Nonlinear hydrodynamic instability and turbulence in pulsatile flow

TL;DR

This paper uncovers a nonlinear instability mechanism in pulsatile pipe flow that can generate turbulence at low flow rates, impacting cardiovascular health by causing shear stress fluctuations and flow reversal.

Contribution

It identifies a new nonlinear instability process involving geometrical distortions leading to turbulence in pulsatile flows, relevant to cardiovascular conditions.

Findings

Flow bursts of turbulence occur at low flow rates due to geometric distortions.

Flow pattern transitions involve helical vortices during deceleration.

Shear stress fluctuations and flow reversal are observed within each cycle.

Abstract

Pulsating flows through tubular geometries are laminar provided that velocities are moderate. This in particular is also believed to apply to cardiovascular flows where inertial forces are typically too low to sustain turbulence. On the other hand flow instabilities and fluctuating shear stresses are held responsible for a variety of cardiovascular diseases. Here we report a nonlinear instability mechanism for pulsating pipe flow that gives rise to bursts of turbulence at low flow rates. Geometrical distortions of small, yet finite amplitude are found to excite a state consisting of helical vortices during flow deceleration. The resulting flow pattern grows rapidly in magnitude, breaks down into turbulence, and eventually returns to laminar when the flow accelerates. This scenario causes shear stress fluctuations and flow reversal during each pulsation cycle. Such unsteady conditions can adversely affect blood vessels and have been shown to promote inflammation and dysfunction of the shear stress sensitive endothelial cell layer.