish, and can drive the theoretical research of robotic fish.

Contents of the invention

The purpose of the present invention is to provide a numerical simulation method for hydrodynamics of a three-joint robotic fish.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A numerical simulation method for hydrodynamics of a three-joint robotic fish, comprising the following steps:

The first step is to establish a physical model; the three-joint robotic fish includes a fish head, a fish body and a fish tail, the fish body includes a fixed part and a swing part, and a control mechanism for controlling the movement posture of the robot fish and wireless charging is installed in the fixed part. The swinging part includes three main driving joints, which are the first main driving joint, the second main driving joint and the third main driving joint arranged in sequence along the body length of the fish. One end of the fixed part is fixedly connected with the fish head, and the other end is connected with the second One main driving joint is fixedly connected, the length ratio of the first main driving joint, the second main driving joint and the third main driving joint is 1:1:1.2;

Each of the main driving joints includes a steering gear and a driving rod, one end of the driving rod is connected to the output shaft of the steering gear, and the other end is fixedly connected to the steering gear of the adjacent main driving joint; the first main driving joint is connected to the second main driving joint. Between the driving joints, and between the second main driving joint and the third driving joint, several slave driving joints are arranged along the length direction of the fish body, and each slave driving joint includes a support ring, a vertebra hinge and a vertebra hinge connecting rod, and the support ring There is a card slot for fixing the spine hinge, the spine hinge has a first connection part and a second connection part, the second connection part is U-shaped, the second connection part is clamped in the card slot of the support ring, the first connection part The second connecting part of the previous slave driving joint is rotationally connected through the vertebral hinge connecting rod, and the second connecting part is rotationally connected with the first connecting part of the latter slave driving joint through the vertebral hinge connecting rod. The vertebral hinges of multiple slave driving joints Connected end to end to form the backbone of the robotic fish.

In the second step, according to the physical model obtained in the first step, determine the impact of the model on the surrounding environment, determine the calculation area, and perform grid division; put the physical model obtained in the first step in a water tank with a certain flow rate for fluid simulation analysis , the flow field diagram and pressure diagram of the three-joint robot fish in the flowing water are simulated by Fluent software, and the calculation area is set to 10 times the size of the three-joint robot fish, and the tetrahedron grid is selected;

The third step is to set the boundary conditions; assuming that the water flow in the experimental environment of the three-joint robot fish is a constant flow, the calculation adopts the MRF method, and the entire calculation area is regarded as a calculation area, and the fluid characteristics around the three-joint robot fish are calculated;

The fourth step is to import the divided grid diagram into the computational fluid dynamics analysis software Fluent, and carry out a series of setting solutions; set the initial conditions: period T=2s, water velocity v=0.5m/s, for the critical moment in the period For simulation analysis, select the pressure field diagram and velocity flow field at five moments t=0, t=1/8T, t=1/4T, t=5/8T, t=3/4T for analysis;

The fifth step is to select, analyze and process the data, and obtain the digital simulation analysis results.

Furthermore, in the fifth step, the maximum pressure change and the maximum water flow at the tail of the three-joint robot fish are drawn in the cycle, and the maximum pressure point of the robot fish in the cycle is 1/3 from the front of the fish head and mouth. Practice has also proved that Breakage can easily occur here.

Further, in order to increase the compressive strength of the head, thereby reducing the damage of the head, on the inside of the mouth of the fish head of the three-joint robot fish, three 4mm diameter rods are arranged at the front 1/3 of the mouth. Power support rod.

After the present invention adopts the above scheme, it has the following technical effects:

The selected calculation area of the present invention is 10 times that of the three-joint bionic robot fish, the grid division adopts the structure tetrahedron, the boundary condition adopts the method of multiple reference models, and the digital simulation simulation analysis is carried out. It can be concluded that within the cycle, the pressure is concentrated on the head and both sides at its maximum, and when t=1/4T and t=3/4T, that is, when the tail flicks to the maximum extent, the pressure received is the largest, and the water flow at the tail is also the largest. The force is the largest; according to the simulation result data, the cause of the actual head damage was found, and the specific location of the damage appeared in the front of the mouth, about 1/3 of the front. Three support frames are designed at the front 1/3 of the inner side to improve the phenomenon of head damage.

Description of drawings

Figure 1 is a schematic flow chart of the simulation method.

Figures 2a and 2b are the pressure field diagram and the velocity flow field diagram at t=0, respectively.

