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

A Direct Numerical Simulation study of turbulence in a channel flow subjected to a sudden change in pressure gradient is carried out to facilitate a direct comparison between unsteady turbulent ramp-up/ down flows. Various turbulent statistics are calculated and compared for the two flow excursions. Large differences are observed between the two flows for 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 deviates much from pseudo-steady values in the ramp down flow. Furthermore, the turbulent fluctuation of the wall shear stress reduces at the beginning of the ramp up flow showing an effect of "relaminarization" associated with flow acceleration. The three components of the turbulent normal stresses behave distinctly different.