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Research Goal and Objectives

FOR 2688 Project Overview

The overall objective of our research unit is to develop a qualitative and quantitative understanding of hydrodynamic instabilities and migration phenomena in pulsatile flows of Newtonian and complex fluids with a special focus on the vascular flow of blood.

Pulsatile flows are ubiquitous in industrial processes and in biological systems. Flows in engines, hydraulic systems, pumping mechanisms and most prominently, in the cardiovascular system are pulsatile. In practice, perfectly steady flow rates are technically difficult to achieve and most flows have an oscillatory, or at least unsteady, component that introduces an external time scale. Nevertheless, most existing studies on flows in pipes and channels at low and high Reynolds (Re) numbers and their instabilities consider constant driving.
Pulsatile driving of the flow leads to qualitatively different transition scenarios, both in Newtonian and in complex fluids. First, acceleration and deceleration could cause new instabilities. Second, in complex fluids the coupling with internal time scales can destabilize the predominant flow and migration patterns. These instabilities may result in severe technical problems because of flow-structure interactions and resonance phenomena. For example, they may lead to a caustic failure of tubing, whereas in the vascular system flow instabilities pose a severe risk of cardiac diseases and thrombosis. For Newtonian fluids, significant progress has recently been made in understanding the transition from laminar to turbulent flow. However, the impact of pulsation on the transition mechanisms is much less well understood. The same holds for the geometry of the fluidic system that is kept simple in most studies, even if geometries in most applications are more complex than straight pipes or channels. Equally, non-Newtonian effects on flow stability are often neglected, although many fluids of practical relevance are complex, such as polymer solutions or suspensions. While in the Newtonian case instabilities are driven by inertia, in complex fluids interactions between the particles or elastic stresses can lead to new instability mechanisms, the most prominent one probably being viscoelastic turbulence. In suspensions, such as slurry or blood, the transport and migration of the particles are strongly coupled with the flow and particles can migrate towards or from the center of the conduit, which affects the macroscopic flow properties. In addition, blood shows strong shear thinning as well as viscoelasticity, and the vascular vessel walls are deformable. It is a major goal of this research unit to understand which of these ingredients dominate instability mechanisms in vascular flow.

We aim to study the effect of unsteady driving, starting with the simplest system, namely the flow of water through a straight pipe. Subsequently, we increase the complexity of the sample by considering hard and soft sphere suspensions, and ultimately blood. Beyond straight pipes, we study different flow geometries such as bends, contraction-expansion and bifurcations, the effect of wall compliance, and models of the aorta. In addition, we perform in-vivo imaging of vascular flow in rodents. Hence, our research unit covers many fluidic systems, from turbulent pipe flow of water up to in-vivo blood flow.

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