Figures 3a and 3b are pressure field diagrams and velocity flow field diagrams at t=1/8T, respectively.

Figures 4a and 4b are pressure field diagrams and velocity flow field diagrams at t=1/4T, respectively.

Figures 5a and 5b are pressure field diagrams and velocity flow field diagrams at t=5/8T, respectively.

Figures 6a and 6b are pressure field diagrams and velocity flow field diagrams at t=3/4T, respectively.

Fig. 7 is a diagram of the change of the maximum value of pressure within a cycle.

Fig. 8 is a graph showing the variation of the maximum value of the flow velocity within a period.

Figure 9 shows the distance from the point of maximum pressure to the top of the head during the cycle.

Figure 10 shows the structure of the three-joint robotic fish.

Figure 11 shows the structure in which the support rods are arranged inside the mouth of the fish head.

detailed description

As shown in Figure 1, a numerical simulation method for hydrodynamics of a three-joint robotic fish includes:

The first step is to establish a physical model based on realistic conditions;

As shown in Figure 10, the three-joint robot fish includes a fish head, a fish body and a fish tail, and the fish body includes a fixed part 1 and a swing part 2, and a control mechanism for controlling the movement posture of the robot fish and wireless charging is installed in the fixed part. The swinging part includes three main driving joints, the first main driving joint, the second main driving joint and the third main driving joint are arranged in sequence along the length direction of the fish body, one end of the fixed part is fixedly connected with the fish head, and the other end is connected with the first main driving joint. The joints are fixedly connected, and the length ratio of the first main driving joint, the second main driving joint and the third main driving joint is 1:1:1.2.

Each main driving joint includes a steering gear and a driving rod. One end of the driving rod is connected to the output shaft of the steering gear, and the other end is fixedly connected to the steering gear of the adjacent main driving joint. It is a reciprocating motion, in which each motion joint is also a reciprocating motion based on the previous motion joint. The overall motion track of the motion joints of the robot fish changes along a sinusoidal function. The second main drive joint follows the first main drive. The reciprocating motion on the basis of joints still obeys the sinusoidal function change. There is a phase difference between the first main driving joint and the second main driving joint. The third main driving joint is the same as the second main driving joint. The reciprocating motion based on the driving joint still obeys the sinusoidal function change, and there is a larger phase difference between the second main driving joint and the third main driving joint when they move.

Between the first main driving joint and the second main driving joint, between the second main driving joint and the third driving joint, between the third driving joint and the fish tail, several slave driving joints are arranged along the fish body length direction, each A slave drive joint includes a support ring 3, a vertebra hinge 4 and a vertebra hinge connecting rod, the support ring has a draw-in slot for fixing the vertebra hinge, the vertebra hinge has a first connecting portion and a second connecting portion, and the second connecting portion is in the shape of a U Type, the second connecting part is clamped in the card slot of the support ring, the first connecting part is rotationally connected with the second connecting part of the previous slave driving joint through the vertebral hinge connecting rod, and the second connecting part is connected with the rear joint through the vertebral hinge connecting rod. The first connecting part of a slave driving joint is rotationally connected, and the spine hinges of multiple slave driving joints are connected end to end to form the spine of the robot fish; the kinematic relationship of the slave driving joints is changed according to the movement joints of the main driving joints, the inner side of the mouth of the fish head, Located at the front 1/3 of the mouth, there are three stressed support rods with a diameter of 4mm.

In the second step, according to the physical model obtained in the first step, determine the impact of the model on the surrounding environment, determine the calculation area, and perform grid division;

Put the physical model obtained in the first step in a water tank with a certain flow rate for fluid simulation analysis, and use the Fluent software to simulate the flow field diagram and pressure diagram of the three-joint robotic fish in the flowing water with a certain regular cycle movement in the cycle. Take the critical moment for analysis, because the larger the calculation area is, the more accurate the calculation will be. If the calculation area is too small, it will affect the calculation data. Considering comprehensively, set the calculation area to be 10 times the size of the three-joint robotic fish.

Tetrahedral grids have strong adaptability to complex regional boundaries and constraints, and are fast and flexible, and the workload of engineers is small, so tetrahedral grids are selected. Considering that the calculation of the flow field near the edge of the three-joint robotic fish is relatively large during grid division, the grid of the boundary part is encrypted to facilitate subsequent calculations. When the calculation area is discretized, the calculation area division method at different times The same grid division method is selected, and Table 1 shows the number of grid divisions at different times.

