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A comparative study of turbulence in a ramp-up and a ramp-down flow

DOI: 10.1615/ICHMT.2009.TurbulHeatMassTransf.1250
9 pages

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

S. He
School of Engineering and Physical Sciences, University of Aberdeen, Aberdeen AB24 3UE; Department of Mechanical Engineering, University of Sheffield, Mappin Street, Sheffield,S1 3JD, UK

Paolo Orlandi
Department of Mechanics and Aeronautics, "Sapienza" University of Rome, Via Eudossiana 18, 00184 Rome, Italy

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

Abstract

Direct Numerical Simulations of a turbulent channel flow subjected to a sudden change in pressure gradient are carried out to facilitate a direct comparative study of ramp-up and ramp down flows. Various turbulent statistics are calculated and compared for the two flow excursions. Large differences are observed between the two flows at low Reynolds numbers. Unsteady wall shear stress in a ramp-up flow overshoots, and then undershoots the pseudo-steady value at the initial stage of excursion, whereas it does not deviate significantly from pseudo-steady values in the ramp-down flow. Furthermore, the intensities of turbulent fluctuation of the wall shear stress reduce at the beginning of the ramp-up flow showing an effect of "relaminarization" associated with flow acceleration. At this stage, the three components of the turbulent normal stresses exhibit distinctly different characteristics.

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