Transition to turbulence in oscillatory flows in stenosed pipes

We studied oscillatory flow in stenosed cylindrical pipesin axisymmetric and to quantify and characterize the onset of turbulence in zero mean oscillatory flows. Direct numerical simulation (DNS) in various degrees of stenosis, flow frequencies and Reynolds numbers were conducted amounting to about 180 simulations. The canonical studies provide basic insight into the factors that lead to onset and sustainment of turbulence. DNS were conducted using the lattice Boltzmann method (LBM) solver Musubi. Stenosis with area reduction of 75%, 60%, 50% and 25% were studied in both axisymmetric and eccentric configurations. Meshes of up to 2.8 billion cells were created and simulations were conducted  on 300’000 CPU cores of the SuperMUC-NG petascale system in Munich, GERMANY. We found: 1. The flow transitions only in higher degrees of stenosis namely 50%, 60% and 75%, where Re~1800 is the approximate threshold for transition. 2. The flow reversal stabilizes the flow field for all pulsation frequencies and degrees of stenoses. 3. A higher pulsation frequency leads to earlier breakdown of flow – a phenomenon that is seen mostly for lower stenoses degrees. 4. The eccentricity of the stenosis causes higher fluctuations.