**Hirotaka Morimatsu and Takahiro Tsukahara \***

Department of Mechanical Engineering, Tokyo University of Science, Chiba 278-8510, Japan; 7515117@alumni.tus.ac.jp

**\*** Correspondence: tsuka@rs.tus.ac.jp; Tel.: +81-4-7122-9352

Received: 31 October 2020; Accepted: 27 November 2020; Published: 30 November 2020 -

**Abstract:** Direct numerical simulations were carried out with an emphasis on the intermittency and localized turbulence structure occurring within the subcritical transitional regime of a concentric annular Couette–Poiseuille flow. In the annular system, the ratio of the inner to outer cylinder radius is an important geometrical parameter affecting the large-scale nature of the intermittency. We chose a low radius ratio of 0.1 and imposed a constant pressure gradient providing practically zero shear on the inner cylinder such that the base flow was approximated to that of a circular pipe flow. Localized turbulent puffs, that is, axial uni-directional intermittencies similar to those observed in the transitional circular pipe flow, were observed in the annular Couette–Poiseuille flow. Puff splitting events were clearly observed rather far from the global critical Reynolds number, near which given puffs survived without a splitting event throughout the observation period, which was as long as 10<sup>4</sup> outer time units. The characterization as a directed-percolation universal class was also discussed.

**Keywords:** subcritical transition; spatiotemporal intermittency; direct numerical simulation
