The study presents an energy performance improvement measure for an Autonomous Underwater Vehicle (AUV) carrying oceanographic equipment for collecting scientific data from the ocean. The required electric energy for the on-board equipment is harvested from tidal energy by using twin horizontal axis turbines which are integrated with thin-wall diffusers to enhance their energy capturing performance. The main focus and hence objective of the paper is the optimal design of the diffusers by using Reynolds Average Navier–Stokes Equations (RANSE) based Computational Fluid Dynamics (CFD) method and the validation of the design using physical model tests. A goal-driven optimisation procedure is used to achieve a higher power coefficient for the turbine while keeping the size and the drag of the diffuser as practically minimum as possible. Two main parameters of the optimisation are selected, the outlet diameter and the expansion section length of the diffusers, which are optimised for the highest flow acceleration ratio at the diffuser throat and for the minimum drag of the integrated diffuser and turbine system which is called as "Diffuser Augmented Tidal Turbine" (DATT) system. The numerical optimisation is validated by two sets of physical model tests conducted with a single turbine without diffuser and the same turbine integrated with the diffuser (DATT) in a cavitation tunnel and a circulating water channel. These tests demonstrated a performance enhancement for the turbine with the optimal diffuser by almost doubling the power coefficient of the turbine without the diffuser. However, the performance enhancement was dependent upon the pitch angle of the turbine

No abstract

No abstract

The tubercles along the leading edges of the humpback whale flippers can provide these large mammals with an exceptional maneuverability. This is due to the fact that the leading-edge tubercles have largely a 3D benefit for the finite hydrofoils, which can maintain the lift, reduce the drag and delay the stall angle. Newcastle University launched a series study to improve a tidal turbine’s performance with the aid of this concept. This paper presents a numerical simulation of the tested hydrofoil, which is representative of a tidal turbine blade, to investigate the flow around the foil and also to numerically model the experiment. This hydrofoil was designed based on an existing tidal turbine blade with the same chord length distribution but a constant pitch angle. The model tests have been conducted in the Emerson Cavitation Tunnel measuring the lift and drag. The results showed that the leading-edge tubercles can significantly improve the performance of the hydrofoil by improving the lift-to-drag ratio and delaying the stall. By applying Shear Stress Transport (SST), Detached Eddy Simulation (DES) and Large Eddy Simulation (LES) via using the commercial CFD solver, Star-CCM+, the tested hydrofoil models were simulated and more detailed flow information has been achieved to complement the experiment. The numerical results show that the DES model is in close agreement with the experimental results. The flow separation pattern indicates the leading-edge tubercles can energize the flow around the hydrofoil to keep the flow more attached and also separate the flow into different channels through the tubercles.

No abstract