Direct Numerical Simulation of Turbulent Open Channel Flow: Streamwise Turbulence Intensity Scaling and Its Relation to Large-Scale Coherent Motions

We conducted direct numerical simulations of turbulent open channel flow (OCF) and closed channel flow (CCF) of friction Reynolds numbers up to \(\textrm{Re}_\tau \approx 900\) in large computational domains up to \(L_x\times L_z=12\pi h \times 4\pi h\) to analyse the Reynolds number scaling of turbulence intensities. Unlike CCF, our data suggests that the streamwise turbulence intensity in OCF scales with the bulk velocity for \(\textrm{Re}_\tau \gtrsim 400\) . The additional streamwise kinetic energy in OCF with respect to CCF is provided by larger and more intense very-large-scale motions in the former type of flow. Therefore, compared to CCF, larger computational domains of \(L_x\times L_z=12\pi h\times 4\pi h\) are required to faithfully capture very-large-scale motions in OCF—and observe the reported scaling. OCF and CCF turbulence statistics data sets are available at https://doi.org/10.4121/88678f02-2a34-4452-8534-6361fc34d06b .