Dazheng Wang

Flow separation impacts on the hydrodynamic performance analysis of a marine current turbine using CFD

Although there are different strategies to control the operation of marine turbines, the so-called ‘stall-regulated strategy’ is one of the most widely used and mature control strategies. Since the stall phenomenon is closely related to flow separation around the turbine blades the treatment of this separation requires great care during the design and performance analysis of turbines when using computational fluid dynamics (CFD). This study investigates appropriate methodologies and approaches to simulate the hydrodynamic performance of horizontal marine turbines with a specific emphasis on the flow separation phenomena. The well-known viscous flow solver ANSYS-CFX was employed as the main CFD code to predict the power extraction coefficient of these turbines. The investigations were carried out by using both numerical and experimental methods applied on tidal stream turbine models tested in the Emerson Cavitation Tunnel of Newcastle University, UK and the circulating water channel of Harbin Institute of Technology, China. The measured power extraction coefficients generally agreed well with the numerically predicted ones except for one of the models with the lower pitch angle which displayed large discrepancies over the entire operating range. The detailed flow analyses from the CFD studies with this turbine and other model at higher pitch angles revealed that large-scale detached vortices developed downstream of the model with the lower pitch angle may have contributed to this large discrepancy. The study therefore draws attention to the importance of the combined use of the CFD and model test-based approaches in the design and performance analysis of marine turbines.

Experimental investigation into the impact of slipstream wash of a podded propulsor on the marine environment

The paper presents the results of an experimental investigation into the velocity field of a podded propulsor carried out by laser Doppler anemometry and the use of this information for assessing the potential impact of a podded propulsor slipstream, in terms of wash velocity, on the marine environment. The investigation was carried out in the Emerson cavitation tunnel of Newcastle University with the roll-on roll-off passenger ship or ferry (ROPAX) hull model as part of the project entitled ‘optimal design and implementation of azimuthing pods for the safe and efficient propulsion of ships’ sponsored by the EU Fifth Framework Programme initiative. Comprehensive velocity field measurements were made in open water, behind the simulated wake of the ROPAX hull and in the bollard pull condition. The mean velocity distributions in the axial and radial directions were calculated from the measured velocity fields to evaluate the potential impact of the slipstream wash of a podded propulsor on the environment. For comparison purposes, some results from similar measurements made with a conventional shafted propeller are also included. The experimental results have revealed that the most significant impact of the propeller slipstream wash on the environment could occur when a ship departs from a port.