Some insights on the unsteady dynamics of streamwise vortices

Streamwise vortices play a significant role in boundary layer transition and wall turbulence. Streamwise vortices in a boundary layer generate low-speed streaks by lift-up. The instability of these low-speed streaks is well studied and can be interpreted in terms of the generation of unsteady streamwise vorticity by tilting mechanisms (Suryanarayanan et al. Phys. Fluids 31, 024101, 2019). Understanding the evolution of streamwise vortices is also important for separation control using vortex generators. It has been recently demonstrated that streamwise vortices can amplify in an unsteady separating flow (Paudyal et al. ASME FEDSM 2025) via potential coupling with the separated shear layer and other instabilities. A series of direct numerical simulations is performed here to obtain further insight into the unsteady amplification. The unsteadiness is introduced by fluctuating the vortex strength or flow speed or both. The scenarios considered include streamwise vortices in a boundary layer as well as over slip surfaces and backward-facing steps to isolate the effects of shear, no-slip, and no-through flow on the vortex evolution. The results are analyzed from a vorticity point of view, and the relevance to boundary layer transition and separation control are discussed.