“…This paves the way for a proper comparison 1 3 121 Page 2 of 8 Luxton 1971; Kameda et al 1998), thus lending credence to the belief that the deficit in −�uv� near the wall is incorrect. Recently, using an extensive flow analysis of PIV data in a turbulent boundary layer over a rough wall made up of 2D square bars with a spacing between the bars of seven times the bar height, Djenidi et al (2014) observed that large outward motions instantaneously deviate from the mean flow direction by up to 50°. This supports the argument that the X-wire is inefficient for detecting these motions, particularly if the apex angle of the X-wire is not optimized for capturing strong flow ejections with large deviations, thus leading to a deficit in −�uv�.…”