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 thirteen waves with durations of 10-20 minutes 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 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.