Abstract
ABSTRACTThe regime of compressible flow generally refers to the super/subsonic case. However, several remarkable cases with low Mach number could not be appropriately described with the incompressible method. It is a similar case for a cavitating jet inside a poppet valve. In order to comprehensively address the discrepancy between incompressible and compressible methods, both non-cavitating and cavitating cases are performed in experiment and calculation based on OpenFOAM. Experiment reveals a transition in flow pattern in both non-cavitating and cavitating flow. For example, for 0.7 mm openness and 30-degree poppet angle, transition happens at approximately 29 and 27 bar pressure drop for the two cases, respectively. In general, results from the compressible method exhibit better agreement with experiment regarding both flow performance and flow structure. By contrast, the incompressible method could not provide an accurate description for the transition process under the applied flow condition. A series of studies are carried out with emphasis on such discrepancy. Firstly, the deviation in flow performance is addressed based on velocity profile and turbulence level. Secondly, the disparity in flow structure is illustrated and the mechanism for cavitation inception is discussed, which combined provide an interpretation of the deficiency of the incompressible method. Thirdly, different inlet boundary conditions are applied, and the results confirm the independence of deficiency of the incompressible method for inlet fluctuation. Finally, a re-examination is proposed concerning traditional notion of compressible flow as well as the applicability of incompressible numerical method.
Highlights
Computational fluid dynamics (CFD) enables more details of flow structure to be captured
The results revealed that flow force under the condition of cavitation is highly dependent on geometric structure, and a two-stage configuration is superior in cavitation suppression but suffers from larger flow force
Flow performance For the non-cavitating flow, no inter-phase transfer is involved in either calculation or experiment, the applied numerical methods are single phase flow
Summary
Computational fluid dynamics (CFD) enables more details of flow structure to be captured. If in the present case the compressible method shows improvement in capturing the evolution of coherent structure, the significance of compressibility at least in numerical calculation is further justified, and extends to the regime in a specific case with low Mach number, which was once commonly simplified as incompressible. Another important motivation comes from the unanswered question of whether cavitation is triggered by inlet turbulence and inappropriate boundary condition contributes to the failure of the incompressible method. It is expected that the discussion will serve as general guidance for numerical studies on small-sized orifices in hydraulic area
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