Three Dimensional Numerical Simulation And Flow Characteristic Analysis Of Eccentric Cylinder Fluid-structure Interaction Flow

Promoting green and low-carbon energy transformation is a key measure to achieve the goal of "dual carbon" and high-quality social and economic development.In many renewable energy sources,Chinese ocean energy resources are abundant,widely distributed,and have a favorable development environment.However,the technology of large-scale and efficient current energy extraction is not mature,especially the extraction of low velocity head current energy.In this paper,N-S-based SST k-ω turbulence model is used to conduct two-and threedimensional numerical simulation of eccentric cylinder flow model under different Reynolds number,different eccentricity and different load damping,so as to explore the motion characteristics and energy conversion efficiency of eccentric cylinder.A dynamic mode decomposition method(DMD)was introduced to extract coherent structures at different frequencies,and the coherent structures and dynamics information of each single frequency were analyzed.The flow around the cylinder is accompanied by the separation of the boundary layer and the alternating shedding of the vortex,which forms a periodic force on the surface of the cylinder.The eccentricity provides an effective moment arm for the fluid force,and the eccentric cylinder starts to self-excited rotation under the action of torque.The rotation of the cylinder will change the phase Angle between the fluid force and the swing.When the phase Angle exceeds a certain threshold,the vortex falls off,and the fluid force changes into resistance to inhibit the continued movement of the cylinder,prompting the cylinder to reverse rotation.The self-excitability and self-limitation of eccentric cylinder rotation realize the periodic oscillation of the cylinder.Combined with the numerical results and experiments,it is found that the oscillation frequency of the cylinder increases with the increase of the Reynolds number of the incoming flow,showing a good linear relationship.When the oscillation frequency is equal to the natural frequency of the eccentric cylinder,the frequency locking phenomenon will also occur.Different eccentricity will also affect the oscillation frequency and Angle of the cylinder,and a better oscillation condition can be achieved in the interval of eccentricity ratio e∈(0.5-0.8).Load damping was introduced to simulate the load generation condition of eccentric cylindrical system,and the energy conversion equation was established to calculate the energy conversion efficiency under different incoming flow Reynolds number,different eccentricity and different load damping.It is found that there is a load threshold under different working conditions.When the load is less than the threshold,the energy conversion efficiency is continuously improved with the increase of damping load,and the optimal energy conversion efficiency under this working condition is obtained at the threshold.In the simulated working condition,the threshold increases with the increase of Reynolds number.Reynolds number Re= 78,000,damping f=1.2(N·m·s/rad),the maximum value is 38.78%.In addition,the three-dimensional effect of the flow around the three-dimensional eccentric cylinder is analyzed and the difference of the two-dimensional and three-dimensional calculation results is explained reasonably.Based on the three-dimensional unsteady calculation results,DMD dynamic mode decomposition method was used to analyze the wake field of different inflow Reynolds numbers(Re= 22000,33000,56000).The results show that the coherence structure has poor stability at low Reynolds number,and the stability increases gradually with the increase of Reynolds number.The spatiotemporal evolution of three dimensional coherent structures at a single frequency of wake field is analyzed.