Instantaneous transport of a passive scalar in a turbulent separated flow

The results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180° bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent large-scale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection

Large-eddy simulation of a vertical axis tidal turbine using an immersed boundary method

Vertical Axis Tidal Turbines (VATTs) are an innovative way of harnessing renewable energy from tidal streams. Herein a novel numerical approach using a refined Large Eddy Simulation (LES) code to simulate the performance of a VATT is presented. The turbine blades are modelled with Lagrangian markers using the Immersed Boundary Method which offers several advantages especially concerning computational effort. Comparisons of the LES results with experimental and numerical data suggest reasonably good accuracy of the code. In addition, the stability of the method for high Reynolds number flows is also discussed