Properties of synthetic and natural streamwise vortex pairs in the near-wall region of turbulent boundary layers

A low-friction-Reynolds-number (Re tau approximate to 400-480) turbulent boundary layer perturbed by additional streamwise vortices is investigated using wall-resolved large-eddy simulation. To better understand how one might passively mimic and/or manipulate the formation of streamwise vortex pairs (SVPs) that naturally occur as part of the near-wall cycle, SVPs are artificially produced by small vortex generators (of height h+ = 30) within the near-wall region. Thus, the present study contrasts the more typical large vortex generators used in turbulent boundary layer manipulation and separation control. Analyses employing a triple decomposition of velocity and pressure and its extension to the incompressible Navier-Stokes equation is used to better understand the coherent and turbulence fields associated with the synthetic and naturally occurring SVPs. Overall, it is observed that the near-wall synthetic and natural streamwise vortices have similar interactions with the surrounding turbulent field, and exhibit similarities in statistical structure. In particular, it is found that the synthetic SVPs exhibit a remarkably similar signature of kinetic energy transport with their natural counterparts. Through the comparisons with the ensembleaveraged natural SVPs, it is found that the natural and synthetic SVPs are identified to own similar spatial scales and evolution characteristics such that when they are inner scaled they exhibit similar magnitudes at comparable streamwise stations. This indicates that the wall response to SVPs may in fact constitute a generic mechanism underlying turbulent transport near the wall. These similarities suggest that embedded small synthetic streamwise vortices are self-contained in the near-wall region and directly interact with the structures in this area to influence the associated turbulent transport.