My interest is in the broad area of Geophysical Fluid Dynamics which focuses on turbulence and internal waves in tidal flow over topography via numerical experiments.

The objective of the present study, based on four phases, is to employ DNS and LES to further understand and quantify the dynamical processes underlying turbulenceduring the generation of an internal wave beam and its subsequent interaction with arealistically-stratified upper ocean.
In the first phase, a study of a stratified non-sloping bottom boundary layer under an oscillating tide has been completed and published by
Gayen et al. *JFM *(2010).
Here, we observed that stratification decreases phase lag with respect to free stream velocity for various boundary layer properties over the course of the cycle.
Turbulence generated internal waves are observed to propagate external to boundary layer. Phase angle of IW changes over tidal cycle. The frequency spectrum of the propagating wave is found to span a narrow band of frequencies corresponding to clustering around 45^{o}.

In second phase, three-dimensional DNS has been performed of generation
by a laboratory-scale slope in the regime of excursion number, *Ex* ≪ 1, and criticality,
e ≃ 1, that shows transition to turbulence. The transition is found to be initiated
by a convective instability which is closely followed by shear instability. Turbulence is
present along the entire extent of the near-critical region of the slope. The wave energy exhibits a temporal cascade to higher harmonics, subharmonics and inter harmonics.
Recently, this work has been published in Gayen & Sarkar, *Phys. Rev. Lett.*(2010).
The work of Gayen & Sarkar, *JFM* (2011) extends the previous work by examining internal wave
energetics as well as the energetics of turbulence in the bottom boundary layer.The peak value of the near-bottom velocity is found to increase with increasing length of the critical region of the topography. The scaling law that is observed to link the
near-bottom peak velocity to slope length is explained by an analytical boundary layer solution that incorporates an empirically obtained turbulent viscosity. The slope length
is also found to have a strong impact on quantities such as the wave energy flux,
turbulent production and turbulent dissipation. These sets of numerical experiments
that explain and characterize turbulence during the generation of internal waves from
tide-topography interaction at near-critical environment constitute the second phase of
work.

Our main goal in the third phase is to scale up the computational domain which has been used in second phase, from O(10)m to O(1) km
in order to reproduce the large scale overturns observed in the ocean and compare the wave energetics, turbulent statistics etc.
with the available measured data. Here, we have taken a beam of 60 m width and amplitude of 0.125 m/s to study the mixing dynamics over a sloping
topography under near critical environment. Large value of *tke* as well as large values of of positive buoyancy flux at elevated level
is observed during the flow reversal from downslope to upslope motion (Ref. Gayen & Sarkar, *Geophys. Res. Lett.* (2011))

The objective of the forth phase is to understand the interaction process between internal tidal beam and realistic upper ocean stratification and to characterize the role turbulence.