Evolution mechanisms of synthetic streamwise vortices in turbulent boundary layers*

Low-friction-Reynolds-number turbulent boundary layers (TBLs) (Re_τ ~ 270 - 300) with embedded synthetic streamwise vortices are investigated using 2D2C-PIV measurement data and well-resolved Large Eddy Simulation (LES). To better understand how additional streamwise vortices evolve in a TBL, synthetic streamwise vortex pairs with different sizes are produced by vane-type vortex generators of height h⁺ ~ 24, 48, 96 in experiments and simulations. Analyses employing a triple decomposition of the velocity field show that the wall-normal location of the peak in the total Reynolds shear stress ((y⁺)_m,zpg)  (≅3(δ⁺)^0.5) in the zero-pressure-gradient (ZPG) TBL is a useful datum for assessing the vortex pair evolution. Since (y⁺)_m,zpg is the onset of the log region in a ZPG TBL, where the mean viscous force loses leading order, a plausible physical explanation of the evolution is proposed here.  Synthetic scales capable of displacing the peak in the total Reynolds shear stress to a position (y⁺)_m >> (y⁺)_m,zpg act as a strong momentum source while displacements to (y⁺)_m << (y⁺)_m,zpg generate an enhanced momentum sink. Correspondingly, the domain of influence of the introduced synthetic scales can either exceed the near-wall region and extend into the outer layer, or tend to be self-contained and decay within the near-wall region. For synthetic scales producing a peak near (y⁺)_m,zpg (i.e., (y⁺)_m ~ (y⁺)_m,zpg), their influence is neither fully limited to the near-wall region nor extends across the entire TBL.