NEAR-WALL BEHAVIOUR OF TRANSIENT FLOW IN A CHANNEL WITH DISTRIBUTED PYRAMID ROUGHNESS

Mehdi Seddighi
School of Engineering, University of Aberdeen, Aberdeen AB24 3UE; Department of Mechanical Engineering, University of Sheffield, Mappin Street, Sheffield,S1 3JD, UK; School of Engineering, Technology and Maritime Operations, Liverpool John Moores University, Liverpool, L3 3AF, United Kingdom

Shuisheng He
School of Engineering The Robert Gordon University Schoolhill, Aberdeen, AB10 1FR, UK; Department of Mechanical Engineering University of Sheffield Sheffield, UK, S1 3JD

Dubravka Pokrajac
School of Engineering University of Aberdeen Aberdeen, UK, AB24 3UE

Tom O'Donoghue
School of Engineering University of Aberdeen Aberdeen, UK, AB24 3UE

Alan E. Vardy
Division of Civil Engineering University of Dundee Dundee, UK, PH14 9SS

Abstract

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.