Observations of the Bottom Boundary Layer Beneath the World's Largest Internal Solitary Waves

Abstract

Measurements in the South China Sea reveal the structure of the bottom boundary layer beneath onshore propagating highly nonlinear internal solitary waves of depression. Offshore directed free stream velocities beneath 13 waves with durations of 10–20 min and velocities up to 1.4 m/s are consistent with the solitary wave solution to the Korteweg-de Vries equation as are phase velocities estimated from pressures and free stream velocities. The measurements indicate a thin layer during free stream acceleration, a thicker layer with an inflected velocity during deceleration, and a long-lived sediment laden wake, after wave passage in the free stream, with velocities opposite to those at maximum flow strength. Reynolds-averaged Navier-Stokes simulations with the k-ε turbulence model reproduce the measured velocities and turbulent Reynolds shear stresses accurately during acceleration and early deceleration. However, differences between the simulations and the measurements during late deceleration and in the wake suggest energetic large-scale turbulence not represented by the simulations. This turbulence might be similar in origin to coherent vortices that have been observed in laboratory experiments and direct numerical simulations at much smaller Reynolds numbers, which have been attributed to absolute and global instabilities resulting from inflected velocity profiles.