A Method for Utilizing Fluid-Induced Vibration Energy Based on an Actively Rotating Elliptic Cylinder

Abstract

The present invention discloses a method for utilizing fluid-induced vibration energy based on an actively rotating elliptical cylinder, comprising: Step 1, introducing a physical model of a rotating elliptical cylinder into a pre-constructed external flow field to obtain a corresponding physical model of the flow field; Step 2, using a high-precision numerical simulation method to simulate the physical model of the flow field to obtain the distribution of the flow field over time and the motion data of the rotating elliptical cylinder; Step 3, analyzing the vibration response results of the rotating elliptical cylinder according to the data obtained in Step 2 to obtain the characteristics of the amplitude change and the law of the reduced velocity varying with the rotation frequency at lock-in. The method provided by the present invention aims to significantly increase the peak amplitude of vibration in the resonance region and enhance the vibration stability by endowing the elliptical cylinder with the degree of freedom of active rotation, optimizing the aspect ratio of the elliptical cylinder, and actively adjusting the rotation speed.

Description

Technical Field

[0001] The present invention relates to the field of utilization of fluid-induced vibration energy, and particularly to a method for utilizing fluid-induced vibration energy based on an actively rotating elliptical cylinder. Background Art

[0002] The energy generated by fluid-induced vibration is a clean energy source with great utilization potential and has received extensive attention in recent years when environmental problems have become increasingly prominent. More and more energy harvesting devices convert fluid-induced vibration energy into electrical energy for utilization through piezoelectric effects and other means, and their principles are mostly based on the self-excited vibration of bluff bodies under the action of oncoming flow to generate vibration energy. Due to its superiority over fossil energy, it has the prospect of large-scale application in the future with the goal of sustainable development.

[0003] The fluid-induced vibration amplitude of traditional non-rotating bluff bodies is usually small. The mechanical energy captured by the energy harvesting device designed based on this directly comes from the vibration of the bluff body, so the energy utilization is limited by the amplitude size. Since the fluid-induced vibration of the bluff body does not reach the maximum amplitude at any oncoming flow velocity or occur uniformly and stably over time, it has a great dependence on the fluid velocity. Wind or ocean currents are ideal objects for the utilization of fluid-induced vibration energy, but usually the flow velocity is not constant, so the stable resonance region is usually very narrow.

[0004] Existing methods for harvesting fluid-induced vibration energy generally ignore the rotational degree of freedom. Even if the rotational degree of freedom is considered, it is usually passive rotation. Passive rotation will affect the vibration stability of the energy harvesting device with axial-flow degrees of freedom, even though it will increase the vibration amplitude under specific conditions.

[0005] Patent document CN114893335A discloses a linear generator with a magnetically levitated oscillator using ocean current-induced vibration, including a fixing plate. Coil assemblies are symmetrically installed at both ends of the top of the fixing plate. There are outer shells outside the coil assemblies. Magnet fixing rods are provided on the inner sides of the outer shells. The bottom of the magnet fixing rods is connected to the fixing plate. Fixed magnets are installed at both the upper and lower ends of the magnet fixing rods. An active magnet is slidably connected to the magnet fixing rod between the fixed magnets at both ends. An oscillator is connected between the active magnets on both sides. This method can provide a greater vibration thrust for the oscillator through the magnetic boundary. However, since the oscillator is restricted on the fixing rod, only the vibration change along the direction of the fixing rod can be realized, thus losing some fluid-induced vibration energy.

[0006] Patent document CN109801538A discloses an experimental device for fluid-induced vibration power generation based on a linear generator, which includes a bearing frame, a spacing adjustment module, a single oscillator module, and a linear generator; the spacing adjustment module is fixed on the bearing frame and fixedly connected to the single oscillator module, and the single oscillator module changes its position through the spacing adjustment module to realize the adjustment of the spacing between multiple oscillator modules. This device can quickly adjust the distance between each oscillator through the single oscillator module to improve the acquisition efficiency, but the adjustment method described in the document is very cumbersome and is easily affected by external factors. Summary of the Invention

[0007] To solve the above problems, the present invention provides a method for utilizing fluid-induced vibration energy, which increases the rotational degree of freedom of energy acquisition, helps to improve the vibration energy obtained by the energy collector, and enhances the vibration stability by actively adjusting the rotational speed.

