| Abstract |
The Performance Assessment of Wave and Tidal Array Systems (PerAWaT) project, launched in October 2009 with £8m of ETI investment. The project delivered validated, commercial software tools capable of significantly reducing the levels of uncertainty associated with predicting the energy yield of major wave and tidal stream energy arrays. It also produced information that will help reduce commercial risk of future large scale wave and tidal array developments.
This report details simulations of a three-bladed axial flow ducted tidal turbine. Additionally, the report describes fundamental work carried out to enable these blade-resolved simulations, including duct design and rotor modelling.
The bi-directional duct is designed with the aid of a computational flow solver. The design process involves the simulation of a range of candidate geometries where camber and thickness are varied methodically, with the rotor modelled as an actuator disk. Device performance is assessedbased on power, thrust and efficiency characteristics under fully turbulent flow conditions. Performance comparisons are made based on overall device dimensions rather than rotor area. The final duct combines desirable features of several candidate designs. The ability of the bi-directional turbines tested to increase both the mass-flow through and the pressure-drop across the rotor is limited. An unducted reference case, of the same outer dimensions,yields a power coefficient 75% greater than that of the best bi-directional ducted design tested.
The selected duct is then modelled incorporating the University of Manchester’s 1/70th scale rotor, operating in the 1m deep EDF flume. We conduct two levels of numerical modelling; fully blade-resolved simulationsand a novel Blade Element Momentum (BEM) theory Computational Fluid Dynamics (CFD) embedded model. We find favourable agreement between the two models. Further we note that the Manchester rotor was designed for unducted operation, and use the BEM embedded model to design a bespoke rotor for operation in the ducted environment.
The three-dimensional, blade-resolved computational modelof the ducted turbines hows excellent agreement with the embedded BEM predictions. One pertinent feature of ducted turbine flows is that the helical tip-vortex structure, readily identifiable in unducted rotor wakes, is not discernible.This is attributed to a bounding effect of the inner duct wall. For ducted turbines, bound circulation is largely maintained to the blade tip, thus limiting the production of a tip vortex. A cylindrical vortex sheet is generated emanating from the downstream edge ofthe duct to account for the momentum loss in the rotor wake. |
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