Skin friction estimation in a strong decelerating flow

The paper presents the analysis of the turbulent boundary layer developed on a flat plate subjected to an Adverse Pressure Gradient (APG) and approaching separation. The aim of the study is to examine the effects of pressure gradient on a non-equilibrium boundary layer while indicating local areas of the equilibrium flow. The emphasis is on the analysis of mean flow velocity and the estimation of skin friction. It is known that accurate measurements of skin friction were considered as a difficult and demanding task despite of various measuring techniques available. A great challenge is especially the measurement of a strong decelerated turbulent boundary layer because of low shear stress and possible large measuring errors. To date, the oil film or oil drop interferometry technique, because of its high accuracy, has become a basis of turbulent-boundary-layer research. In our research, this technique has been used as a reference method for comparing with the traditional Clauser chart method, which generally is considered as not suitably for non-canonical flows. In the paper, however, a correction of the method is proposed, which allows one to increase its range of applicability. This corrected Clauser chart method (CCCM) involves only one iteration while other proposed in the literature methods employ a twofold iterative procedure. The comparison of the methods for the non-canonical turbulent boundary layer, i.e. adverse pressure gradient with a strong flow history effect has been presented. It has been shown that CCCM can be successfully used for small and medium pressure gradients, where the Clauser-Rotta pressure gradient parameter β does not exceed level close to 11