Temporal and spatial intermittencies within channel flow turbulence near transition

Direct numerical simulations (DNS) of plane Poiseuille flow are performed in an extended domain at friction Reynolds numbers ranging from 70 to 100. In minimal domains, turbulence in this Reynolds number range displays substantial intermittency that is associated with chaotic movement of turbulent trajectories between lower and upper branch invariant solutions known as exact coherent states (ECS). The present work aims to address the relationship between temporal dynamics in minimal channels and spatiotemporal dynamics in extended domains. Both temporal and spatial analyses of the turbulent velocity fields are performed, the latter using image analysis methods. These analyses partition the flow characteristics into low-, intermediateand high-drag classes; we present the differences between flows fields in these classes in terms of simple quantities like mean velocity, wall shear stress, and flow structures. The temporal and spatial analysis methods, although completely independent of one another, yield very similar results for both lowand high-drag regions. In particular, the conditional mean profiles in regions of low drag closely resemble those found in low-drag temporal intervals in the minimal channel. Finally, we address the possibility of similarities between turbulence and exact coherent states in two ways: (1) comparing wall shear stress in localized patches the size of minimal channels in large domains with those in actual minimal channel and (2) comparing conditional mean velocity profiles during low-drag events with mean profiles from lower branch ECS. These analyses show that both the local near-wall flow structure in the low-drag patches of the large domain and the conditional mean profiles in the region y+ 30 resemble those of a lower branch minimal domain ECS. In summary, the results presented here suggest that spatiotemporal intermittency in transitional channel flow turbulence is related to temporal intermittency, and by extension to the state space structure, in the minimal channel.