Experimental And Numerical Study On The Flow Structures And Mixture Property Of Cloud Cavitation

Cavitation is the break-up phenomenon of liquid medium under locally low pressure that often occurs on the surfaces of propellers and appendages.Cavitation leads to undesirable effects such as erosion,mechanical vibration and noise,and severely restrict the safety and stealth performance of ships and submarines.Among the various forms of cavitation,the cloud cavitation is the one of great complexity.For one thing,the cloud cavitating flows are associated with strong unsteadiness and are often accompanied by irregular cavity shedding and various flow structures.For another,the water and vapor phases within cloud cavities are disorderly mixed,with the distribution of multiple-size vapor bubbles.The properties of vapor-water mixture are still not fully resolved.Aiming at the two aspects of cloud cavitating flows,systematic experimental and numerical investigations are conducted in this study.Experiments on sheet/cloud cavitating flows were performed in the cavitation tunnel under conditions with two hydrofoils of different chord lengths and various cavitation numbers.Two high-speed cameras were used to record the cavitation images.By utilizing modal decomposition methods,the cavity shedding patterns in both streamwise and spanwise views were extracted and analyzed.The optical fiber probe,which is capable of resolving water and vapor phases,was used to intrusively acquire the mixture characteristics inside the cavities which were not easily obtained by traditional optical approaches.Based on Open FOAM package,the cavitating flows with the same model and conditions with experiments were numerically investigated by Detached Eddy Simulation(DES).The numerical results supplemented the information of cavitating velocity fields that could not be acquired in the experiments.In order to obtain quantitative information from the high-speed photography of cavitating flows,the modal decomposition approaches are used to extract the cavity shedding structures and associated frequencies from the cavitation images.In the cases with different cavitation numbers,the primary shedding modes and shedding frequencies are determined by using the dynamic mode decomposition(DMD)method.The shedding behavior of cavitation on hydrofoils with different chord lengths is normalized by the modified Strouhal number St~*.The spanwise cavitation images are then analyzed by combining the proper orthogonal decomposition(POD)and DMD approaches.The cavity shedding patterns along the span can be categorized into alternate shedding,uniform shedding and irregular small-scale break-up,and the significance of each pattern varies in different cases according to the shape parameter of the cavity.Moreover,the frequency-weighted DMD method is proposed on the basis of the classic DMD algorithm,and the inaccessible high-frequency DMD modes are successfully extracted.These modes with higher frequencies indicate the instantaneous small-scale vapor structures in the cloud cavitating flow and are closely related to the high-frequency noise and vibration.The phase-resolved signals measured by the optical fiber prober are analyzed with multiple time scales.The time history and frequency features of vapor-water signals,the average void fractions over the entire measurement time,and the locally averaged void fractions within isolated cavitating clouds are investigated to obtain the global and local features of vapor-water mixture inside the cavitation region.Statistical descriptions of the distributions of bubble velocity and size are given.The log-normal probability density function is used to fit the bubble size distribution.The resultant distribution parameters agree well with the results of another experiment on cloud cavitation,indicative of a uniform bubble size distribution between two experiments.The volume-weighted probability density function is calculated to reveal the contributions of bubbles with different sizes to the total vapor volume.Furthermore,the scaling laws of bubble size distribution are analyzed.Analogous to the case of breaking waves,two scaling laws of different mechanisms,which are controlled by large-scale coherent structures and turbulent eddies of inertial sub-range scale respectively,are found in the cloud cavitating flow.Thus,it is indicated that turbulence plays a key role in the formation of bubble size distribution.Based on the finite volume method and homogeneous mixture model,the cloud cavitating flows around the hydrofoil are finely simulated.The DMD modes extracted from numerical snapshots are in good agreement with the experimental ones.After the quantitative comparison and validation of numerical results,the velocity fields are investigated to supplement additional information of the flow field inside the cavitation region that was not measured in the experiments.The coherent structures extracted from the velocity field in the mid-span plane have similar characteristic frequencies with those of cavity shedding.The profiles of average void fraction and streamwise velocity at different streamwise positions are sampled to investigate the spatial variation of time-averaged flow field.