Jiongyang Wu

Sensitivity Evaluation of a Transport-Based Turbulent Cavitation Model

A sensitivity analysis is done for turbulent cavitating flows using a pressure -based Navier-Stokes solver coupled with a phase volume fraction transport model and non-equilibrium k-e turbulence closure. Four modeling parameters are adopted for evaluation, namely, Ce1 and Ce2, which directly influences the production and dissipation of turbulence kinetic energy, and Cdest and Cprod, which regulate the evaporation and condensation of the phases. Response surface methodology along with design of experiments is used for the sensitivity studies. The difference between the computational and experimental results is used to judge the model fidelity. Under non-cavitating conditions, the best selections of Ce1 and Ce2, exhibit a linear combination with multiple optima. Using this information, cavitating flows around an axi -symmetric geometry with a hemispherical fore-body and the NACA66(MOD) airfoil are assessed. Analysis of the cavitating model shows that the favorable combinations of Cdest and Cprod, are inversely proportional to each other for the geometries considered. A set of cavitation numbers is selected for each of the geometries to demonstrate the predictive capability of the present modeling approach for attached, turbulent cavitating flows.

Evaluation of Richardson extrapolation in computational fluid dynamics

The concept of Richardson extrapolation is evaluated for improving the solution accuracy of two well-investigated two-dimensional flow problems: (1) laminar cavity flows with Re = 100 and 1,000; and (2) the Reynolds-averaged backward-facing step turbulent flow with Re = 10 6 , aided by the widely used k- k two-equation model with wall function. Uniform grid systems are employed in all cases to facilitate unambiguous assessment. By systematically refining the grid, computational fluid dynamics (CFD) solutions with different resolutions are first obtained, then extrapolated from a finer to a coarser grid using Lagrangian interpolation with either a 9-point or a 16-point formula. For laminar flows, Richardson extrapolation does not exhibit consistent trends in order of accuracy. Furthermore, the relative performance of Richardson extrapolation, in comparison with solutions obtained directly from mesh refinement, is not sensitive to the level of residuals contained in each computation, the detailed interpolation formula between grids, the choice between second-order central difference and upwind convection schemes, and the selection of the error norms. For turbulent flow computations, large jumps in velocity profiles between the wall and the first node cause difficulties in interpolation, and Richardson extrapolation performs unsatisfactorily under such situations. The present study indicates that Richardson extrapolation does not work consistently in approaches typically employed for engineering CFD applications.

Impact of Turbulence and Compressibility Modeling on Three-Dimensional Cavitating Flow Computations

The large density ratio, up to 1000 in water, between liquid and vapor, turbulence with complicated interface dynamics and fast and multiple time scales make the computation of cavitating flows difficult. Utilizing a pressure-based algorithm extended for such flows, the present study focuses on the non-equilibrium and nonstationary aspects in the turbulence model, and the compressibility effects in the cavitation model. Assessment of several modeling concepts has been made in the context of the Favre-averaged Navier-Stokes equations, along with a transport equation-based cavitation model and the e − k two-equation turbulence model. The three-dimensional turbulent cavitating flow in a hollow-jet valve is adopted as the physical focus. While the non-equilibrium and nonstationary modification to the turbulence closures do not seem to influence the qualitative characteristics of the simulation, the compressibility modeling can cause the result to vary substantially.

Filter-Based Unsteady RANS Computations for Single-Phase and Cavitating Flows

The widely used Reynolds-Averaged Navier-Stokes (RANS) approach, such as the k-e two-equation model, has been found to over-predict the eddy viscosity and can dampen out the time dependent fluid dynamics in both single- and two-phase flows. To improve the predictive capability of this type of engineering turbulence closures, a consistent method is offered to bridge the gap between DNS, LES and RANS models. Based on the filter size, conditional averaging is adopted for the Navier-Stokes equation to introduce one more parameter into the definition of the eddy viscosity. Both time-dependent single-phase and cavitating flows are simulated by a pressure-based method and finite volume approach in the framework of the Favre-averaged equations coupled with the new turbulence model. The impact of the filter-based concept, including the filter size and grid dependencies, is investigated using the standard k-e model and with the available experimental information.Copyright