Laterally converging flow. Part 2. Temporal wall shear stress

Abstract

Instantaneous measurements of the wall shear stress were made in the laterally converging duct also used for mean measurements in part 1 and were analysed by conditional sampling and by conditional averaging. The sidewalls of the duct were adjusted to provide (i) a straight duct of constant rectangular cross-section and (ii) laterally (spanwise) converging ducts resulting in streamwise acceleration of the flow. The Reynolds number varied from 7600 to 47 200 and the dimensionless acceleration parameter Kv = (ν/V2)dV/dx ranged from 0 to 3·4 × 10−6, yielding a variation of the flow regime from fully turbulent to nearly laminar. The typical burst pattern, or conditionally averaged time history of the wall shear stress, resembled the time history of the streamwise velocity component deduced at y+ = 15 by Blackwelder and Kaplan using the same general technique. For fully developed flows, inner or wall scaling of the bursting frequency was found to be less dependent upon Reynolds number than outer scaling; other characteristics examined varied with both inner and outer scaling. For converging flows measurements of bursting characteristics essentially confirmed the indicated flow regimes deduced in part 1 and showed that the measured characteristic that was most affected by acceleration was the bursting frequency. All characteristics varied with acceleration, but the variation was generally less when normalized by wall variables rather than when normalized by outer variables.