Experimental evidence of internal solitary wave-induced global instability in shallow water benthic boundary layers

Abstract

Experimental evidence is presented in support of the theoretical prediction of Diamessis and Redekopp [J. Phys. Oceanogr. 36, 784 (2006)] for wave-induced vortex shedding at the lower solid boundary of a stratified fluid system as a result of global instability. The time-dependent boundary layer induced by a strongly nonlinear internal wave of depression in shallow water is examined experimentally. Measurements of the velocity field close to the bottom boundary illustrate coherent periodic shedding of vortex structures at the lower boundary in the adverse pressure gradient region aft of the wave. The vortical structures ascend high into the water column and cause significant benthic turbulence. It is shown that global instability has a critical threshold dependent on the Reynolds number of the flow and the amplitude of the wave. The critical amplitudes observed are approximately half that predicted by Diamessis and Redekopp [J. Phys. Oceanogr. 36, 784 (2006)], indicating that internal wave-induced benthic mixing may be even more prominent than previously thought.

Benthic Turbulence and Mixing Induced by Nonlinear Internal Waves

“…The two NLIWs recorded by the SeaBASS on JD 242.21 and 242.23 have amplitudes 0.07 and 0.15, respectively. Both of these amplitudes exceed the critical value for global instability extrapolated to the field Reynolds number of 10 7 (based on the findings of Diamessis and Redekopp (2006a) and adjusted according to the laboratory observations of Carr et al (2007) for fully non-linear internal waves). Thus, the distinct qualitative similarities between DNS and CMO 96 observations and the low critical wave amplitude value (approximately 10%) at oceanically relevant Reynolds numbers suggest that NLIWs capable of producing distinct benthic eruptions are relatively low-amplitude waves and occur frequently on the continental shelf.…”

Boundary layer flow beneath an internal solitary wave of elevation

“…Carr and Davies 25 and Carr et al 26 presented experimental evidence of a similar flow reversal at the lower boundary under an ISW of depression. The reverse flow also occurred in the decelerating part of the wave-induced flow, which, for an ISW of depression, is in an adverse pressure gradient region.…”

“…25,26 The lower layer was filled first with a prepared solution of brine of prescribed density 3 ͑typically 1048 kg/ m −3 ͒. The top two layers were then carefully added via a floating sponge arrangement by directly filling with fresh water of density 1 ͑typically 997 kg/ m −3 ͒.…”

“…Once the fluid behind the gate was mixed to the required density, the gate was lifted and removed quickly, thereby generating a wave of elevation that propagated along the density interface into the main section of the tank. The results of previous work 25,26 on ISWs of depression were used to estimate the gate position, the trapped volume V and the constituent layers to achieve large amplitude ISWs of elevation. A range of V and background stratification were chosen such that an array of outputs was given.…”

“…Figure 2 provides a flow regime classification plot of the observations of type ͑i͒ waves in terms of the wave Reynolds number and nondimensionalized wave amplitude. For comparative purposes counterpart observations of global instability ͑ϫ͒ and no global instability ͑᭺͒ underneath an ISW of depression from Carr et al 26 are plotted alongside the observations of no global instability ͑vortices͒ underneath an ISW of elevation of type ͑i͒ ‫ء͑‬ ͒. For a fixed Reynolds number, it can be seen that waves of elevation have been generated at higher amplitudes than the critical amplitude in the depression case yet no instability was seen in the boundary layer.…”