Internal waves generated by the wake of Gaussian hills

Depending on their structure and dynamics, the wakes of various obstacles can generate different kinds of internal waves in stratified fluids. Experiments on waves emitted by three-dimensional Gaussian models towed uniformly in a linearly stratified fluid were carried out. Beyond a critical value Fr_c of the Froude number Fr, the developed wake was observed to radiate an internal wave field shorter than the lee waves of the hill. The emergence of such waves is correlated with the periodic shedding of three dimensional vortical structures at Fr>Fr_c. Measurements show that the wavelengths of these short waves are constant in space and time, and proportional to Fr. Their phase and group velocities are proportional to the coherent structure velocity which is estimated from velocity measurements inside the wake. All those spatio-temporal characteristics prove that these short waves are generated by the displacement of the coherent structure inside the wake.