Bypass transition beneath internal solitary waves in turbulent boundary layers

Process-based studies of the transition to turbulence beneath internal solitary waves (ISWs) have typically assumed a laminar bottom boundary layer (BBL) and two-dimensional flow. In natural environments, however, ISWs often propagate over turbulent three-dimensional BBLs. Here, we perform three-dimensional high-resolution spectral numerical simulations of periodic ISWs of depression at momentum-thickness Reynolds numbers representative of laboratory ( upper R e Subscript upper I upper S upper W Baseline equals 300 R e I S W = 300 ReISW=300 ) and lake ( upper R e Subscript upper I upper S upper W Baseline equals 900 R e I S W = 900 ReISW=900 ) scale conditions to examine how ambient turbulence modifies the transition to turbulence beneath ISWs. When the ambient flow is laminar, this transition is governed by a modal instability. At upper R e Subscript upper I upper S upper W Baseline equals 300 R e I S W = 300 ReISW=300 , the instability generates localised vortices that subsequently break down, while at upper R e Subscript upper I upper S upper W Baseline equals 900 R e I S W = 900 ReISW=900 it produces sustained vortex shedding and a turbulent wake in the lee of the wave. In contrast, when the ambient flow is turbulent, the modal pathway is bypassed. Free-stream turbulence excites near-wall streamwise streaks (Klebanoff modes) beneath the front shoulder of the wave, which amplify, undergo secondary instability and break down into turbulent bursts. The resulting turbulence develops upstream of the modal-instability region. With increasing Reynolds number and stronger ambient turbulence, the onset of turbulence shifts progressively toward the ISW trough. These results show that ambient turbulence triggers a bypass mechanism fundamentally different from the modal-instability-driven transition in laminar environments. By suppressing the modal instability and shifting turbulence generation closer to the wave trough, bypass transition provides an explanation for field observations that cannot be reconciled with laminar-transition models. Accurate prediction of near-bed turbulence and sediment transport in natural environments therefore requires accounting for bypass transition.