Transitional Flow Regime and its Significance in Physiology
AUTHOR: Kartik Jain1,2
1Biomedical Fluid Dynamics, Faculty of Engineering Technology, University of Twente, The Netherlands
2Institute for Computational Physics, University of Stuttgart, Germany
BACKGROUND: Presence of turbulence in physiological flows is known since a long time. Recent computational studies found such flow regime in blood flow in aneurysms, flow in the left ventricle as well as the oscillatory cerebrospinal fluid (CSF) flow in the spinal canal. The physical, and physiological significance of such a flow regime remains debatable thereby raising a question whether numerical methods should be appropriately chosen and resolved to characterize such a phenomena. In this work we present fully resolved direct numerical simulation (DNS) of transitional and turbulent flow regimes with steady, pulsatile and oscillatory inflow conditions on basic geometries like the FDA nozzle and a stenosis and discuss the significance of such regimes in detail.
METHODS: DNS were conducted using the lattice Boltzmann method (LBM) solver Musubi. We took the FDA nozzle benchmark to evaluate its efficacy as an experimental benchmark, and to explore if our solver would reproduce previous experimental results. In addition, we conducted simulations on a stenosed pipe in axisymmetric and eccentric configurations to explore the role of geometry on flow transition. Simulations with steady, pulsatile and oscillatory flow were conducted at various Reynolds numbers with meshes of up to 2.8 billion cells, and were executed on 300’000 CPU cores of the SuperMUC-NG petascale system installed in Munich, Germany.
RESULTS: LBM computed quantities matched well against the experiments for throat Re of 2000 and 3500 for the FDA nozzle. We did not find considerable difference in flow quantities and jet breakdown locations with different resolutions. For the stenosis case with steady and pulsatile flow, critical Re for transition and flow quantities agreed with previously conducted experiments. For the oscillatory flow, however, the critical Re for transition tripled compared to the unidirectional pulsating flow attributed to flow stabilization during flow reversal.
DISCUSSION & CONCLUSIONS: The results advocate that transition to turbulence is a possibility in physiologic flows, and the geometry of the conduit is the most prominent factor that results in the onset of turbulence. Such a flow will have significance in mechanobiology. Whether a numerical setup should be tuned to capture such a phenomena would depend entirely on the research questions. For quantities like averaged wall shear stress or pressure drop, for example, it would be unnecessary to conduct detailed DNS whereas for quantites like local flow field that might influence formation of thrombus, or, could cause hemolysis it would be important to conduct detailed DNS. The LBM allows for efficient computations in this flow regime in complex geometries, and a detailed comparison of methods should be carried out in future.