Abstract:Wind velocity distribution and the vortex around the wind turbine present a significant challenge in the development of straight-bladed vertical axis wind turbines (VAWTs). This paper is intended to investigate influence of tip vortex on wind turbine wake by Computational Fluid Dynamics (CFD) simulations. In this study, the number of blades is two and the airfoil is a NACA0021 with chord length of c = 0.265 m. To capture the tip vortex characteristics, the velocity fields are investigated by the Q-criterion iso-surface (Q = 100) with shear-stress transport (SST) k-ω turbulence model at different tip speed ratios (TSRs). Then, mean velocity, velocity deficit and torque coefficient acting on the blade in the different spanwise positions are compared. The wind velocities obtained by CFD simulations are also compared with the experimental data from wind tunnel experiments. As a result, we can state that the wind velocity curves calculated by CFD simulations are consistent with Laser Doppler Velocity (LDV) measurements. The distribution of the vortex structure along the spanwise direction is more complex at a lower TSR and the tip vortex has a longer dissipation distance at a high TSR. In addition, the mean wind velocity shows a large value near the blade tip and a small value near the blade due to the vortex effect.

The flow structures and the pressure fluctuations in a centrifugal pump at both design and off-design points are presented in this paper. The flow structures are numerically obtained by solving the viscous incompressible Navier-Stokes equations with a 3D unsteady flow model. A sliding mesh technique is applied to take into account the impeller-volute interaction. The pressure fluctuations are obtained by both numerical simulations and experimental measurements. The former data denote all the flow field characteristics including those in the impeller, the interface, the volute and the diffuser, while the latter ones that acquisitioned by a high-frequency pressure sensor show only one position at the outlet flange. The FFT technology is adopted to process those dynamic pressure data to reveal the frequency features. All the power spectrum results present that the dominant frequencies are the blade passing ones, while the subdominant ones are varied depending on the flow rates and the positions. At design condition, the mean velocity field in both impeller and volute show well behaved flow with no separation, while subtle separation is found along the tongue side in the diffuser. Some unknown high-frequency contents are discovered in the flow field.

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