Fluid-structure interaction in pulsatile flow of non-Newtonian fluids

• Applicant: Professor Dr.-Ing. Markus Holzner, Ph.D.
• Subject Area: Fluid Mechanics ; Hematology, Oncology
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 349558021

The interaction between blood flow and the elastic wall boundaries plays an important role in cardiovascular circulation as it drives pulse-wave propagation in arteries and assists in the maintenance of organ perfusion during diastole when cardiac ejection ceases. In the highly compliant aorta the elastic walls strongly influence the flow velocity, pressure and wall shear stress distributions and may affect the appearance of turbulence, which is relevant for the development of pathophysiological states. However, even in simple geometries like flexible pipes it is not yet fully understood under which conditions the coupling of the flow and wall movement is most relevant and when it promotes a dampening or rather an amplification of flow disturbances and turbulence. The aim of this project is to elucidate flow-structure interaction and the transition to turbulence in pulsatile flow through compliant conduits and aorta replicas. In the first project phase we established novel methodologies to advance the understanding of flow-structure interactions in flexible pipes and aorta replicas and we set up a new test rig that can host 3D-printed pipe and aorta geometries driven by a piston pump through a high-precision electrical cylinder allowing for steady and pulsatile forcing with arbitrary waveforms over a wide range of Womersley and Reynolds numbers. Real blood is not a Newtonian fluid and in Phase II we now include fluid rheology and aim to elucidate the impact of flexible walls on transition to turbulence in non-Newtonian fluids. This will be achieved by looking at the stability of pulsatile flow in elastic pipes and flexible aortas and we will now consider shear thinning fluids and viscoelastic polymer solutions. As a first step, we will investigate the impact of flexible walls on the stability of pulsatile flow of polymer solutions. This will provide insights on the synchronisation time scales between flow and wall motion in a system where elastic energy can be stored 'externally' (walls) and 'internally' (polymers). As a second step, we will study the transition to turbulence in flexible aortas focusing on wall elasticity and rheology and we will focus on the descending aorta where the effect of inflow conditions fades away but other factors (flexible walls, pulsatility, rheology) persist. These results will provide comprehensive evidence on the origin of disordered blood flow in realistic cardiovascular geometries. Upon completion of this project, we expect to understand how the transition to turbulence depends on characteristics of wall material and non-Newtonian fluid properties. This will be relevant for understanding the relation between blood flow and vascular diseases as it will improve our ability to predict blood transport efficiency with decreasing aortic compliance in atherosclerosis.

• DFG Programme: Research Units
• Subproject of: FOR 2688: Instabilities, Bifurcations and Migration in Pulsatile Flows
• International Connection: Switzerland
• Major Instrumentation: High speed camera
• Instrumentation Group: 5430 Hochgeschwindigkeits-Kameras (ab 100 Bilder/Sek)
• Partner Organisation: Schweizerischer Nationalfonds (SNF)