Table 1 Number of grid divisions at different times

The third step is to set the boundary conditions; assuming that the water flow in the experimental environment of the three-joint robotic fish is a constant flow, the calculation adopts the multiple reference model method, that is, the MRF method (moving reference frame). MRF is a steady-state algorithm, and the calculation amount is relatively small The advantage of MRF is that it can realize the overall numerical simulation of the experimental tank without experimental data, and it is suitable for the small system of the three-joint robotic fish in the water. During the simulation, the entire calculation area is regarded as a calculation area, and the fluid characteristics around the three-joint robotic fish are calculated.

The fourth step is to import the divided grid diagram into the computational fluid dynamics analysis software Fluent, and perform a series of setting solutions;

Set the initial conditions: period T = 2s, water velocity v = 0.5m/s. Perform simulation analysis on the key moments in the cycle, and select the pressure field diagram and velocity flow at five key moments t=0, t=1/8T, t=1/4T, t=5/8T, t=3/4T Field, as shown in Figure 2-6 shows the pressure field diagram and velocity flow field diagram at critical moments in the unit cycle.

When the three-joint robot fish is swinging periodically, when the water velocity v=0.5m/s, it can be seen from Figure 2a that when t=0, the maximum pressure point appears directly in front of the head, which is P ₁ =1.45×10 ^{- 1} Pa, it can be seen from Figure 2b that when t=0, the maximum water flow appears on both sides of the head, which is v ₁ =6.65×10 ^-1 m/s;

It can be seen from Figure 3a that when t=1/8T, the maximum pressure point appears on the right side of the head, which is P ₂ =1.49×10 ^-1 Pa. It can be seen from Figure 3b that when t=1/8T, the maximum water flow appears at On the right side of the tail, v ₂ =7.46×10 ^-1 m/s;

It can be seen from Figure 4a that when t=1/8T, the maximum pressure point appears on the right side of the head, which is P ₃ =1.56×10 ^-1 Pa. It can be seen from Figure 4b that when t=1/8T, the maximum water flow appears at the left rear of the tail, which is v ₁ =8.59×10 ^-1 m/s;

It can be seen from Figure 5a that when t=5/8T, the maximum pressure point appears on the left side of the head, which is P ₄ =1.49×10 ^-1 Pa. It can be seen from Figure 5b that when t=5/8T, the maximum water flow appears at On the left side of the tail, v ₄ =8.35×10 ^-1 m/s;

It can be seen from Figure 6a that when t=3/4T, the maximum pressure point appears on the left side of the head, which is P ₅ =1.58×10 ^-1 Pa. It can be seen from Figure 6b that when t=3/4T, the maximum water flow appears at On the right side of the tail, v ₅ =8.42×10 ^-1 m/s. Because the movement of the three-joint robot fish is continuous, although the above is only processing the key moments, its data can be processed continuously according to the key points in the cycle. The following is the specific processing of the data of the simulation analysis.

The fifth step is to select, analyze and process the data, and obtain the digital simulation analysis results;

The maximum change of pressure and the maximum change of water flow at the tail of the three-joint robotic fish cycle are shown in Figure 7 and Figure 8, respectively:

It can be seen from Figure 7 that when the swimming cycle of the three-joint robotic fish is 2s, the maximum pressure in the cycle is mainly concentrated at P=0.140-0.160Pa, and the maximum peaks appear at t=1/4T and t=3/4T time, the maximum trough appears at time t=1/2T; it can be seen from Figure 8 that the maximum value of the flow field around the bionic robotic fish is mainly concentrated between v=0.66～0.85m/s, and the maximum peak also appears at At time t=1/4T and t=3/4T, the minimum value trough appears at time t=1/2T.

When t=1/4T and t=3/4T, the swing angles of the three main drive joints reach the limit. At this time, the bionic robot fish is bending its body to the limit, and it is performing the largest tail flick action, and the pressure it receives is the greatest. , the water flow at the tail is also the largest, and the propulsion force is the largest; when t=1/2T, the swing of the three main drive joints is very small, close to zero. At this time, the body bending of the bionic robotic fish is the smallest, the pressure it receives is the smallest, and the water flow at the tail is also the smallest.

