Sanda-Carmen Georgescu

Jet drops ejection in bursting gas bubble processes

The numerical simulation using a boundary element method is presented for a gas bubble bursting at a free surface in a potential flow with a viscous fluid assumption. Systematic comparisons are given with experimental data on the first “jet drop” size in relation with the parent bubble size, and on the critical bubble radius above which no jet drop forms. The computations were made for different liquids. It is pointed out that an exact description of the jet formation and break up requires the complete Navier–Stokes equations only in the final phase of the evolution.

Efficiency of Marine Hydropower Farms Consisting of Multiple Vertical Axis Cross-Flow Turbines

This study focuses on the Achard turbine, a vertical axis, cross-flow, marine current turbine module. Similar modules can be superposed to form towers. A marine or river hydropower farm consists of a cluster of barges, each gathering several parallel rows of towers, running in stabilized current. Two-dimensional numerical modelling is performed in a horizontal cross-section of all towers, using FLUENT and COMSOL Multiphysics. Numerical models validation with experimental results is performed through the velocity distribution, depicted by Acoustic Doppler Velocimetry, in the wake of the middle turbine within a farm model. As long as the numerical flow in the wake fits the experiments, the numerical results for the power coefficient (turbine efficiency) are trustworthy. The overall farm efficiency, with respect to the spatial arrangement of the towers, was depicted by 2D modelling of the unsteady flow inside the farm, using COMSOL Multiphysics. Rows of overlapping parallel towers ensure the increase of global efficiency of the farm.

HBMOA computational efficiency assessed for a hydropower optimization problem

A simple hydropower optimization problem is used to compare the computational efficiency of the Honey Bees Mating Optimization Algorithm (HBMOA), with the efficiency of other evolutionary algorithms, namely 3 recent ones: firefly algorithm, cuckoo search algorithm and bat-inspired algorithm (BA). The selected case study is a hydropower development on the Arges river, in Romania, consisting of Vidraru Reservoir (upstream) and Vidraru Hydro-Power Plant (downstream). Under specific conditions, Newton-Raphson method gives an accurate solution of the above optimization problem — that solution can be used as reference solution when assessing the computational efficiency of the above algorithms. From the overall performance viewpoint, BA is the most efficient algorithm, while from the hydropower viewpoint, HBMOA gives the preferred result: its annual energy production is the closest to the imposed reference value.

Influence of duct geometry on Achard turbine efficiency

In order to increase the efficiency of the Achard turbine, we propose a new duct geometry yielding to a shrouded Achard turbine: a turbine running within a channeling device with S-shaped vertical walls, in a free flow domain. Several Achard turbines, superposed, form towers. For such towers, the flow is almost unchanged in horizontal planes along the vertical axis, allowing thus 2D numerical modeling in a cross-section of the tower. The 2D modeling of the transient flow inside the shrouded Achard turbine with straight blades was performed with COMSOL Multiphysics. The studied case focuses on a row of two towers, placed across the water flow direction. In order to depict the influence of the duct geometry on turbines efficiency (power coefficient), our results were compared with other results obtained for shrouded Achard turbines running within a different type of duct: a symmetric two-foiled channeling device. Both duct geometries proved to increase the turbine efficiency with respect to the case of an isolated Achard turbine.