**Jinsheng Liu 1, Yue Xiao 2, Mogeng Li 3, Jianjun Tao 2,\* and Shengjin Xu 1,\***


Received: 26 October 2020; Accepted: 8 December 2020; Published: 11 December 2020

**Abstract:** The intermittent distribution of localized turbulent structures is a key feature of the subcritical transitions in channel flows, which are studied in this paper with a wind channel and theoretical modeling. Entrance disturbances are introduced by small beads, and localized turbulent patches can be triggered at low Reynolds numbers (*Re*). High turbulence intensity represents strong ability of perturbation spread, and a maximum turbulence intensity is found for every test case as *Re* ≥ 950, where the turbulence fraction increases abruptly with *Re*. Skewness can reflect the velocity defects of localized turbulent patches and is revealed to become negative when *Re* is as low as about 660. It is shown that the third-order moments of the midplane streamwise velocities have minima, while the corresponding forth-order moments have maxima during the transition. These kinematic extremes and different variation scenarios of the friction coefficient during the transition are explained with an intermittent structure model, where the robust localized turbulent structure is simplified as a turbulence unit, a structure whose statistical properties are only weak functions of the Reynolds number.

**Keywords:** subcritical transition; channel flow; turbulence fraction; moment
