Numerical modeling and simulation of turbulence-cavitation interactions in a venturi geometry

Aditya R Chunekar, Purdue University

Abstract

Partial cavitation is an unsteady cavitation phenomenon involving shedding and coherent collapse of large vapor structures. A better understanding of the cavitation dynamics and the complex turbulence-cavitation interactions occurring in the partial cavitation phenomenon can be gained through numerical simulation of a turbulent cavitating flow in a venturi nozzle. In the present study, two approaches for turbulence simulation were used viz. Reynolds Averaged Navier-Stokes (RANS) modeling approach and the Large Eddy Simulation (LES) approach. The commercial software FLUENT was used for the 2-D unsteady RANS simulations. A Renormalization Group (RNG) k − ε turbulence model was used along with the “full cavitation model” available in FLUENT. The turbulent viscosity was modified as proposed by Delgosha et al. [1] to account for the compressibility effects of the two-phase region. Although the simulations predict the classical vapor cloud shedding, the time-averaged distributions of streamwise velocity and void fraction in the two-phase region deviate considerably from the experimental measurements, especially in the turbulent cavity closure region. This can be attributed to the inherent unsteadiness of the cavitation phenomenon and significant influence of turbulent flow structures on cavitation which the RANS approach cannot capture. Hence LES computations were conducted on a 3-D venturi nozzle geometry using an in-house research code. The code solves the fully compressible Favre-filtered Navier-Stokes equations with the dynamic Smagorinsky model for the closure of the sub-grid scale terms. Cavitation is modeled using a homogeneous equilibrium assumption. The stiffness of the system due to the incompressible liquid phase is addressed by artificially increasing the Mach number. The simulations predict the formation of a vapor cavity at the venturi throat with an irregular shedding of the small scale vapor structures near the turbulent cavity closure region. It is found that the formation of vapor in the throat region suppresses the velocity fluctuations. The collapse of the vapor structures in the downstream region is observed to be a major source of vorticity production resulting in the formation of hair-pin vortices. A detailed analysis of the vorticity transport equation shows a decrease in the vortex stretching term due to cavitation. A substantial increase in the baroclinic torque is observed in the regions where the vapor structures collapse leading to vorticity generation. A spectra of the pressure fluctuations in the far-field downstream region shows an increase in the acoustic noise at high frequencies due to cavitation.

Degree

M.S.M.E.

Advisors

Frankel, Purdue University.

Subject Area

Mechanical engineering

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