Instantaneous wall-shear-stress measurements: advances and application to near-wall extreme events

In this article we provide an overview of widely used methods to measure the mean and fluctuating components of the wall-shear stress in wall-bounded turbulent flows. We first note that it is very important to perform direct measurements of the mean wall-shear stress, where oil-film interferometry (OFI) provides the highest accuracy with an uncertainty level of around 1%. Nonetheless, several indirect methods are commonly used due to their straightforward application and these are reviewed in the light of recent findings in wall turbulence. The focus of the review lies, however, on the fluctuating wall-shear stress, which has over the last decade received renewed interest. In this respect, it is interesting to note that one near-wall feature that has received attention is the so-called backflow event, i.e. a sudden, strong short-lived reverse-flow area, which challenges measurement techniques in terms of temporal and spatial resolution, as well as their dynamic range and multi-directional capabilities. Therefore, we provide a review on these backflow events as well as commonly used techniques for fluctuating wall-shear-stress measurements and discuss the various attempts to measure them. The review shows that further development of the accuracy and robustness of available measurement techniques is needed, so that such extreme events can be adequately measured.