On near-wall hot-wire measurements

A specially constructed hot-wire probe was used to obtain very near-wall ve locity measurements in both a fully developed turbulent channel flow and fl at plate boundary layer flow. The near-wall hot-wire probe, having been cal ibrated in a specially constructed laminar flow calibration rig, was used t o measure the mean streamwise velocity profile, distributions of streamwise and spanwise intensities of turbulence and turbulence kinetic energy k in the viscous sublayer and beyond; these distributions compare very favorably with available DNS results obtained for channel flow. While low Reynolds n umber effects were clearly evident for the channel flow, these effects are much less distinct for the boundary layer flow. By assuming the dissipating range of eddy sizes to be statistically isotropic and the validity of Tayl or's hypothesis, the dissipation rate epsilon (iso) in the very near-wall v iscous sublayer region and beyond was determined for both the channel and b oundary layer flows. It was found that if the convective velocity U-c in Ta ylor's hypothesis was assumed to be equal to the mean velocity (U) over bar at the point of measurement, the value of (epsilon (+)(iso))(1) thus obtai ned agrees well with that of (epsilon (+))(DNS) for gamma (+) greater than or equal to 80 for channel flow; this suggests the validity of assuming U-c = (U) over bar and local isotropy for large values of gamma (+). However, if U, was assumed to be 10.6u(tau), the value of (epsilon (+)(iso))(2) thus obtained was found to compare reasonably well with the distribution of (ep silon (+)(iso))DNS for gamma (+) less than or equal to 15.