Actuator discs may be used as a simple method for simulating horizontal axis tidal turbines, both in experiments and CFD models. They produce a similar far wake to a real turbine, but eliminate some of the scaling issues which occur in experiments, and reduce the mesh density required in CFD simulations. This paper examines methods for applying a simple actuator disc in a commercial CFD code, Ansys CFX, and compares the wake produced with experimental results for similar values of disk thrust coefficient (C T). The results show that the CFD model gives reasonable agreement with the experimental results. The main factors affecting the wake structure are the initial C T value, the ambient turbulence levels, and potentially the disc induced turbulence. The main differences between the models and experiments were in terms of the turbulence levels throughout the model. With further development, it is considered that the CFD actuator disc could be an accurate and validated method for numerically modelling tidal turbines.

The actuator disc-RANS model has widely been used in wind and tidal energy to predict the wake of a horizontal axis turbine. The model is appropriate where large-scale effects of the turbine on a flow are of interest, for example, when considering environmental impacts, or arrays of devices. The accuracy of the model for modelling the wake of tidal stream turbines has not been demonstrated, and flow predictions presented in the literature for similar modelled scenarios vary significantly. This paper compares the results of the actuator disc-RANS model, where the turbine forces have been derived using a blade-element approach, to experimental data measured in the wake of a scaled turbine. It also compares the results with those of a simpler uniform actuator disc model. The comparisons show that the model is accurate and can predict up to 94 per cent of the variation in the experimental velocity data measured on the centreline of the wake, therefore demonstrating that the actuator disc-RANS model is an accurate approach for modelling a turbine wake, and a conservative approach to predict performance and loads. It can therefore be applied to similar scenarios with confidence.

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A method is described for measuring the mineralization of an organic solute (14C-glucose) by the heterotrophic indigenous bacteria in lake sediments. Since there is no suitable procedure for the determination of in situ microbial activities in sediments, the procedure described is probably the best devised so far and may serve as a base for a more definitive procedure. The surface sediments of aquatic ecosystems are the boundary between a circulating dynamic medium primarily dominated by properties of water and its solutes and a structurally more stable medium with properties much like soil. This boundary area is the site of intense microbial activity in many natural waters. A great variety of bacteria can be isolated from sediments, and they are present in high concentrations, but only limited information can be obtained by culturing these bacteria. Methods are needed that give information on biological and chemical processes promoted by the whole community of microorganisms in the sediment environment. One important function of sediment micro