Bypass transition in the boundary layer beneath internal solitary waves in the presence of free-stream turbulence

Experiments and numerical simulations, of the transition to turbulence beneath internal solitary waves (ISWs), typically consider the waves progressing through a laminar bottom boundary layer (BBL). However, in coastal oceans, laminar BBLs are uncommon, and ambient turbulence is frequently present. This study investigates how ambient turbulence influences the transition to turbulence in the BBL beneath ISWs. We performed direct numerical simulations of periodic ISWs at a Reynolds number characteristic of lake environments to examine the three-dimensional transition under both laminar and turbulent upstream conditions. When the BBL preceding the ISW was laminar, the growth of the most unstable BBL mode led to vortex shedding and transition to turbulence in the lee of the wave. When the ISW propagated through a turbulent wake, this classical modal instability was bypassed. Near-wall streamwise streaks of velocity perturbations were excited by the ambient turbulence. The streaks formed under the favorable pressure gradient region beneath the front shoulder of the ISW, grew downstream, and eventually triggered secondary instabilities that led to an earlier transition to turbulence, closer to the ISW trough. Our findings suggest that ambient turbulence can explain the observed variability in the location of peak sediment resuspension beneath ISWs in natural settings, relative to experimental and numerical studies using laminar conditions.