Near-wall hot-wire measurements Part II: Turbulence time scale, convectivevelocity and spectra in the viscous sublayer

This work continues the studies of Khoo et al. (Exp. Fluids 29: 448-460, 20 01), where experiments were performed in turbulent-channel and flat-plate b oundary-layer flows using near-wall hot-wire probes. The probability densit y function (pdf) of the wall-shear stress and streamwise velocity fluctuati ons in the viscous sublayer, buffer region and beyond were compared and ana lyzed. The convective velocity U-c of the streamwise velocity fluctuations in the very near-wall region was obtained using a two-point correlation tec hnique. It was found that in the viscous sublayer, U-c is approximately con stant at 13u(tau) and 15u(tau), respectively, for the channel and boundary- layer flows. Spectra data for the viscous sublayer are presented for the fi rst time, and the normalized spectral plots for different flow conditions c ollapse at high frequencies or wavenumbers, thus indicating the possible pr esence of small-scale universality at different Reynolds numbers. The integ ral time scale corresponding to the streamwise velocity fluctuations in the viscous sublayer is also presented.