Coordination Funds

• Applicant: Professor Dr. Christian Wagner
• Subject Area: Fluid Mechanics ; Cardiology, Angiology ; Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 349558021

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 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. 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, especially for large pulsation amplitudes. The same holds for the geometry of the fluidic system that is kept simple in most studies, even though geometries in most applications are not straight, rigid 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. 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 underlie instability mechanisms in vascular flow. In the first funding period, we reached already a substantial understanding on the effect of unsteady driving on the most fundamental system: the flow of water through a straight pipe, including large-amplitude pulsation and viscoelastic flow. We have also made first progress in the understanding of the migration dynamic in suspension of soft and hard particles, and flow in models of the aorta. With respect to the flow of blood, we have obtained a quantitative understanding on the effect of pulsation on the shape transitions on the single cell level and we already could clarify the cause for the highly irregular flow in the capillary network by means of in-vivo imaging of vascular flow in rodents. A special focus in the second funding period will be now set on the effect of specific flow geometries such as bents, junctions and models of the aorta. In addition, viscoelasticity and wall compliance will be added to our systems. Also, for the case of blood, we will now study the effect of polymers, including the clinical case of volume replacement fluids.

• DFG Programme: Research Units
• Subproject of: FOR 2688: Instabilities, Bifurcations and Migration in Pulsatile Flows