Simulation of Cloud Cavitation: Evaluation and Qualitative Assessment of Turbulence Predicted by k-omega SST and k-omega SAS Models

Abstract

Cloud cavitation is a strongly unsteady two-phase flow in which vapor formation, cavity shedding, collapse, and turbulence interact over a wide range of spatial and temporal scales. In numerical simulations, these processes are commonly modeled using Reynolds-averaged or scale-adaptive turbulence closures, but agreement in mean flow or shedding frequency does not necessarily imply agreement in turbulent momentum transport. This thesis evaluates cloud-cavitating Venturi-flow simulations obtained using k-omega SST and k-omega SST-SAS models and compares them with experimental velocity and grayscale image data. The experimental dataset corresponds to a 48 L/min cloud-cavitating Venturi case. High-speed grayscale images were used to estimate an apparent vapor-fraction field and an apparent mixture-density field. These fields were combined with measured velocity fluctuations to form an apparent density-weighted Reynolds shear stress. The numerical SST and SST-SAS datasets were compared using spectral diagnostics, time-averaged velocity and density fields, density-weighted stress fields, and phase-averaged quantities over the cavitation cycle. A spatial filtering-based decomposition was also used to define experimental resolved-equivalent and residual stress contributions for diagnostic comparison with SST-SAS resolved and modelled stress terms. The spectral analysis showed that the experiment had a dominant shedding frequency near 95.21 Hz, while SST-SAS produced a close peak near 97.65 Hz. SST remained in the same low-frequency cavitation range but showed a shifted dominant response near 117.17 Hz. The mean velocity fields showed that both simulations reproduced the broad cavity-induced acceleration and flow deflection. However, the density weighted stress comparisons revealed larger differences. The experimental stress was weaker and more localized, while SST and SST-SAS produced stronger and more organized stress along the cavity shear layer and downstream closure region. In SST-SAS, this difference was present in both resolved and modelled stress contributions. The results show that matching the dominant cavity-shedding time scale is not sufficient to reproduce density-weighted turbulent momentum transport. The main discrepancies are associated with local vapor distribution, wall-normal interface motion, re-entrant-flow organization, u'-v' covariance, and the eddy-viscosity-based modelled stress response. The thesis therefore demonstrates the value of density-weighted and phase-resolved diagnostics for qualitative assessment of cavitating-flow turbulence predictions.