On the Generation of Large-Scale Vortical Flow Structures in Pipe Multifurcations

Abstract

In many hydraulic power plants, the water supplied through the penstock needs to be distributed to multiple turbines. The application of pipe multifurcations is motivated by economic and efficiency reasoning. However, flow instabilities and large-scale vortical flow structures can occur for some operating conditions, causing high-amplitude pressure and mass flow rate fluctuations. To enhance the understanding of the flow instability inception, we investigate the unsteady flow in a pipe multifurcation with six branches for different operating conditions. The unsteady flow is simulated employing the large eddy simulation approach, where the numerical mesh is constructed such that y+ < 1 and a recycling-type boundary condition is utilised to provide the turbulent inflow conditions. While the general flow pattern are described by the time-averaged results, the work focuses on analysing the unsteady physical phenomena occurring in the pipe multifurcation. At part-load operation, the results clearly show the formation of large-scale vortical flow structures stretching into the active branches. The vortices are observed to move around and eventually burst. When one vortex breaks down, the other large-scale vortices reaching into the other active branches are likely to burst and the flow pattern changes. Such events come with decreased turbulence intensity propagating through the branch line, flow separation at the branch line junction, and mass flow rate fluctuations. After a short time, large-scale vortices were established again.