The shortest channels that sustain short-streak turbulence

It is often assumed that infinitely long streaks of the streamwise velocity play a significant role in the sustainment of the turbulent energy cycle in channel flow, particularly as drivers of bursting. However, some channels are known to remain turbulent even when a numerical torque is applied to suppress the infinitely long streaks, calling their necessity into question. Here, we aim to understand how channels damped in this way remain turbulent by studying the effect of damping on the size of the minimal computational box sustaining turbulence at Re_τ ≈ 200. It is found that damped channels need to be at least 400 wall units long, approximately twice as much as canonical ones, and that they are characterized by an enhanced energy of the first streamwise spectral mode of the streamwise velocity. This suggest that this first mode acts as a substitute of the infinitely long streaks in the cycle sustainment. Intriguingly, both damped and undamped minimally short channels need to be wider than classical longer ones, suggesting that a collective interaction of several turbulence units is involved.