• Influence of upstream turbulence on the wake characteristics of a tidal stream turbine.

      Ahmadi, Mohammad H.B.; University of Derby (Elsevier, 2018-08-20)
      The influence of the upstream turbulence intensity on the flow characteristics downstream of a laboratory-scale horizontal axis tidal stream turbine is investigated in this study. Three test cases with the same mean velocity and different turbulence intensities are simulated numerically using the hybrid large eddy simulation/actuator line modelling technique. The mean velocity components, mean turbulent fluctuations, velocity deficit and wake extension are compared along the streamwise direction to examine the upstream turbulence effects. The inflow conditions are generated by the mapping method using the mean velocity and turbulent profiles experimentally obtained for a turbulent open channel flow. Comparing results for the mean velocity and turbulent fluctuations shows that the upstream turbulence level strongly affects the flow characteristics downstream of the turbine by influencing the tip vortices breakdown process and in turn wake recovery. The comparison also reveals that the ambient turbulence level strongly influences the velocity deficit but it does not significantly affect the streamwise velocity and the radial location of tip vortices in the flow.
    • Numerical simulations of wake characteristics of a horizontal axis tidal stream turbine using actuator line model.

      Baba-Ahmadi, Mohammad H.; Dong, Ping; University of Dundee; University of Liverpool (Elsevier, 2017-06-10)
      The wake of a laboratory scale tidal stream turbine in a shallow water channel with a turbulent inflow is simulated using the hybrid LES/ALM technique, which combines large eddy simulation with the actuator line method. The turbulent inlet conditions are generated using the mapping method to avoid a precursor running and large space for saving data. The numerical results demonstrated the usefulness of the mapping technique as well as some shortcomings that still remain to be addressed. Good agreement between numerical predictions and experimental data is achieved for both the mean and turbulent characteristics of the flow behind the turbine. The examination of changes in turbulence intensity and turbulent kinetic energy in the streamwise direction confirms the existence of a peak and transition to a highly turbulent flow about three diameters downstream of the turbine, which means that the distinct characteristics of the streamwise changes of turbulence intensity or turbulent kinetic energy may serve as an effective indicator for the flow regime transition and wake behaviour.
    • Validation of the actuator line method for simulating flow through a horizontal axis tidal stream turbine by comparison with measurements

      Baba-Ahmadi, Mohammad H.; Dong, Ping; University of Dundee; University of Liverpool (Elsevier, 2017-05-20)
      The purpose of the present work is to evaluate the capability of the Actuator Line Method (ALM) to simulate flow through a horizontal axis tidal stream turbine. A numerical model combining the ALM with large eddy simulation technique is developed and applied to compute the flow past a laboratory-scale tidal stream turbine. The flow field is analysed in terms of streamwise mean velocity, turbulence intensity, turbulent kinetic energy and the decay rate of the maximum turbulent kinetic energy behind the turbine. It is found that the ALM performs well in predicting the mean flow and turbulence characteristics behind the turbine. The flow field predicted show a clear transition from an organised vorticity region near the turbine to a highly turbulent flow downstream. The location of this transition and the controlling parameters are discussed but further investigation, both numerical and experimental is required in order to clarify its effects on the flow structure and the performance of downstream turbines in tidal turbine arrays.