Near-wall behaviour of transient flow in a channel with distributed pyramid roughness

DNS has been performed to investigate near-wall behaviour of turbulence for a rapid 'turbulent-to-turbulent' transient flow in a channel with a smooth top surface and a rough bottom surface made of close-packed pyramids. The transient flow is studied following a rapid change in flow rate from Re = 2800 to Re = 7400. The equivalent roughness heights normalised by the wall units, k⁺ _s, of the initial and final flows are, respectively, 14.5 and 41. The results show that near-wall behaviour of turbulence in the early stages of the transient process for the rough wall differs significantly from that over the smooth-wall. The early transient process over the rough-wall is in the form of a single cycle of birth, evolution and eventually breakdown of strong primary counter-rotating hairpin structures in the region very close to the roughness crests. Similar to that in a steady flow, the direct effect of roughness in a transient flow is confined to a region up to approximately three times of roughness height above the crest. Though the transient process starts from an initially fully-developed turbulent flow, the early transient process exhibits a roughness-induced laminar-turbulent transition. Various statistical quantities, including the three components of r.m.s. of velocity fluctuations and also turbulent shear stress, confirm the visualisation results.