Research On Dynamic Response Of Flexible Foil In Complicated Flow

Lightweight,flexible materials are increasingly used in the field of micro-aircraft and marine applications.Different from traditional alloy materials,these materials are relatively low-mass compared to the surrounding fluid medium and are greatly affected by the surrounding fluid loads and inertial forces.In particular,the unavoidable cavitation flow in the marine and marine fields makes the dynamic response characteristics of lightweight flexible applications more complicated.However,there are few researches on the fluid-structure interaction response characteristics of lightweight,flexible materials.Aiming at the above engineering background,this paper takes a two-dimensional foil as the research object,and studies the stability of lightweight flexible foil and the fluid-structure interaction response in cavitation flow.The research contents and conclusions of this paper are as follows:A two-way fluid-structure interaction method was established to simulate the dynamic response of flexible foil.In this paper,the three-dimensional foil is simplified into a two-dimensional structure,and the two degrees of freedom of bending and torsion are studied.The dynamic response characteristics of the flexible foil are simulated by a combination of CFX flowfield calculation software and Fortran field calculation program.The research on stability of flexible wing was carried out.The results show that LHC algorithm is basically consistent with the experimental results.With the increase of relative mass ratio,the failure modes of flexible foil followed by static divergence,dynamic divergence and flutter.Static divergence is a linear static instability,where the deformation increase without oscillations,and the twisting deformation dominates.Dynamic divergence is similar to static divergence,the mean divergence deformations also increase with time,and at the same time dynamic divergence has an oscillation frequency,which decays with increasing deformation.When the flutter occur,the flexible foil amplitude oscillation amplitude,the total damping coefficientis zero,the structure is in the unstable edge.The dynamic response of flexible foil in cavitation was studied.The dynamic response characteristics of flexible foil with different relative mass ratios in sheet cavitation and cloud cavitation flow were analyzed.The results show that:In the sheet cavitation flow,the cavitation period gradually increase with the increase of the relative mass ratio,and the frequency of cavitation cycles is consistent with the bending frequency of the flexible foil.The amplitudes of the lift coefficient,torque coefficient,bending deformation and torsional deformation showed the trend of first increasing and then decreasing;with the corresponding mean value overall decreased.The deformation of the flexible foil is dominated by bending deformation and the torsional deformation is small,so there is a long delay in bending and the lift force of the flexible foil in the sheet cavitation cavitation flow.The wetted natural torsional frequency and the sub-harmonics of the wetted natural frequencies were excited,and the corresponding amplitudes gradually increase with the increase with the relative mass ratio.In the cloud cavitation flow,the cavitation evolution period increases with the increase of the relative mass ratio,but the amplitude is small.The oscillation amplitude of the lift coefficient,the torque coefficient,then bending deformation and the torsional deformation increases with time,and the corresponding average value overall upward trend.Due to the large load of cloud cavitation,the delay between bending deformation and lift force is small.Vibration of the flexible foil is mainly affected by the development of cavitation,the natural frequency of torsion and the sub-harmonics of the wetted natural frequencies,and the corresponding amplitudes gradually increase with the relative mass ratio.