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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