In order to study the interaction between the internal waves (IWs) and the surface waves (SWs) with different periods over a slope-shelf, laboratory experiments and numerical simulations by using plunger wave maker are conducted in the paper. Laboratory results reveal that the wave period of IWs shows significant decrease during this process at long incident; at short incident, the wave period of IWs and SWs keep the same value. The results of the numerical simulation show that the short interaction time causes the weak internal hydraulic jump on front slope when IWs propagates over a slope-shelf.
In a stratified fluid, the barotropic waves (i.e., Surface waves; SWs) are generated by external forces (ex., wind or flow) in surface layer and then baroclinic waves (i.e., Internal waves; IWs) are induced in interface d due to the different pressure variations. The field observations reveal, in the ocean, the IWs are generated by the interaction between flow and submerged topography (Hsu et. al., 2000); in the lake, the wind plays an important role on producing the surface pressure difference and internal waves are generated indirectly (Pannard et. al., 2011); at the estuary, the internal waves are generated by the interaction between waves and flow. Internal waves have significant effect on ecology, environment and engineering when they propagate over varied topography (Bourgault et al., 2014; Lamb, 2014). Internal solitary wave (ISW), which is a special type of internal waves, has been researched by many literatures about its generation, transport and dissipation (Grimshaw and Helfrich, 2018; Hsieh et al. 2016, Lamb, 2014). On the other hand, as short flow is indiced by larger amplitude of ISW, the resulting surface waves become small and can be ignored during this interaction.
In lake and estuary, IWs generated by surface pressure variations may affect stratified fluid mixing, SWs' transmission, bottom nutrient pumping and lakeside scouring. Up to now, the existing literature about the interaction between surface and internal waves or together with a submerged obstacle is less and unclear. The mechanism for IWs' or SWs' generation, propagation and dissipation are the important topics for research. Although sequential data can be recorded by ADCP or CTD in the field observations, it is difficult to clearly analyze the interaction between IWs and SWs. Accordingly researchers often adopted both theoretical analysis, laboratory experiments and numerical simulations. However, for theoretical researchers studying the interaction between surface and internal waves, they almost adopt two wave functions directly in the surface and interface and ignore the density mixing in calculating the fluid field (Das et. al., 2018; Lee et. al., 2007; Shermeneva et al, 2015). For laboratory experiments, several wave-making mechanisms are followed to generate surface and internal gravity waves or internal waves: Vlasenko and Hutter (2001) used two pistons in upper and lower layers to study internal solitary waves transformation over a sill; Dalziel et al., (2007) adopted gate-type wavemaker to observe the internal solitary waves transformation on flat bottom; Ko and Cho (2017) used plunger type wavemaker (cylindrical shape) to find the interactions between surface and internal waves on flat bottom. Cheng et al. (2018a) discussed the variations of wave amplitude and wave period for the SWs and IWs that are generated by plunger type wave maker and propagate over a submerged ridge.
