Laboratory experiments were conducted to study the interaction between two downward propagating internal wave rays with identical properties but opposite horizontal phase velocities. The intersection of the rays produced a velocity field with stagnation points, and these points propagated vertically upwards within the intersection region. Nonlinear non-resonant interactions between the two rays produced evanescent modes, with frequencies greater than the ambient buoyancy frequency, trapped within the intersection region. These evanescent modes provided a mechanism whereby energy could accumulate locally and, even though the vertical wavelength of the primary resultant wave remained the same, the local isopycnal displacements increased in time. Eventually, the isopycnals were forced to overturn in the region just above the stagnation points by the variation with depth in the local horizontal strain rate.The gravitationally unstable overturning ultimately broke down releasing its available potential energy and generating turbulence within the intersection region. The results showed that the release of available potential energy was disrupted by the wave motions and even the dissipative scales were directly affected by the ambient stratification and the background wave motion. The distribution of the centred displacement scales was highly skewed towards the Kolmogorov scale and the turbulent Reynolds number Ret was low. Thus, the net buoyancy flux was very small and almost all turbulent kinetic energy was dissipated over the parameter range investigated. The results also showed that for such dissipative events the square of the strain Froude number (ε/νN20) and the turbulent Reynolds number Ret can be less than one.

The interaction between two downward propagating internal wave rays in a linearly stratified fluid was examined numerically and experimentally. The numerical simulations employed a SIMPLE scheme with a third order QUICK discretization for the advective terms and second order Crank-Nicholson scheme on a non-staggered grid to solve the full unsteady equations of motion in an open domain with boundary conditions based on the Sommerfield radiation condition. The laboratory experiments were performed in a glasswalled tank filled with a linearly stratified salt solution with two identical wave paddles used to generate two internal wave rays. The interaction between the two wave rays was visualized by bull eye's rainbow schlieren and shadowgraph techniques, in conjunction with velocity measurements made by particle image velocimetry. Good agreement was found between the observed and simulated interaction mechanisms. The nonlinear nonresonant interaction of two wave rays with identical properties, but opposite horizontal phase velocities, led initially to the formation of small spatial scales followed by the development of evanescent modes. The evanescent modes, with frequencies greater than the local buoyancy frequency, were trapped within the intersection region. The energy transferred to the trapped evanescent modes ultimately causing overturning of the density field in the intersection region.