Spanwise vorticity structure in turbulent boundary layers

Four-element hot-wire probe measurements are used to examine the structure of the spanwise vorticity in the inner region of low Reynolds number zero pressure gradient turbulent boundary layers. Single-probe measurements were made over the range 1010 less than or equal to R(theta) less than or equal to 4850(R(theta) = theta U-proportional to/nu, where theta is the momentum deficit thickness, and U-proportional to, is the free-stream velocity), while two probe measurements were made at R(theta) approximate to 1010. The present results indicate that for y(+) < 50 statistical moment profiles of omega(z) scale on inner variables. Event duration analyses indicate that a nearly logarithmic increase in inner normalized time scales of the omega(z) bearing motions occurs with increasing R(theta). Outside the buffer region, this R(theta) dependence is effectively removed if the Taylor time scale is used to normalize the event durations. Two-point correlations with probe separations in the spanwise as well as wall-normal direction are presented. In addition, the structure of the associated two-dimensional (2-D) probability distributions are examined to reveal the statistically most significant contributions underlying these correlations. Wall-normal probe separation measurements indicate the increasing prevalence of adjacent regions of opposing sign omega(z) as the wall is approached. Spanwise probe separation experiments indicate the predominance of single-sign contributions, as well as increasing spatial coherence nearer the wall. The present results are interpreted to indicate that the organized spanwise vorticity-bearing motions are distributed in planes parallel to the wall for y(+) less than about 12, and decrease to a nearly fixed scale outside the buffer region.