Effect of the forcing term in the direct numerical simulation of turbulent channel flow

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Luca Boscagli

The present work studies the influence of the forcing strategy, constant flow rate (CFR) and constant pressure gradient (CPG), in direct numerical simulation (DNS) of a turbulent channel flow.

The choice of the forcing strategy is known to be of no consequences on one- point one-time statistics, at least if one considers a fully developed turbulent flow in large computational domains. In practice, however, the computational box is always finite, fluctuations of the forcing term become more significant with smaller box sizes, and may affect the flow dynamics and its statistics.

The problem is here investigated via DNS of a turbulent plane channel flow in a Minimal Flow Unit (MFU), which emphasises possible consequences of the choice of the driving strategy, in order to bring to light an indirect dynamical effect of the different temporal fluctuations of the forcing term on near-wall turbulence.

Due to high statistical error unavoidably affecting the results, no firm conclusions can be made when the problem is analyzed at a Reynolds number close to Re = 5772 (known to be the limit value for linear stability), even if a different trend in the growth of the friction coefficient appears. On the other hand, studying a sudden reduction of the Reynolds number in a subcritical regime, and investigating the transition process, those turbulent flow fields obtained with CPG strategy are characterized by a lower probability to become laminar with respect to those obtained CFR.