A comparative study of near-wall turbulence in high and low Reynolds number boundary layers

The present study explores the effects of Reynolds number, over three order s of magnitude, in the viscous wall region of a turbulent boundary layer. C omplementary experiments were conducted both in the boundary layer wind tun nel at the University of Utah and in the atmospheric surface layer which fl ows over the salt flats of the Great Salt Lake Desert in western Utah. The Reynolds numbers, based on momentum deficit thickness, of the two flows wer e R-theta=2x10(3) and R(theta)approximate to 5x10(6), respectively. High-re solution velocity measurements were obtained from a five-element vertical r ake of hot-wires spanning the buffer region. In both the low and high R-the ta flows, the length of the hot-wires measured less than 6 viscous units. T o facilitate reliable comparisons, both the laboratory and field experiment s employed the same instrumentation and procedures. Data indicate that, eve n in the immediate vicinity of the surface, strong influences from low-freq uency motions at high R-theta produce noticeable Reynolds number difference s in the streamwise velocity and velocity gradient statistics. In particula r, the peak value in the root mean square streamwise velocity profile, when normalized by viscous scales, was found to exhibit a logarithmic dependenc e on Reynolds number. The mean streamwise velocity profile, on the other ha nd, appears to be essentially independent of Reynolds number. Spectra and s patial correlation data suggest that low-frequency motions at high Reynolds number engender intensified local convection velocities which affect the s tructure of both the velocity and velocity gradient fields. Implications fo r turbulent production mechanisms and coherent motions in the buffer layer are discussed. (C) 2001 American Institute of Physics.