CYCLIC MODULATION OF REYNOLDS STRESSES AND LENGTH SCALES IN PULSED TURBULENT CHANNEL FLOW

Recent data obtained in an unsteady turbulent; channel flow is reviewed. Results concerning the modulation characteristics of the Reynolds shear stresses, of the structural parameters and of the length scales inferred from unsteady spatial correlations are discussed. The close examination of both the amplitude and the phase shifts of the Reynolds shear stresses confirms the existence of three distinct inposed frequency regimes, namely the quasi-steady regime, the relaxation regime, in which the amplitudes decrease and which is accompanied by large time lags, and a subsequent third regime wherein the modulation characteristics change considerably. The fine structure of the near-wall turbulence response through quadrant analysis reveals large cyclic variations of the contributions of ejections and sweeps to the Reynolds shear stress. The reaction of the spanwise extent of the near-wall structures is investigated through the spanwise correlation coefficient between the wall shear stress and the streamwise velocity, and the resulting length scales. A temporal filtering of both signals shows that the inactive motions respond uniformly in the whole imposed frequency regime. A strong correlation is found between the modulation characteristics of the streak spacing and the ejection frequency.