**Daniel Feldmann \*, Daniel Morón and Marc Avila**

Center of Applied Space Technology and Microgravity (ZARM), University of Bremen, Am Fallturm 2, 28359 Bremen, Germany; daniel.moron@zarm.uni-bremen.de (D.M.); marc.avila@zarm.uni-bremen.de (M.A.)

**\*** Correspondence: daniel.feldmann@zarm.uni-bremen.de

Received: 4 December 2020; Accepted: 24 December 2020; Published: 30 December 2020 -

**Abstract:** Despite its importance in cardiovascular diseases and engineering applications, turbulence in pulsatile pipe flow remains little comprehended. Important advances have been made in the recent years in understanding the transition to turbulence in such flows, but the question remains of how turbulence behaves once triggered. In this paper, we explore the spatiotemporal intermittency of turbulence in pulsatile pipe flows at fixed Reynolds and Womersley numbers (*Re* = 2400, *Wo* = 8) and different pulsation amplitudes. Direct numerical simulations (DNS) were performed according to two strategies. First, we performed DNS starting from a statistically steady pipe flow. Second, we performed DNS starting from the laminar Sexl–Womersley flow and disturbed with the optimal helical perturbation according to a non-modal stability analysis. Our results show that the optimal perturbation is unable to sustain turbulence after the first pulsation period. Spatiotemporally intermittent turbulence only survives for multiple periods if puffs are triggered. We find that puffs in pulsatile pipe flow do not only take advantage of the self-sustaining lift-up mechanism, but also of the intermittent stability of the mean velocity profile.

**Keywords:** unsteady shear flow; turbulence intermittency; helical instability; puff dynamics
