Time-accurate Numerical Simulations of Swirling Flow with Rotor-stator Interaction

Abstract

A series of numerical simulations is undertaken to study a highly swirling turbulent flow generated by rotor-stator interaction in a swirl generator. The purpose is to assess the applicability of different turbulence models in 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 URANS, two low-Reynolds URANS and three hybrid URANS-LES. These are the standard k − , SST k − ω, realizable k − , RNG k − , Launder-Sharma k − , Lien-Cubic k − , delayed DES Spalart-Allmaras, DDES SST k − ω and improved DDES-SA. 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 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 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. The flow contains a strong disintegration of the vortex rope which is predicted well by the hybrid RANS-LES models. The hybrid methods also capture the blade wakes better than the other models, elucidating the wake interaction with the vortex rope. The frequency of the vortex rope is predicted well and the total turbulence (resolved and modeled), suggested by DDES-SA, corresponds reasonably well to the experimental results.

Active flow control of the vortex rope and pressure pulsations in a swirl generator

“…The flow in water turbines operating at the best efficiency point condition has a moderately low level of swirl forming a stable on-axis structure (Javadi, Bosioc, Nilsson, Muntean, & Susan-Resiga, 2016;Javadi & Nilsson, 2015a). The flow at conditions away from the best efficiency point often contains a high level of swirl which may cause a recirculation region.…”

“…Javadi and Nilsson (2014a) studied the Scale-Adaptive Simulation (SAS) (Menter & Egorov, 2010) approach in highly swirling flows and reported that the approach predicts the massively separated swirling flows very well. Detached-Eddy Simulation (DES) (Spalart, Jou, Strelets, & Allmaras, 1997) is another promising hybrid URANS-LES strategy which is studied for application to swirling flows (Javadi & Nilsson, 2015a, 2015bJavadi et al, 2016). The Delayed DES (DDES) methodology was proposed by Spalart et al (2006) to identify the boundary layer thickness and to extend the URANS mode compared to that of the initially proposed DES methodology.…”

“…This method is appropriate for flows with large internal structures at practical Reynolds numbers. Javadi and Nilsson (2015a) studied the highand low-Reynolds number models and the DDES and LES models in a flow similar to that in a Francis turbine operating in the part load condition. The hybrid URANS-LES simulations were up to two orders of magnitude more expensive than those with the eddy-viscosity models.…”

“…They also concluded that the DDES-SA model plausibly predicts the highly swirling turbulent flow in hydraulic machinery. The precession happens far from the wall effects, and thus the DDES method is an applicable approach for highly swirling flows (Javadi & Nilsson, 2015a). Javadi and Nilsson (2015b) showed that the DDES method is still sensitive to wall-parallel grid spacing and, in the case of coarse spacing, the method does not switch to the LES mode at all.…”

“…The jets are injected in a direction opposite to that of the main swirl of the flow. The study is performed with the DDES-SA turbulence model and the results are compared with the results of Javadi and Nilsson (2015a), and Javadi et al (2016).…”