[0008] A method for utilizing fluid-induced vibration energy based on an actively rotating elliptical cylinder includes:

[0009] Step 1: Introduce a physical model of a rotating elliptical cylinder into a pre-constructed external flow field to obtain a corresponding physical model of the flow field;

[0010] Step 2: Use a high-precision numerical simulation method to simulate the physical model of the flow field to obtain the distribution of the flow field over time and the motion data of the rotating elliptical cylinder;

[0011] Step 3: Analyze the vibration response results of the rotating elliptical cylinder according to the data obtained in Step 2 to obtain the amplitude change characteristics and the law of the reduced velocity varying with the rotation frequency at lock-in.

[0012] The present invention discovers through numerical simulation that rotation can greatly increase the peak amplitude of the elliptical cylinder, and endowing the elliptical cylinder with the degree of freedom of active rotation can actively control the vibration at the current flow velocity so that it is always in the resonance range, thereby enhancing the vibration stability and further meeting the requirements of large-scale clean energy utilization.

[0013] Specifically, in Step 1, the physical model of the rotating elliptical cylinder includes the rotational angular velocity of the elliptical cylinder, the aspect ratio, and the elastic coefficient and structural damping in the flow direction.

[0014] Specifically, in Step 1, the vibration constraint equation of the physical model of the rotating elliptical cylinder is as follows:

[0015]

[0016] In the formula, k is the elastic coefficient, c is the structural damping, m is the mass, and t is the time.

[0017] Specifically, in step 1, the external flow field includes a velocity inlet, a pressure outlet, and upper and lower symmetric boundaries, and the transverse length of the external flow field is set to satisfy a congestion rate of less than 1%.

[0018] Specifically, in step 2, the high-precision numerical simulation method includes solving the second-order vibration equation by the fourth-order Runge-Kutta method and solving the flow field by the overlapping grid method, and performing simulations at a preset time step and a constant rotational speed.

[0019] Specifically, the value range of the time step is Δt ≤ 0.000255T rot , T rot is the time for the elliptical cylinder to rotate one week.

[0020] Specifically, in step 2, the motion data of the rotating elliptical cylinder includes the vibration response results corresponding to at least 10 reduced velocities for each combination of the aspect ratio and the rotational speed.

[0021] Specifically, in step 3, the amplitude change characteristics include the distribution of the peak amplitude in the flow direction of the actively rotating elliptical cylinder for different combinations of the aspect ratio and the rotational speed, and the comparison of the amplitude data of the rotating elliptical cylinder with the amplitude results of the non-rotating elliptical cylinder at similar Reynolds numbers.

[0022] Specifically, in step 3, the expression of the law of the reduced velocity changing with the rotation frequency at locking is as follows:

[0023]

[0024] In the formula, f rot is the rotation frequency, U is the actual flow velocity, D is the major axis length of the rotating ellipsoid, and U * is the reduced velocity corresponding to the current rotational speed at locking.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] (1) By means of numerical simulation, the present invention adds an active rotation degree of freedom to the elastically mounted elliptical cylinder vibrating in the flow direction with different aspect ratios. The calculation results show that the amplitude in the flow direction of the elliptical cylinder increases significantly, and the peak vibration amplitude can be further increased by optimizing the rotational speed and the aspect ratio.

[0027] (2) The present invention gives the relationship between the locking position of the rotating elliptical cylinder, the flow velocity, and the rotation frequency. By actively adjusting the rotational speed, the flow-induced vibration under the current oncoming flow velocity can be made uniform, stable, and close to the ideal amplitude, providing a new method for stably and efficiently utilizing clean energy. Description of the Drawings

[0028] Figure 1 is a flowchart of a method for utilizing flow-induced vibration energy based on an actively rotating elliptical cylinder provided by the present invention;

[0029] Figure 2 Schematic diagram of the flow field physical model provided in this embodiment;

[0030] Figure 3 Schematic diagram of elliptical cylinders with different length-diameter ratios provided in this embodiment;

[0031] Figure 4 Schematic diagram of the amplitude results under different combinations of length-diameter ratios and rotational speeds provided in this embodiment;

[0032] Figure 5 Relationship diagram of the dimensionless amplitude and dimensionless rotational speed at the predicted locking point provided in this embodiment;

[0033] Figure 6 Provided in this embodiment Figure 5 Curve graph of the displacement corresponding to each locking position changing with time in Detailed implementation manners

[0034] The present invention will be specifically described below in conjunction with embodiments and the accompanying drawings. It should be clear that the description of specific parameters is only exemplary and is not intended to limit the scope of the present invention. In addition, the present invention does not focus on simulation algorithms and specific mechanisms, so the parts related to specific calculation methods and mechanism analysis will not be specifically described.

