  This paper report on the design of an experiment to establish an experimental
self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL)
flow at the verge of separation in order to characterise its statistical
structure and the contribution of the Reynolds stresses to the wall shear
stress via quadrant analysis decomposition in conjunction with the Renard &
Deck (RD) decomposition of the mean skin friction. In this study
high-spatial-resolution (HSR) two-component - two-dimensional (2C-2D) particle
image velocimetry (PIV) is employed to measure the self-similar APG-TBL flow.
The measurements of the self-similar APG-TBL at the verge of separation, which
is referred to as 'strong APG-TBL' is compared with similar HSR 2C-2D PIV
measurements of a TBL under zero and mild APG to characterise the effect of the
APG on the TBL. In the strong APG-TBL, the outer scaled profiles of the first-
and second-order statistics, as well as the defect velocity exhibit
self-similarity over the measured streamwise domain of 3.3{\delta}. The
quadrant decomposition of the velocity fluctuations shows that the formation of
the outer peak in the Reynolds shear stress profile under APG is due to the
energisation of sweep motions of high-momentum fluid in the outer region of the
boundary layer. The RD decomposition of the mean skin friction shows that the
skin friction has larger contributions from ejections than sweeps. This is due
to the emergence of the outer peaks with a correspondingly diminishing of the
inner peaks in the turbulence production profiles. The skewness and flatness
profiles show that the zero skewness and minimum flatness locations of the
streamwise and wall-normal velocity fluctuations collapse with the location of
the maximum Reynolds streamwise stress in the TBLs irrespective of the pressure
gradient imposed on the TBL.
