Use of PIV to highlight possible errors in hot-wire Reynolds stress data over a 2D rough wall

Abstract

Particle image velocimetry (PIV) measurements are carried out in a turbulent boundary layer over a 2D rough surface consisting of transverse square bars. The aim of this work is to investigate a possible cause for the near-wall X-wire measurement errors observed on similar rough surfaces. The PIV measurements do not show the anomalous near-wall deficit of Reynolds stresses as measured with X-wires over the same surface. An extensive flow visualization analysis of the PIV data for a spacing between the roughness elements of p = 7k (k is the roughness element height) shows the occurrence of large-scale inward (sweeps) and outward (ejections) motions with a period of about 10.6delta/U0 (delta and U0 are the boundary layer thickness and the free-stream velocity). While these motions dominate the near-wall region and contribute almost equally to the Reynolds shear stress -‹uv›, the mean outward deviation from the mean flow direction is stronger than the inward deviation. Also, when the roughness spacing is reduced to p = 3k, the outward deviation reduces significantly more than the inward deviation. The results support the argument that the outward motions, which can have an instantaneous deviation angle of more than 50 deg. in the case p = 7k, make the X-wire probe inefficient for detecting the ejection events (associated with the outward motions), particularly if the apex angle of the X-wire is not optimized for capturing the strong flow ejections with large deviations.The results explain in part the disparate information on the effect of the roughness on the Reynolds stresses in the outer region of the turbulent boundary layer over rough walls.