[0035] As Figure 1 shown, a flow-induced vibration energy utilization method based on an actively rotating elliptical cylinder includes:

[0036] Step 1, as Figure 2 shown, the elliptical cylinder has degrees of freedom in two directions, namely the flow direction and rotation, and is elastically installed. c represents the structural damping of the system, k represents the elastic coefficient, the angular velocity of rotation is Ω, the mass is m, the length of the major axis of the elliptical cylinder is fixed at D, and the minor axis is adjusted accordingly with the change of the length-diameter ratio; the distance from the center position of the elliptical cylinder to the inlet is 50D, the distance from the center position of the elliptical cylinder to the outlet is 100D, and the transverse length is 100D, ensuring the full development of the wake flow and the blockage ratio less than or equal to 1%. The inlet is a uniform oncoming flow in the flow direction with a velocity of U. The inlet boundary condition is a velocity inlet, the outlet boundary condition is a pressure outlet, the upper and lower sides are symmetric boundaries, and the surface of the elliptical cylinder is a no-slip boundary condition, obtaining the corresponding flow field physical model;

[0037] Step 2: Change the aspect ratio and the active rotation speed of the elliptical cylinder, and use the high-precision numerical simulation method in ANSYS FLUENT to calculate the given working conditions, obtaining the time-dependent flow field distribution and the displacement and velocity of the elliptical cylinder at each time step. The high-precision numerical simulation method includes solving the second-order vibration equation using the fourth-order Runge-Kutta method, solving the flow field using the overlapping grid method (ensuring that the height of the first layer of the boundary layer of the elliptical cylinder is 0.024D and the grid expansion ratio is less than 1.07 during the foreground grid division), and controlling the time step within Δt ≤ 0.000255T rot , T rot represents the time for the elliptical cylinder to rotate one week. To obtain the maximum amplitude result, structural damping is ignored in the numerical calculation; keep the Reynolds number Re = 100 and the mass ratio m * = 20 (the mass ratio ρ is the mass per unit length of the elliptical cylinder) unchanged;

[0038] As Figure 3 shown, when the constant dimensionless rotation speed α = 0.5, change the aspect ratio (∈) of the elliptical cylinder so that its shape approaches a flat plate from a uniform cylinder, taking ∈ = 1, 0.75, 0.5, 0.25; perform numerical calculations for the given working conditions to obtain the vibration response results and the flow field distribution corresponding to no less than 10 reduced velocities at each aspect ratio; keep the Reynolds number Re = 100 and the mass ratio m * = 20 unchanged. When the constant aspect ratio ∈ = 0.25, change the dimensionless active rotation speed (α) of the elliptical cylinder so that its rotation speed range covers low, medium, and high speeds, taking α = 0.2, 0.5, 1; perform numerical calculations for the given working conditions to obtain the vibration response results and the flow field distribution corresponding to no less than 10 reduced velocities at each rotation speed;

[0039] Step 3: As Figure 4 shown, the dimensionless maximum amplitude A is the maximum amplitude value, that is, the maximum displacement value in the sampling time series. The maximum dimensionless streamwise amplitude value when ∈ = 0.25 and α = 0.2 is about 0.5 times the diameter. Reference refers to the numerical simulation results for comparison, that is, the streamwise amplitude results at different reduced velocities obtained after numerically simulating the non-rotating cylinder with streamwise vibration for Reynolds number Re = 180, mass ratio m* = 20, and zero structural damping. Its peak streamwise amplitude is only 0.01 times the diameter;

[0040] The reduced velocities at the lock-in points for each combination of aspect ratio and rotation speed satisfy:

[0041]

[0042] In addition, the reduced velocity is defined as This means that for a specific oncoming flow velocity, if the natural frequency of the system cannot be changed, changing the rotational speed can make the corresponding reduced velocity fall within the locking (resonance) range, thus making the vibration uniform and stable. Six different dimensionless rotational speeds are selected to verify this law, as Figure 5 shown, and the maximum dimensionless amplitude distribution corresponding to the predicted reduced velocity points for each dimensionless rotational speed is obtained.

