  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.
