Study On Flow-induced Vibration Responses And Bending-Twisting Coupling Characteristics Of Flexible Hydrofoil

Hydrofoil is widespread in the hydraulic machinery and ocean engineering.To simplify the design and analysis process,the traditional hydrofoil,which exposes many problems when facing high speed shipping requirement and complex hydrological environment,for example,the structure damage caused by cavitation of hydrofoil,work in off-design conditions when faces the harsh currents et al.The above disadvantages of rigid hydrofoil limits its application range.For the very reason,the flexible hydrofoil made by composite materials can be a promising solution.However,flexible hydrofoil will undergo flow-induced bending and twisting deformation and flow-induced vibration（FIV）under unsteady hydrodynamic loads.The nonlinear responses of the structure are not clear and many problems need to be further studied due to the complexity of fluid-structure interaction in FIV for flexible hydrofoil.Therefore,the flow-induced bending and twisting of flexible hydrofoil is of important significance to academic and engineering fields.In this thesis,the studies of flow-induced bending and twisting vibration of flexible NACA0015 hydrofoil caused by unsteady loads were investigated by numerical and experimental methods.The bending and twisting responses,and the near-wake structures transition are discussed in this work.The influence of different parameters（the incoming velocity of flow,the initial attack angle of hydrofoil,the degree of freedom for the vibration,and the moment of inertia）on the flow-induced bending and twisting of flexible hydrofoil are also analyzed.Meanwhile,the fluid-structure interaction mechanism of flexible hydrofoil is also revealed.To obtain the vibration responses of flexible foil and the parameters of flow field,the unsteady Reynolds-averaged Navier-Stokes equations and coupled with the shear stress transport（SST）k-ωturbulence model were solved.To further reveal the coupling mechanism of the bending and the twisting vibration,the hydrodynamic responses of flow pass fixed rigid hydrofoil the forced pitching rigid foil,the one-degree of freedom FIV responses of bending hydrofoil and the twisting hydrofoil,and the FIV responses of flexible hydrofoil were carried out for comparison.In addition,the piezoelectric control method is employed to control the flow-induced vibration of hydrofoil based on the coupling mechanism of the bending and the twisting vibration.The piezoelectric control method is confirmed to be an effective way to control the bending and twisting of flexible hydrofoil.The performance of the piezoelectric control method was also carried out by experimental studies.The main conclusions can be drawn as follows:Firstly,the force characteristics and near-wake structures of flow pass stationary and forced pitching rigid NACA0015 hydrofoil were analyzed,the hydrodynamic load responses and thrust efficiency of hydrofoil are also studied,which provide reliable references for the study of fluid-structure coupling mechanism of flexible hydrofoil.The results show that the lift force of the stationary hydrofoil with low initial attack angle（α₀）is significantly affected by the velocity of incoming flow,while the standard deviation(δ_Cl)of lift coefficient almost does not change with the reduced velocity（U^*）at high initial attack angle.For forced pitching hydrofoil,the value ofα₀ can significantly influence the tendency of ratio of time-averaged lift coefficient and drag coefficient(C_lmean/C_dmean).For given reduced vibration frequency and amplitude,the value of C_lmean/C_dmean for foil with high initial attack angle is far lower than that of foil with low initial attack angle.The thrust efficiency（η）of forced pitching hydrofoil increases with the rise of vibration amplitude and frequency in the whole and the value ofηalways keeps in negative value for high initial attack angle（α₀≥30°）.Secondly,when only the bending or twisting vibration of the NACA0015 hydrofoil is allowed,the amplitude,frequency,and force response of the flow-induced vibration of the hydrofoil were numerically investigated.The effects of flow conditions and initial attack angle on the flow-induced bending vibration of the hydrofoil were analyzed combined with the characteristics of the near-wake structures.The results show that the bending amplitude(A_peaks^*)of bending hydrofoil does not change much with the change of U^*andα₀ whenα₀<25°;the boundary layer separation points on the surface of bending hydrofoil moves towards the leading edge of the hydrofoil whenα₀≥25°,which reduces the lift force of the hydrofoil and further suppresses the high vibration amplitude of bending hydrofoil.In addition,the near-wake structures change from“U”pattern to the2S and 2S^*pattern with the rise ofα₀.The twisting angle amplitude(θ_peaks^*)increases with the increase of U^*overall,the twisting vibration mode of hydrofoil transforms from a stable harmonic response to a multi-amplitude and multi-frequency co-existence mode with the increase of the initial attack angle.Moreover,the reduced twisting frequency（f_θ^*）of hydrofoil decreases with rising of the initial attack angle,and the value of f_θ^*closes to1 with the increase of the reduced velocity.Furthermore,as for the flexible NACA0015 hydrofoil which is allowed to bend and twist,the flow-induced bending and twisting vibration responses,and the influence of initial attack angle and condition of incoming flow were numerically studied.The influence of near-wake structures and the attack angle on the bending-twisting vibration is analyzed.Moreover,the fluid-structure interaction mechanism of flow-induced bending and twisting is revealed.The structures and fluid dynamic responses of hydrofoil with different degree of freedom are compared to reveal the coupling mechanism of bending and twisting vibration.The results show that the moment of inertia for flexible hydrofoil can affect the flow-induced bending and twisting responses.High moment of inertia can effectively suppress the bending and twisting vibration of flexible hydrofoil under high reduced velocity range.The bending amplitude of the flexible hydrofoil is very low in the range of low reduced velocity range.When U^*reaches a certain critical value,the bending amplitude begins to increase.Whenα₀>10°,the twisting amplitude increases monotonically with the increase of U^*due to that the standard deviation of torque coefficient(δ_Cm)rises when hydrodynamic elastic instability of flexible hydrofoil rises with the increase of U^*.In addition,the transition of center for the distribution of local pressure coefficient can also directly affect the hydrodynamic elastic instability of flexible hydrofoil.The limit cycle oscillations（LCO）of bending and twisting responses for flexible hydrofoil is relatively stable,which mainly composed of harmonic response.Moreover,the harmonic response range of twisting vibration is narrow when compared with that of bending vibration.For both the bending and twisting vibration,the LCO of hydrofoil becomes more and more unstable with the rising of U^*andα₀.By comparing the hydrodynamic responses of NACA0015 hydrofoil with different degrees of freedom,it’s found thet the bending and twisting coupling of flexible hydrofoil is mainly reflected in the excitation of twisting vibration by bending vibration,which leads to the high pressure fluctuation on the surface of flexible hydrofoil.Finally,the piezoelectric control method was employed to control the flow-induced vibration of NACA0015 hydrofoil based on the coupling mechanism of the bending and the twisting vibration.The flow-induced bending and twisting of flexible hydrofoil without control,with open circuit piezoelectric system,and with piezoelectric energy dissipation system were studied and compared by flume experiments.The piezoelectric control method was confirmed to be feasible for the control of flow-induced bending and twisting of flexible hydrofoil,the control characters of the method were also carried by flume experiments.The experimental results show that the add of structural damping and dynamic piezoelectric damping by the method can suppress the vibration of hydrofoil,especially in high initial attack angle and high reduced velocity range.