[0043] In addition, as the rotational speed decreases, the maximum amplitude continuously increases. When ∈ = 0.25 and α = 0.125, the amplitude is approximately 0.73D.

[0044] As Figure 6 shown, it shows the displacement-time curve corresponding to each data point in Figure 5 . In the figure, (a) is the displacement-time curve when ∈ = 0.25, α = 0.125, and U * = 12.6, (b) corresponds to the displacement-time curve when ∈ = 0.25, α = 0.2, and U * = 7.5, (c) corresponds to the displacement-time curve when ∈ = 0.25, α = 0.25, and U * = 6.3, (d) corresponds to the displacement-time curve when ∈ = 0.25, α = 0.3, and U * = 5.3, (e) corresponds to the displacement-time curve when ∈ = 0.25, α = 0.5, and U * = 3.14, and (f) corresponds to the displacement-time curve when ∈ = 0.25, α = 1, and U * = 1.5.

[0045] It can be found that each time series presents stable and uniform vibration, and both the amplitude size and frequency remain constant. This indicates that after the rotational speed is changed, according to the formula the vibration at the selected reduced velocity position is uniform, stable, and meets the requirements of energy harvesting, verifying the accuracy of the above law.

Claims

1. A method for utilizing fluid-induced vibration energy based on an actively rotating elliptical cylinder, characterized in that, Including: Step 1: Introduce a rotating elliptical cylinder physical model into a pre-built external flow field to obtain a corresponding flow field physical model; Step 2: Use a high-precision numerical simulation method to simulate the flow field physical model to obtain the time distribution of the flow field and the motion data of the rotating elliptical cylinder. The high-precision numerical simulation method includes using the fourth-order Runge-Kutta method to solve the second-order vibration equation and the overlapping grid method to solve the flow field, and performing simulations at a preset time step and a constant rotational speed; The motion data of the rotating elliptical cylinder includes the vibration response results corresponding to at least 10 reduced velocities for each combination of aspect ratio and rotational speed; Step 3: Analyze the vibration response results of the rotating elliptical cylinder according to the motion data obtained in Step 2 to obtain the amplitude change characteristics and the law of the change of the reduced velocity with the rotational frequency at lock-in.

2. The flow-induced vibration energy utilization method based on an actively rotating elliptical cylinder according to claim 1, characterized in that In Step 1, the rotating elliptical cylinder physical model includes the angular velocity of rotation of the elliptical cylinder, the aspect ratio, and the elastic coefficient and structural damping in the flow direction.

3. The flow-induced vibration energy utilization method based on an actively rotating elliptical cylinder according to claim 1 or 2, characterized in that In step 1, the vibration constraint equation of the rotating elliptical cylinder physical model is as follows: ; where is the elastic coefficient, is the structural damping, is the mass, is the time.

4. The flow-induced vibration energy utilization method based on an actively rotating elliptical cylinder according to claim 1, characterized in that In Step 1, the external flow field includes a velocity inlet, a pressure outlet, and upper and lower symmetric boundaries. The transverse length of the external flow field is set to satisfy a blockage ratio of less than 1%.

5. The flow-induced vibration energy utilization method based on an actively rotating elliptical cylinder according to claim 1, wherein The value range of the time step is , which is the time for the elliptical cylinder to rotate one week.

6. The flow-induced vibration energy utilization method based on an actively rotating elliptical cylinder according to claim 1, wherein In Step 3, the amplitude change characteristics include the distribution of the peak amplitude in the flow direction of the actively rotating elliptical cylinder under different combinations of aspect ratio and rotational speed, and the comparison of the amplitude data of the rotating elliptical cylinder with a Reynolds number of 100 and the amplitude results of the non-rotating elliptical cylinder with a Reynolds number of 180.

7. The flow-induced vibration energy utilization method based on an actively rotating elliptical cylinder according to claim 1, wherein In step 3, the expression of the law that the reduced speed at locking varies with the rotational frequency is as follows: ; where is the rotational frequency, is the actual flow velocity, is the major axis length of the rotating ellipsoid, is the reduced speed at locking corresponding to the current rotational speed.

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