WALL SHEAR-STRESS AND VELOCITY IN A TURBULENT AXISYMMETRICAL BOUNDARY-LAYER

Instantaneous streamwise fluctuations of the wall shear stress have be en measured using a hot-element probe in a thick axisymmetric turbulen t boundary layer on a cylinder aligned parallel to the flow. The measu rements were made at a momentum-thickness Reynolds number R(theta) = 3 050 and a ratio of boundary-layer thickness to cylinder radius of delt a/a = 5.7. The ratio of the r.m.s. of the fluctuation to the mean valu e of the wall shear stress, tau(rms)/tauBAR, is about 0.32, a value sl ightly lower than that for recent measurements for flow over a flat pl ate. The probability density function of the wall shear stress is simi lar to that for planar wall-bounded flows within experimental error. T he power spectral density of the wall shear stress shows that a cylind rical boundary layer contains less energy at lower frequencies and mor e energy at higher frequencies than other wall-bounded flows. Analysis of simultaneous measurements of the streamwise wall shear stress and the streamwise velocity using VITA and peak detection suggests that tr ansverse curvature has little effect on the near-wall burst-sweep cycl e compared to planar wall-bounded flows. The angle of inclination of t he structures is similar to that measured for large-scale structures i n planar wall-bounded flows. However, measurements of the cross-correl ation between the shear stress and the velocity suggest the existence of smaller structures yawed to the axis of the cylinder. The coherence between shear stress and velocity shows a low frequency associated wi th the inclined structures and a higher frequency associated with the yawed structures. The yawed structures could have an arrowhead or half -arrowhead shape and may be associated with fluid from the outer flow washing over the cylinder.