Instability Mechanism Of Rotating Cascade Cavitation Flow In Non-inertial Systems

Cavitation is a common phenomenon in fluid machinery.This phenomenon is one of the important issues that engineering designers need to consider in the design and operation of hydraulic machinery.The cavitation process is accompanied by vibration,noise,cavitation and other unfavorable factors that affect the operation of the machine.At the same time,the aggravation of cavitation will also cause the rapid decline of head and efficiency,resulting in the waste of energy.Therefore,the cavitation mechanism Research matters.The main content and results of this paper are as follows:(1)Numerical simulation and experimental comparison of NACA0015 hydrofoil cloud cavitation by open source software OpenFOAM.The research shows that the cavitation morphology of the two-dimensional and threedimensional hydrofoils is quasi-periodic.Due to the complexity of the threedimensional structure,the shape development of the cavitation on the threedimensional hydrofoil in the span direction is not the same.The cavitation instability mechanism of the hydrofoil has been studied.For the twodimensional hydrofoil,the back jet flow generated by the reverse pressure gradient and the cavitation interface work together to cut off the attached cavitation bubbles on the surface of the hydrofoil,causing them to lose stability and fall off;for the three-dimensional hydrofoil wing,due to the wall effect,the direction of the return jet is divided into lateral and forward return jets,and the joint action of the two return jets makes the cavitation destabilize and fall off.The study compares the advantages and disadvantages of the current thirdgeneration vortex identification technology used in 3D hydrofoil cavitation.The third-generation vortex identification technology represented by the omega criterion can accurately identify the existence of vortex in the recommended threshold range.The determination of the strength of the vortex core and the location of the vortex core have high accuracy.(2)In order to explore the influence of non-inertial system on cavitation instability,the cavitation development process of stationary cascade was studied first.The development of cavitation on the back of the blade of the static cascade under low cavitation number also presents the quasi-periodic development of four stages in the three-dimensional hydrofoil,but the cavitation process of the static cascade does not appear in the process of cavitation shedding and collapse.Instead of cloud cavitation in large flakes,shedding starts from cavitation near the front cover and near the surface of the blades.The research compares the cavitation instability mechanism of the three-dimensional hydrofoil cloud and the cavitation instability mechanism of the stationary cascade under low cavitation number,and shows that both of the instability mechanisms are due to the retrojet flow.The flow area of the static cascade expands continuously,and the return jet at the end of the cavity presents quasi-three-dimensional flow characteristics,which is why the cavitation shape of the static cascade is different from that of the three-dimensional hydrofoil.(3)Finally,the cavitation development process of the rotating cascade in the non-inertial system is studied,and the influence of the non-inertial system on cavitation is compared.Through comparison,it is found that the abstract blade can greatly reduce the influence of blade curvature and other factors on cavitation instability,which is convenient for further research on its instability mechanism.The cavitation instability mechanism of the rotating cascade has been preliminarily found out,indicating that the co-action of Coriolis force and return jet flow is the cause of cavitation instability.