Analysis And Prediction Research On Energy Harvesting Efficiency Of Flow-induced Vibration With Two Tandem Vibrators Containing Roughness

Marine current energy has many advantages,such as abundant reserves,wide distribution and easy prediction of current state,so it has become the focus of Marine energy utilization research.In order to utilize the low velocity Marine current energy which is widely distributed in China,the low velocity Marine current energy generation technology based on the flow induced vibration to absorb the kinetic energy of the current has been widely paid attention.In the large-scale exploitation of ocean current energy by fluid-induced vibration,the traditional experimental methods cost a lot of money,and the numerical simulation methods require a lot of computational resources,so the two methods can not quickly obtain all the calculation results under the flow rate and working conditions.Therefore,in this thesis,the fluid-induced vibration response of the vibration device is simulated and analyzed,and the fitting model and neural network prediction model are built to quickly and comprehensively predict and analyze the vibration under various working conditions.The main work contents and conclusions of this thesis are as follows:Firstly,in order to study the energy capture effect of the flow-induced vibration energy capture device under water and the parameters when achieving the optimal energy capture effect,the wake vibrator model was adopted in this thesis to simulate the vortex-induced vibration of a smooth single cylinder under the elastic support of single degree of freedom,and the fluid-structure coupling parameters of the vibrators with different mass ratios and damping ratios were calculated at different flow rates.The coupling parameters of amplitude ratio and frequency ratio are substituted into the formula to calculate the energy harvesting efficiency of the vibrator.According to the fitting analysis of the simulation results,the fitting formula of the maximum capturing efficiency changing with damping ratio and mass ratio is proposed when the maximum capturing efficiency is reached.Finally,the numerical models of smooth oscillator and rough oscillator are established,and the response characteristics and energy capture results of smooth oscillator and smooth oscillator with roughness distribution are compared and analyzed.The results show that with the increase of the mass ratio,the maximum value of the energy capture efficiency decreases gradually under different reduction velocity and different damping ratio,but there is an optimal damping ratio to make the maximum value of the energy capture efficiency under different mass ratio and reduction velocity.The fitting empirical formula can predict the optimal damping ratio well,and has a good reference function for studying the parameters of the flow-induced vibration energy capture device to achieve the optimal energy capture effect.Secondly,in order to reduce the cost and achieve the maximum energy utilization,the three-dimensional transient Reynolds time-mean method and the overlapping mesh technique were adopted to establish the two-vibrator vibration model with roughness under the serial arrangement of different spacing ratios.The vortex discharge combination of the vibrator and the corresponding vibration response characteristics are analyzed from five aspects,namely,tail vortex discharge,vibrator pulsating pressure,displacement time history curve,vibration response amplitude and vibration response frequency.Finally,the integral calculation method is adopted to realize the correct calculation of energy capture efficiency under high vortex induced force.Calculation results show that the oscillator falls off a single cycle of vortex on the number of increases with the increase of flow velocity,when low flow velocity,oscillator of the trailing vortex loss is given priority to with “2S”,as the flow rate increased to “2T”,eventually converted to galloping state,single vortex lift and drag of vibrator impact is not big,oscillator motion affected by the flow field of the overall situation.In addition,the trailing vortices of the upstream part of the oscillator attach to the surface of the downstream oscillator at a small spacing ratio,which directly affects the local pressure changes on the surface of the downstream oscillator,thus affecting the vibration response of the downstream oscillator.Finally,in order to realize all kinds of the vibration response under the condition of rapid,comprehensive prediction,set up and compared the general feedforward neural network and strengthen the topological evolution optimization neural network feedforward type can absorb energy efficiency of the flow prediction model,the model results show that the damping ratio in the process of the oscillator can capture performance to promote role,the oscillator can capture efficiency is greater than the influence of the stiffness and spacing ratio.In addition,the maximum value of the vibrator is obtained at the moderate stiffness K=700N/m and the large spacing ratio L/D=2.56 in the VIV stage of low flow rate.In the galloping phase of high velocity,the larger stiffness and smaller spacing ratio are beneficial for the vibrator to capture more water energy.