Numerical Study of A Generic Tidal Turbine Using BEM Optimization Methods

Three blade-geometry optimization models derived along with assumptions from the blade element momentum(BEM)approach are studied by using a steady BEM code to improve a small horizontal-axis rotor of three blades that has been previously used in experiments.The base rotor blade has linear-radially varying chord length and pitch angle,while the other three models noted as Burton,Implicit and Hansen due to their references and characteristics yield blades of non-linearly varying chord length and pitch angle.The aim is to compare these rapid models and study how assumptions embedded in them affect performance and induction factors.It is found that the model that has the least assumptions(Hansen)and which considers the blade-profile drag in its optimization procedure yields the highest power coefficient,C_(P),at the optimal tip speed ratio(TSR),about 7%higher than the base one and also higher C_(P) at high TSR.It produces an axial induction factor distribution along the blade that is closest to the 1 D optimal value of 1/3.All optimized tangential induction-factor distributions along the blade closely vary as inverse to the square of the radial distance,while being mildly higher than the base distribution.It shows that sufficient swirl is necessary to increase power but at a level causing not too much energy loss in unnecessary swirl of the wake.At high TSR,all optimized rotors adversely produce higher thrust than the base one,but the one with most embedded assumptions(Burton)produces the highest thrust.Details of all three optimization models are given along with the distributions of the power,thrust,blade hydrodynamic efficiency and induction factors.

Numerical investigation of particle saltation in the bed-load regime

This paper numerically investigates particle saltation in a turbulent channel flow having a rough bed consisting of 2–3 layers of densely packed spheres.In this study,we combined three the state-of-the-art technologies,i.e.,the direct numerical simulation of turbulent flow,the combined finite-discrete element modelling of the deformation,movement and collision of the particles,and the immersed boundary method for the fluid-solid interaction.Here we verify our code by comparing the flow and particle statistical features with the published data and then present the hydrodynamic forces acting on a particle together with the particle coordinates and velocities,during a typical saltation.We found strong correlation between the abruptly decreasing particle stream-wise velocity and the increasing vertical velocity at collision,which indicates that the continuous saltation of large grain-size particles is controlled by collision parameters such as particle incident angle,local rough bed packing arrangement,and particle density,etc.This physical process is different from that of particle entrainment in which turbulence coherence structures play an important role.Probability distribution functions of several important saltation parameters and the relationships between them are presented.The results show that the saltating particles hitting the windward side of the bed particles are more likely to bounce off the rough bed than those hitting the leeside.Based on the above findings,saltation mechanisms of large grain-size particles in turbulent channel flow are presented.