Forced unsteady turbulent wall flows a review of experimental results and closure models

Experimental results on pulsed turbulent channel/pipe Hows are reviewed and analysed in terms of the forcing amplitude and frequency. The amplitudes and phase or time-shifts of quantities such as the velocity and wail shear-stress oscillations and the modulation of turbulent fluctuations of the \vall shear-stress and of the Reynolds stresses arc examined in some detail. Since the modulations of the turbulent quantities do not decay monoloncously with increasing frequency there arc three distinct regimes of the turbulence response, a quasi-steady, a relaxation and a high frequency regime. In the latter the response of the modulations increase and tend to be in-phase accross the wall layer. Closure models developed for unsteady turbulent flow are finally discussed i especially the models based on rapid di.storsion ideas.