Nikolaos Th. Fourniotis

Effect of bed dunes on spatial development of open-channel flow

The spatial development of turbulent, sub-critical open-channel flow over five identical dunes is studied by the numerical solution of the RANS equations utilizing theVOF free-surface formulation and the k–e or Spalart–Allmaras turbulence models. Results are presented for smooth and rough walls and several dune dimensions. One of the cases was also studied experimentally. The flow separation at each dune crest generates a recirculation in the dune lee-side and reattachment at a distance, which increases with increasing dune height and decreasing dune length. Turbulence does not fully develop over the dunes, while the majority of turbulent kinetic energy production takes place in the recirculation region. The spatially mean, free-surface level decreases in the flow direction over the dunes, while the free-surface amplitude increases with increasing dune height. The dune drag coefficient increases with increasing dune height, while the contribution of form resistance on the drag increases with increasing dune height and decreasing dune length.

Simulation of the Winter Meso-Scale Wind and Tidal Circulation in the Gulf of Patras (Greece)

The Gulf of Patras is a shallow embayment (of 80 m mean depth), in western Greece, leading to the Ionian Sea on the west, and, through the straits of Rio-Antirio, to the Gulf of Corinth, on the east. In the present study, the three-dimensional modelling system [1] was applied to investigate the tide- and wind-driven circulation in the complex bathymetry of the natural basin of the Gulf of Patras. Numerical simulations have been conducted for different scenarios for wind speed and direction, and the results of the circulation forced by a uniform wind stress corresponding to a wind speed of 4 m/s and two different directions are reported. In addition, tidal records measured at both ends of the Gulf were used as forcing in order to simulate the tidal circulation. The numerical study corroborated that the tidal currents at the Rio-Antirio straits are stronger than those induced by the mean wind and are among the strongest tidal currents to be found in Greek waters. Comparisons between the three-dimensional model predictions and available field measurements have shown reasonable agreement both qualitatively and quantitatively.© 2008 ASME

Wintertime Tidal Hydrodynamics in the Gulf of Patras, Greece

ABSTRACT Horsch, G.M. and Fourniotis, N.Th., 2017. Wintertime tidal hydrodynamics in the Gulf of Patras, Greece. Three-dimensional numerical simulations have been applied to simulate the winter barotropic hydrodynamic response that develops in the Gulf of Patras due to tidal and wind forcing. The simulations were performed using the code MIKE 3 flexible mesh hydrodynamic module. They reproduce already-known features of the tidal flow entering the Gulf of Patras from the Ionian Sea, such as reflection of the tidal wave at the Rio-Antirio Straits and the cove of the Bay of Nafpaktos. Furthermore, details of the flow structure are revealed, such as the gyre that develops during the transition from ebb to flood tide in front of the Messolonghi Lagoon and the gyre in the Bay of Nafpaktos from flood to ebb. The action of prevailing winds on tidal circulation is to create strong nearshore currents, which dictate the sense of rotation of gyres that develop nearshore at both the northern and southern shores of the gulf, when the wind opposes the tide. The exchange flow, which develops between the Gulf of Patras and the adjacent Nafpaktos Bay, was found to be dominated, in general, by the tide, rather than by the wind; this nevertheless creates a lag in exchange flow when the wind opposes the tide. Finally, tidal circulation was found to create upwellings during the wintertime regime, reported herein for the first time, at the same locations in the Gulf where upwellings are known to develop in the summertime baroclinic flow, although not under the same wind conditions.