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We revisit the grid-point requirement estimates in Choi and Moin [Phys.Fluid, 24, 011702 (2012)] and establish more general grid-point requirementsfor direct numerical simulations (DNS) and large-eddy simulations (LES) of aspatially developing turbulent boundary layer. We show that, by allowing thelocal grid spacing to scale with the local Kolmogorov length scale, thegrid-point requirement for DNS of a spatially developing turbulent boundarylayer is $N\sim Re_{L_x}^{2.05}$ rather than $N\sim Re_{L_x}^{2.64}$ assuggested by Choi and Moin, where $N$ is the number of grid points and $L_x$ isthe length of the plate. In addition to the grid-point requirement, we estimatethe time-step requirement for DNS and LES. We show that, for a code that treatsthe convective term explicitly, the time steps required to get convergedstatistics are $N_t\sim Re_{L_x}/Re_{x_0}^{6/7}$ for wall-modeled LES and$N_t\sim Re_{L_x}/Re_{x_0}^{1/7}$ for wall-resolved LES and DNS (with differentprefactors), where $Re_{x_0}$ is the inlet Reynolds number. The grid-point andtime-step requirement estimates allow us to estimate the overall cost of DNSand LES. According to present estimates, the costs of DNS, wall-resolved LESand wall-modeled LES scale as $Re_{L_x}^{2.91}$, $Re_{L_x}^{2.72}$, and$Re_{L_x}^{1.14}$.

Grid-point and time-step requirements for direct numerical simulation and large-eddy simulation