Investigations Of The Flow Mechanisms Associated With Cloud Cavitation In The Hydraulic Machinery

Cloud cavitation is the complex pressure-driven phase change phenomenon,which widely exists in various hydraulic machinery,including pumps,turbines,and marine propellers.Among them,the sudden and repeated formation and collapse of the cloud cavity can induce mechanical vibration,noise,deterioration of hydraulic performance,and erosion,seriously affecting the unit's high efficiency and stable operation.Nowadays,researchers can repeat the development process of cloud cavitation using the current numerical techniques,including the breakup of attached cavity and the collapse of bubble clusters.In addition,controlling the cloud cavitation with an obstacle has been experimentally proven to weaken the unsteadiness effectively.However,due to the limitations of the experimental measurements,the underlying physical mechanisms of the controlling process are still not clear.Therefore,this dissertation firstly carried out a large amount of numerical work to investigate the flow mechanisms of cloud cavitation and instability suppression around the hydrofoils,revealing the corresponding spatial-temporal characteristics,spectral characteristics,hydrodynamic characteristics,vortex structures,and mode characteristics.Two numerical solutions were considered,including the Reynolds-averaged turbulent model in the commercial software ANSYS CFX and Large Eddy Simulation in the open-source Open FOAM.Finally,the modified Reynolds time-averaged turbulent model was employed to study the flow mechanisms of cloud cavitation inside the impeller of a centrifugal pump and the effects of placing the obstacle on the blade.This work can be summarized as follows:1.The numerical theory of homogeneous cavitating flow was introduced in detail in conjunction with ANSYS CFX and Open FOAM.Firstly,the open-source code was interpreted to enhance understanding of the Finite Volume Method and corresponding interpolation methods.Then,the governing equations of the cavitation solver were summarized,and the implementation of the Volume of Fluid method was highlighted.Finally,the Navier-Stokes(NS)equations of Reynolds-averaged model and spatial-filtered model were derived.Thus,the Shear Stress Transport(SST)k-? model and the Wall Adapting Local Eddy-viscosity(WALE)model were introduced to close the equations.2.The instability mechanisms of the cloud cavity around the Twist-11 hydrofoil equipped with the free-slip end wall were discussed firstly.The calculation indicated that modifying the turbulent viscosity of the SST k-? model can successfully reproduce the cloud cavitation,which agreed well with the experimental observations.Moreover,the spectrum characteristics and the underlying physical mechanisms were explored with Fast Fourier Transform(FFT),Bispecturm,Dynamic Mode Decomposition(DMD),and Proper Orthogonal Decomposition(POD).The spatial-temporal evolution of the cavity and the oscillating characteristics of the local flow rate were discussed based on recording the shedding processes of the dynamical cavity by arranging the monitor points and monitor surfaces around the hydrofoil.The results reported that the growing process of the sheet cavity could be divided into smooth and shattered stages.Analyses of Fast Fourier Transform(FFT),Bispecturm,Dynamic Mode Decomposition(DMD),and Proper Orthogonal Decomposition(POD)showed the existence of harmonics of the shedding frequency,of which the twice and triple frequency were captured.However,only the fundamental frequency dominated the cavitating field.3.Then,the instability mechanisms of the cloud cavity around the Clark-Y hydrofoil equipped with the non-slip end wall were discussed.The calculation also indicated that modifying the turbulent viscosity of the SST k-? model can successfully reproduce the cloud cavitation,which agreed well with the experimental observations.Moreover,the evolution of the cavitation vortex was discussed in depth by combining the Eulerian and Lagrangian perspectives,revealing that the Kelvin-Helmholtz(K-H)instability appearing inside the attached cavity was induced by the reversed flow.The perturbation of the water-vapor interface within the attached sheet cavity originated from the K-H instability and induced instability in the strain region during the development process,eventually forming streamwise vortices.Based on the concept of the scale factor,the correlation between the second-order derivative of the cavity volume and the pressure was established.4.The cavitating field around the trailing-blunt NACA 0009 hydrofoil was investigated using Wall Adapting Local Eddy-viscosity Large Eddy Simulations.The results showed that the main structural features of the cavities and vortices predicted by the large eddy simulation were consistent with those predicted in the previous chapters using Reynolds-averaged simulations,indirectly confirming that the conclusions derived based on the modified SST k-? turbulence model were plausible.In addition,the blunt structure of the trailing edge generated the vortex cavitation in the wake region,and the mechanisms associated with the transient hydrodynamic were different from that of the sharp-trailing hydrofoil.Due to the vortices shedding from the trailing edge of the hydrofoil was asymmetrical,the growth and shedding mechanisms of the vapor were also different.5.A trailing-blunt NACA 0009 hydrofoil was selected as the examination target to explore the flow mechanisms behind the suppression of the cloud cavitation field using an obstacle,which provided theoretical support for engineering applications.The influences of the obstacle on the evolution of cloud cavity were revealed based on the analyses of the Lagrangian Coherent Structures.The results showed that installing the obstacle at the different locations along the suction surface would affect the partial cavity oscillation in different ways.When the barrier was located at 10% and 20% chord length away from the leading edge,it behaved similarly to the vortex generator,enlarging the shedding frequency.However,when the barrier moved to 30% and40% chord length away from the leading edge,the development of the attached sheet cavity was significantly disturbed by the cloud cavity.Moreover,POD analyses of the hydrofoil equipped with the obstacle showed similar flow patterns but with the shortened size of each structure.6.Numerical simulations and experimental measurements were combined to investigate the flow mechanisms of cloud cavitation inside the centrifugal pump operating at the design point and the influences of placing the obstacles on the impeller blade.The results showed that as the NPSHa decreased,the sheet cavity attached to the impeller blade would become the cloud cavity.At the operating condition of the cloud cavity,the spatial-temporal diagram of vapor fraction along the suction surface had a clear boarding line dividing the cavity into two parts: the high vapor content region at the head and the low vapor content region at the tail.Moreover,the power spectral density peak shifted toward the blade's trailing edge as the NPSHa decreases.Except for the region nearby the connection between the pressure blade and the shroud,the power spectral density of pressure at the operating point of the sheet cavity was significantly different from the cloud cavity.The installation of an obstacle effectively reduced the cavity volume and thus the risk of cavitation erosion before the steep drop of head was observed.Most importantly,there was no significant change in the point of steep drop in head.However,the placement of the obstacle did not weaken the power spectral density of pressure at the suction surface but instead enhanced the pressure pulsation downstream of the cavity.