The maximum value in the cycle of the three-joint robotic fish mainly occurs at the head, with the top of the fish head being 0, and the fish head pointing to the fish tail as the positive direction. It can be seen from the figure that the maximum pressure point in the cycle is mainly concentrated at L=0~16.5mm from the head, the maximum moment appears at t=3/4T, and the minimum moment appears at t=1/2. In the design of the three-joint bionic robot fish, the length of the head is designed to be L ₁ =125mm and the length of the mouth is designed to be L ₂ =50mm according to the actual size of the Arowana. It can be seen that L=0~16.5mm is mainly concentrated in front of the mouth In the middle of the front 1/3 of the mouth, it is easy to be damaged here, and three supporting rods 5 with a diameter of 4mm can be designed inside, as shown in Figure 11, the structure of the supporting rods can increase the compressive strength of the head , thereby reducing damage to the head.

Inspired by the above-mentioned ideal embodiment according to the present invention, through the above-mentioned description content, relevant workers can make various changes and modifications within the scope of not departing from the technical idea of the present invention. The technical scope of the present invention is not limited to the content in the specification, but must be determined according to the scope of the claims.

Claims (5)

1. A three-joint robot fish hydrodynamics numerical simulation method, is characterized in that: comprises the following steps: The first step is to establish a physical model; the three-joint robotic fish includes a fish head, a fish body and a fish tail, the fish body includes a fixed part and a swing part, and a control mechanism for controlling the movement posture of the robot fish and wireless charging is installed in the fixed part. The swinging part includes three main driving joints, which are the first main driving joint, the second main driving joint and the third main driving joint arranged in sequence along the body length of the fish. One end of the fixed part is fixedly connected with the fish head, and the other end is connected with the second One main driving joint is fixedly connected, the length ratio of the first main driving joint, the second main driving joint and the third main driving joint is 1:1:1.2; In the second step, according to the physical model obtained in the first step, determine the impact of the model on the surrounding environment, determine the calculation area, and perform grid division; put the physical model obtained in the first step in a water tank with a certain flow rate for fluid simulation analysis , the flow field diagram and pressure diagram of the three-joint robot fish in the flowing water are simulated by Fluent software, and the calculation area is set to 10 times the size of the three-joint robot fish, and the tetrahedron grid is selected; The third step is to set the boundary conditions; assuming that the water flow in the experimental environment of the three-joint robotic fish is a constant flow, the calculation adopts the MRF method, and the entire calculation area is regarded as a calculation area, and the fluid characteristics around the three-joint robotic fish are calculated; The fourth step is to import the divided grid diagram into the computational fluid dynamics analysis software Fluent, and carry out a series of setting solutions; set the initial conditions: period T=2s, water velocity v=0.5m/s, for the critical moment in the period For simulation analysis, the pressure field diagram and velocity flow field at five moments t=0, t=1/8T, t=1/4T, t=5/8T, t=3/4T are selected for analysis; The fifth step is to select, analyze and process the data, and obtain the digital simulation analysis results. 2. The hydrodynamic numerical simulation method of the three-joint robotic fish according to claim 1, characterized in that: in the fifth step, the maximum pressure change and the maximum water flow at the tail of the three-joint robotic fish are drawn in the cycle to obtain the cycle of the machine. The maximum pressure point of the fish is 1/3 from the front of the fish head and mouth. 3. The hydrodynamic numerical simulation method for three-joint robotic fish according to claim 1, wherein each main drive joint includes a steering gear and a driving rod, and one end of the driving rod is connected to the output shaft of the steering gear, The other end is fixedly connected with the steering gear of the adjacent main drive joint. 4. The hydrodynamic numerical simulation method for three-joint robotic fish according to claim 3, characterized in that: between the first main driving joint and the second main driving joint, and between the second main driving joint and the third driving joint Several slave drive joints are arranged along the body length of the fish, each slave drive joint includes a support ring, a vertebra hinge and a vertebra hinge connecting rod, the support ring has a draw-in slot for fixing the vertebra hinge, and the vertebra hinge has a first connecting portion And the second connecting part, the second connecting part is U-shaped, the second connecting part is clamped in the slot of the support ring, the first connecting part is rotationally connected with the second connecting part of the previous slave driving joint through the vertebra hinge connecting rod , the second connecting part is rotationally connected with the first connecting part of the latter slave driving joint through a vertebra hinge connecting rod, and a plurality of vertebrae hinges of the slave driving joint are connected end to end to form the spine of the robotic fish. 5. The numerical simulation method for hydrodynamics of a three-joint robotic fish according to claim 1, wherein three joints with a diameter of 4 mm are arranged on the inner side of the mouth of the three-joint robotic fish and located at the front 1/3 of the mouth. force support rod.