Hydrodynamic stability of pulsatile flow of complex fluids

Pulsatile flows are widespread in nature and technology and a prominent example is cardiovascular blood flow. The flow rates in such periodic processes often exceed typical thresholds at which turbulence may occur. Our interest is in the hydrodynamic stability and the transition to turbulence in pulsatile flows. With respect to blood flows, it quickly becomes clear that this question is much more complex than that regarding the stability of ordinary fluid flows with constant driving in simpler geometries. Even in simple (steadily driven) pipe flow, in which only a single control parameter (the average flow velocity or more precisely the Reynold number) occurs, and understanding of the transition to turbulence poses a considerable challenge. In the case of pulsation, in addition to the frequency and amplitude, the waveform (which often deviates significantly from a sinusoidal shape) also plays an important role. In addition, fluid properties (suspension of blood cells, elastic and shear-thinning properties) also influence the turbulence transition. Moreover, geometric deviations, e.g. curvature, constrictions bifurcations can stabilize or destabilize flows. In our previous project on pulsatile flows, our group determined the influence of pulsation and the dependence of frequency, amplitude and waveform on the flows stability in straight pipes. We succeeded in identifying a previously unknown instability mechanism that interestingly already occurs at significantly lower flow velocities and we were able to detect its signature in blood flows. In this new project we will now explore the influence of complex fluid properties (specifically shear thinning, particle concentration, viscoelasticity) and geometry (curvature and bifurcations) on pulsating flows, in order to determine how these changes affect the stability of the flow and whether other types of instability may occur.