CFD Simulations AC6-14
Swirling flow in a conical diffuser generated with rotor-stator interaction
Application Challenge AC6-14 © copyright ERCOFTAC 2024
Overview of CFD Simulations
A series of numerical simulations was undertaken to study a highly swirling turbulent flow generated by rotor-stator interaction in a swirl generator \cite{Javadi2015c}. The purpose was to assess the applicability of different turbulence models in a swirling flow with a high level of unsteadiness and a significant production and dissipation of turbulence in the flow away from the wall. Nine turbulence models are compared: four high-Reynolds number models URANS, two low-Reynolds number models URANS and three hybrid URANS-LES models. These are the standard $k-\epsilon$, SST $k-\omega$, realizable $k-\epsilon$, RNG $k-\epsilon$, Launder-Sharma $k-\epsilon$, Lien-Cubic $k-\epsilon$, delayed DES Spalart-Allmaras \cite{Spalart2006}, DDES SST $k-\omega$ \cite{Gritskevich2012} and improved DDES-SA \cite{Shur2008} models. The URANS models are capable of capturing the main unsteady feature of this flow, the so-called helical vortex rope, which is formed by the strong centrifugal force and an on-axis recirculation region. However, the size of the on-axis recirculation region is overestimated by the URANS models. Although the low-Reynolds number URANS formulations resolve the boundary layers in the runner and the draft tube more accurately, they still encounter difficulties in predicting the main flow features in the adverse pressure gradient region in the draft tube. It is shown that a more detailed resolution, which is provided by the hybrid URANS-LES methods, is necessary to capture the turbulence and the coherent structures.
SIMULATION CASE CFD
Solution Strategy
The calculations reported herein are made using the finite-volume method in the OpenFOAM-1.6-ext CFD code. The second-order central differencing scheme is used to discretize the diffusion terms. The linear-upwind differencing is used in URANS simulations to approximate the convection term. The blended numerical scheme is used in the hybrid method. The scheme is a combination of linear-upwind differencing in the URANS region and a limited linear total variation diminishing (TVD) scheme with a conformance coefficient in the LES region. The convection term in the LES region is interpolated by 15\% linear-upwind differencing and 85\% central differencing. The larger the part of the central differencing in the LES region is, the smaller the time-step required. Furthermore, the second-order van Leer TVD scheme is used to approximate the convection term in the hybrid method. The numerical schemes have only small effects on the time-averaged values. Time marching is performed with an implicit second-order accurate backward differentiating scheme.
Computational Domain
Boundary Conditions
Application of Physical Models
Numerical Accuracy
CFD Results
References
Contributed by: A. Javadi, A. Bosioc, H Nilsson, S. Muntean, R. Susan-Resiga — Chalmers University of Technology
© copyright ERCOFTAC 2024