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Turbulence Heat and Mass Transfer 6. Proceedings of the Sixth International Symposium On Turbulence Heat and Mass Transfer
September, 14-18, 2009, Rome, Italy

DOI: 10.1615/ICHMT.2009.TurbulHeatMassTransf


ISBN Print: 978-1-56700-262-1

ISSN: 2377-2816

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

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