Large Eddy Simulation of Turbulent Swirling Flow Through a Sudden Expansion

Turbulent swirling flow through a sudden expansion is investigated numerically using Large Eddy Simulation (LES). The flow resembles the flow in a draft tube of a water turbine that is working at part load. The swirling inflow is subject to a strong adverse pressure gradient and the symmetry of the flow breaks down close to the inlet. This gives rise to an oscillating, helicoidal vortex core which in turn creates a highly unsteady and turbulent flow field. In this work, the large-scale turbulent structures are numerically resolved, and detailed information about the flow characteristics is obtained. The oscillating flow is analysed using Fourier transforms of the wall pressure at different downstream locations. The most dominant frequency corresponds to the rotational rate of the precessing vortex core, and it is found that this frequency is constant throughout the domain. The results of two simulations using numerical discretization schemes of different order are compared. It is shown that the frequency of the precessing vortex core is not sensitive to the choice of discretization. However, the lower frequencies of the flow depend to a higher extent on the numerical accuracy. To validate the results, the computed velocities are averaged and compared to experimental data. The agreement is good. The Reynolds stress tensor is also computed and analysed. It is found that large degrees of turbulent anisotropy are found only in the region that is dominated by the oscillating vortex core. Further downstream, the degree of turbulent anisotropy is almost negligible despite the relatively higher level of swirl