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.