GALCIT Colloquium

Large-Eddy Simulations (LES) of smooth and rough-wall channel flows and smooth-wall pipe flows at Reynolds numbers Re= O(10²) - O(10⁸)  are presented and discussed. Roughness effects are modeled by introducing a subgrid, semi-empirical, roughness-corrected wall model that dynamically transmits roughness-induced drag information at the wall to the outer LES, which uses a stretched-vortex subgrid-scale model operating in the bulk of the flow. The LES is first applied to fully turbulent, uniformly smooth and uniformly rough-wall channel flows in order to capture the flow dynamics over smooth, transitionally rough and fully rough regimes. Results include a Moody-like diagram for the wall averaged friction factor, believed to be the first of its kind obtained from LES. Next, we describe LES of channel flow with sequential rough and smooth wall strips - each of length 64 half-channel heights at various roughness and Reynolds number levels.  Recovery of mean quantities and turbulent statistics after both smooth-to-rough and rough-to-smooth transitions are discussed qualitatively as well as quantitatively. Finally, we extend the present LES methodology to smooth-wall, turbulent pipe flow at bulk-flow Reynolds numbers up to O(10⁸). Computed friction factors have shown favorable agreement with Superpipe data and estimates of the Kármán constant from LES closely match values obtained from these data.