TURBULENT MIXING IN THE GULF OF TRIESTE UNDER CRITICAL CONDITIONS
The present dissertation is part of a research project aimed at investigate the characteristics of the turbulent mixing in the Gulf of Trieste under critical conditions, namely when the forcings acting on the water column are: the tidal current, the effect of the Earth rotation and the presence of a vertical stable stratification due to heat fluxes supplied at the free surface. Critical conditions can occur both in winter, when only tidal current and rotation influence the flow, and in summer, when also the effect of stratification plays a very important role. The former case is the object of the thesis. Since the Reynolds number of the oceanographic system is too high to be studied by means of the present numerical techniques, the numerical experiment is carried aut at Re = 1.6 x 106 , one arder of magnitude smaller than the effective one, still considering the flow in a turbulent regime and keeping constant the physical parameters characterizing the actual flow, i.e. the Keulegan-Carpenter and the Rossby numbers. The filtered governing equations describing the oscillating, rotating turbulent flow are solved by means of resolved large-eddy simulation (LES), modeling the subgrid-scale stresses through a dynamic-mixed model. Due to the burdensome computational efforts required far such study, the code adopted is implemented in a parallel framework. The work is organized as follows: the first chapter presents a general introduction describing the Gulf of Trieste and the forcings, chapter 2 is devoted to the formulation of the problem and the mathematical model adopted, chapter 3 describes the parallel implementation together with validation tests. The results are presented in two different chapters: the purely oscillating flow is given in chapter 4, whereas the rotating-oscillating one is discussed in chapter 5. Finally, conclusions are given in chapter 6. It has to be remarked that the present simulation of the Stokes boundary layer represents the first numerical study that investigates the details of the turbulent field in a purely oscillating flow at a Reynolds number such that most of the cycle of oscillation is characterized by the presence of fully developed turbulence. Our results are in good agreement with the experimental observations and corroborate the findings of the relevant experimental studies. Moreover, to the best of our knowledge, the turbulent rotating-oscillating flow has been never investigated. The rotation of the reference frame induces a destabilizing effect on the flow, depending on the forcing current direction, which agrees with theory an d precedent studi es regarding the turbulent neutral steady Ekman layer. Furthermore, a highly anisotropie character of turbulence can be drawn from our simulations. The results of the present dissertation show that: mixing processes in the Gulf of Trieste under critical conditions during the winter season are characterized by an intense turbulent activity during the central phases of both the half-periods of oscillation of the M2 tide, along more than half the water column. Levels of turbulence peculiar of the second half-period (tidal current flowing from NEto SW) appear remarkably stronger than those of the first, and the vertical extension where turbulent activity can be observed results increased. The role played by rotation is of fundamental importance in the enhancement of horizontal and vertical mixing throughout the whole tidal cycle of oscillation. Unlike the purely oscillating case, the three fluctuating components are mutually correlated, and turbulent intensities contribute to intensify mixing also near the surface. From the numerical point of view, the present dissertation has also shown t ha t a resolved LES gives accurate results in the case of unsteady boundary layers. Moreover, the dynamic-mixed model adopted appears to be a robust tool for simulating both the Stokes boundary layer, since it is able to adjust to the local and instantaneous characteristics of the flow field, and its rotating counterpart.