Speaker 2 :  Magda Carr  (Newcastle University)
    Date & Time : February 18th (Fri.), 2022 / 10:50-11:30PM (Tokyo (JST)) 
                  (--> February 18th (Fri.), 2022 / 1:50-2:30PM (London (GMT)) )
    Title : The effect of stratification on shoaling Internal Solitary Waves
    Abstract : 
         Internal solitary waves (ISWs) are finite amplitude waves of permanent form that travel 
       along density interfaces in stably stratified fluids. They owe their existence to an exact 
       balance between non-linear wave steepening effects and linear wave dispersion. They are 
       common in all stratified flows especially coastal seas, straits, fjords and the atmospheric 
       boundary layer. In the ocean, they are thought to be a source of mixing, and are important 
       in re-suspension of sedimentary materials, and mixing processes in the benthic boundary 
       layer. They are subsequently of interest from both an environmental and offshore engineering 
       point of view.
         In this presentation a combined experimental and numerical study will illustrate the effect 
       of stratification form on the shoaling characteristics of internal solitary waves propagating 
       over a smooth, linear topographic slope. It is found that the form of stratification affects 
       the breaking type associated with the shoaling wave. In a thin tanh stratification (homogeneous 
       upper and lower layers separated by a thin pycnocline), good agreement is seen with past studies. 
       Waves over the shallowest slopes undergo fission. Over steeper slopes, the breaking type changes 
       from surging, through collapsing to plunging with increasing wave steepness Aw/Lw for a given 
       topographic slope, where Aw and Lw are incident wave amplitude and wavelength, respectively.
       In a surface stratification regime (linearly stratified layer overlaying a homogeneous lower 
       layer), the breaking classification differs from the thin tanh stratification. Plunging dynamics 
       are inhibited by the density gradient throughout the upper layer, instead collapsing-type 
       breakers form for the equivalent location in parameter space in the thin tanh stratification. 
       In the broad tanh profile regime (continuous density gradient throughout the water column), 
       plunging dynamics are likewise inhibited and the near-bottom density gradient prevents the 
       collapsing dynamics as well. Instead, all waves either fission or form surging breakers. 
       As wave steepness in the broad tanh stratification increases, the bolus formed by surging 
       exhibits evidence of Kelvin-Helmholtz instabilities on its upper boundary. In both two- and 
       three-dimensional simulations, billow size grows with increasing wave steepness, dynamics 
       not previously observed in the literature.

       J. Fluid Mech. (2022), vol. 933, A19, doi:10.1017/jfm.2021.1049