Advanced numerical prediction of strongly swirling turbulent flows

Övrigt konferensbidrag, 2013

The strongly swirling turbulent flow through an abrupt expansio n is investigated using highly resolved LES and hybrid RANS- LES, to shed more light on the stagnation region and the helical vortex breakdown. The vortex breakdown in an abrupt expansion resembles the so-called vortex rope occurring in hydro power draft tubes. It is know n that the large-scale helical vortex structures can be captured by re gular RANS turbulence models. However, the spurious suppression of the small-scale structures should be avoided using less diffusive methods [1]. The present work compares LES and hybrid RANS-LES results with the experimental measurement of Dellenback [2]. The com putations are conducted using a general non-orthogonal, block- structured, finite-volume method with a fully collocated storage available in the OpenFOAM CFD code. The dynamics of the flow is studied at two Reynolds numbers, Re =6.0×10 4 and Re =10.0×10 4 to study the effect of high Reynolds turbulent flow with almost constant high swirl number ( S r =1.16 and S r =1.23, respectively). The delayed detached eddy simulation (DDES) Spalart-Allmaras model [3] and the dynamic one-equati on LES model are used to predict the coarse (8.2×10 6 cells) and fine (12×10 6 cells) discretized computational domain. The averaged velocity field, pressure field and the root mean square of the velocity fluctuations are captured and investigated qualitatively. Fig. 1 shows the instantaneous pressure iso-surface for the two operating conditions. The flow with lower Reynolds number gives much weaker outburst although the frequency of the structures seems to be constant for the plateau swirl number.