The diagnostic plot as a measure of departure from equilibrium in turbulent wing-body junction flows

The ‘alternate’ diagnostic plot introduced by Alfredsson et al. (Phys. Fluids, vol. 23, 041702, 2011) is used as a benchmark to locally evaluate the departure of turbulent wing-body junction flow wakes from ‘equilibrium’ boundary layers. Experimental measurements of the streamwise and transverse velocity components have been acquired in three spanwise/wall-normal planes in the wakes of both a streamlined ‘wing’ and a bluff ‘wing’ junction. Both obstacles produce a secondary flow of Prandtl’s first kind, which disrupts the equilibrium implied by the universality of the alternate diagnostic plot. The obstacle wake itself (away from the junction) also disrupts this equilibrium. It is found that with downstream development the flow eventually recovers to the base zero-pressure-gradient boundary layer ‘equilibrium’, and that this recovery process emanates from the near-wall region.