A numerical method for calculation of power output from ducted vertical axis hydro-current turbines

Abstract

This paper investigates effects of ducting on power output from vertical axis hydro-current turbines. A numerical two-dimensional method based on the potential flow theory is developed for calculation of non-dimensional power output from these turbines. In this method, the blades are represented by vortex filaments. The vortex shedding from the blades is modeled by discrete vortices. A boundary element method is used to incorporate the duct shape which is represented by a series of panels with constant distributions of sources and doublets. The aerodynamic loading on the blades are calculated using a quasi-steady modeling. A time-marching scheme is used for implementation of the numerical method. The results of this method are compared with experimental results for a turbine model. A good correlation between the numerical and experimental results is obtained for tip speed ratios equal and higher than 2.25. However due to a lack of dynamic stall modeling, the numerical method is not able to predict power output accurately at lower tip speed ratios wherein effects of dynamic stall are significant. Both numerical and experimental results also showed that the power output from a turbine can increase significantly when it is enclosed within a well-designed duct. The maximum power output of the turbine model investigated in this paper showed a 74% increase when the turbine is operating within the duct relative to the case it is in free-stream conditions.