Over the last 30 years flow visualisation has evolved from qualitative to quantitative and in many circumstances is replacing more traditional flow measurement techniques. In concert over a much longer period has been the investigation of turbulent wall-bounded flows because of their eminence in all forms of transport as well as geo-physical flows. In any experimental investigation of turbulence wall-bounded flows high accuracy data are of great importance to highlighting the influence of new parameters such as roughness, pressure gradient, etc.. However, the true statistics of wall-bounded flows is still often subject to controversies and this is particularly true at high Reynolds numbers. One of the points of discrepancy is that it is often difficult to compare accurately different experimental conditions.
Comparisons of previous experimental data sets have shown significant discrepancies between different studies even if the Reynolds number were the same. The quality of the flow is also of importance as external turbulence level or pressure gradient close to zero can affect the comparison. The classical way to compare is to use a judicious scaling and to match the Reynolds numbers. In this case, the difference can come from the accuracy of the measurement of the statistics, which is generally good, but also of the scaling parameter such as the friction velocity, which is used to normalize the profiles in the inner region.
This seminar will highlight some of the advances in qualitative flow visualisation as embodied in high-spatial resolution particle image velocimetry in the different flavours that are possible and their application with particular emphasis to high Reynolds number turbulent wall-bounded shear flows. Some of the advantages of these approaches as well their pitfalls will be briefly addressed to enable us to address the reasons for discrepancies between different experiments of nominally the same turbulent wall-bounded flows and minimise them.