Numerical simulation and shape improvement of a novel vertical axis autorotation current turbine

Abstract

The ultimate objective of this study is to improve the performance of the VAACT by changing its shape, i.e., to find the best values of inner length and flapped angle that yield a higher mean power coefficient. Firstly, the uniform flow over a nominally two-dimensional normal thin flat plate with blockage ratio of 0.214 is numerically resolved by URANS (Unsteady Reynolds-averaged Navier-Stokes) based on the RKE (Realizable k ") model, URANS based on the SST (k ! Shear Stress Transport) model and DES (Detached Eddy Simulation). A comprehensive comparison against earlier experimental data shows that URANS-SST method yields a correct Strouhal number but overestimates the mean drag coefficient, while URANS-RKE and DES methods succeed in giving accurate predictions of the both. Moreover, DES is able to capture irregular small-scale structures and reproduce the three-dimensionality of vortex shedding. Then, DES is used for the numerical simulation of the rotation of the flapped VAACT. The good agreement between the simulation and experimental results indicates the applicability and accuracy of the numerical methods used. The free surface effect is found to be insignificant when the fluctuation of the free surface is small relative to the height of the turbine submerged into the water surface. Finally, shape improvement of the flapped VAACT based on the response surface model obtained through CFD and second-order polynomial regression are successfully performed. The best combination with inner length of 10.8 cm and flapped angle of 47.2 degrees are found.