LABORATORY STUDY OF THE INTERACTION BETWEEN 2 INTERNAL WAVE RAYS

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 ray s produced a velocity field with stagnation points, and these points p ropagated vertically upwards within the intersection region. Nonlinear non-resonant interactions between the two rays produced evanescent mo des, with frequencies greater than the ambient buoyancy frequency, tra pped within the intersection region. These evanescent modes provided a mechanism whereby energy could accumulate locally and, even though th e 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 stagna tion 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 wit hin the intersection region. The results showed that the release of av ailable potential energy was disrupted by the wave motions and even th e dissipative scales were directly affected by the ambient stratificat ion and the background wave motion. The distribution of the centred di splacement scales was highly skewed towards the Kolmogorov scale and t he turbulent Reynolds number Re, 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 fo r such dissipative events the square of the strain Froude number (epsi lon/nu N-0(2)) and the turbulent Reynolds number Re-t can be less than one.