Physical And Numerical Investigation Of Cavitating Flow-induced Vibrations

As higher performance and efficiencies of ocean resources exploitation,marine propulsion and hydropower system were demanded,structures became more flexible and were subjected to higher flow rates,flow-induced vibration problems have become one of the major issues in a wide range of applications,such as aerospace engineering,energy,structural mechanics and bioengineering.The transient flow with fluid structure interaction is a kind of highly unsteady phenomenon,which will produce strong structural vibrations with different flow-excitation mechanisms,including turbulence,vorticity shedding,multiphase flow,wake interference of double-row cascade and rotor-stator interaction.As the flow-induced vibration may cause structural unstability and even fatigue failures in a relatively short time,it has become one of the major issue for the safe and stable operation,especially when the local pressure decreases to the saturated vapor pressure of the liquid,cavitation occurs,involving complex interaction between phase-change and vortex structures.The unsteady breakdown and shedding of the cavities will induce strong transient loads and lead to further hydrodynamic instabilities,even structure failures.In order to address the critical technical issue related to the marine propulsion,hydraulic machinery and application of composite materials,the unsteady cavitating flow-induced vibration and the effect of material properties have been investigated via combined physical and numerical studies.A simultaneous sampling system,which combines the high-speed visualization and unsteady vibration measurement,has been established,so that the cavitating flow patterns will be synchronized with the transient structural vibration velocities to give a comprehensive understanding of the cavitating flow-induced vibration mechanism.The vibration signal analysis has been made to the modal test and vibration measurement results to obtain the flow-induced resonance characteristics.A hybrid coupled fluid structure interaction model has been established.A revised k-? SST turbulence model combined with ?-Re? transition model has been proposed to simulate the unsteady cavitating flow in consideration of local compressibility and laminar to turbulence transition.A structural dynamic model of the two-degree-of-freedom system with bending and twisting deformation has been applied.A hybrid coupled algorithm has been adopted to account for the fluid added mass effect and avoid the over-estimation of the structural displacement.The effect of different flow parameters and structural excitation on the structural stability has been evaluated.The evolution of the transient cavitating behaviors and corresponding cavitating flow-induced vibration has been investigated.The transient cavitation development strongly affects the vibration response and the dominant flow-induced vibration frequency decreases with the reducing of cavitation number.The unsteady break-off of the cavity,which is caused by the re-entrant jet,generates significant vibration fluctuations and the periodical shedding of the large cloud cavities have strong impacts on the structural vibrations.When the cavity develops to the trailing edge of the hydrofoil,the shock wave released by the collapse of the cloud cavity and its induced collapse of the residual attached cavity on the suction side aggravates the vibration of the foil,with larger amplitude and lower frequency.The coupled investigation of cavitating flow mechanism and corresponding cavitating flow-induced vibration has been conducted.The interaction between the evolution of transient cavitating flow patterns and structural hydroelastic responses has been investigated via combined physical and numerical analysis.The cavitation has enahnced the foil deformation and meanwhile,the hydroelastic response has also affected the cavitation development and the vortex structure interactions.A manufacturing concept for advanced composite blades has been adopted and subsequent manufacture has been made.The modal characteristics and damping capacity of the composite hydrofoil has been tested and the dramatic increase of damping behavior has been obtained.The transient flow-induced vibration mechanism of the composite hydrofoil has been investigated and the results showed that,as the excitation source,the flow velocity under lock-in conditions of composite hydrofoil decreases,resulting to the decrease of the resonance strength and reducing the possibility of structural failures.