Vertical axis wind turbines (VAWT) have recently received growing interest for application in urban environments due to their omni-directional capabilities. However, further research is required to optimize their performance. The central shaft is an inseparable part of a VAWT whose effect on turbine performance is currently not known. In this paper the effect of the shaft on the power coefficient (CP) and thrust coefficient (C-T) of a VAWT is studied for different shaft-to-turbine diameter ratios (δ) and normalized surface roughness heights (ks/ds) using unsteady Reynolds-averaged Navier-Stokes (URANS) Computational Fluid Dynamics (CFD) simulations validated with experiments. The turbine power loss is found to increase by 3.5% by changing δ from 4 to 12%. This is due to the higher width of the shaft wake: blades pass through a larger region with lower velocity in the downwind area. The turbine CP is improved by up to 2% for a rough shaft (ks/ds = 0.08) due to a shift in the flow regime over the shaft from sub-critical to critical. The findings of the present study mean that for Reynolds numbers relevant to small VAWTs, roughening the shaft is an effective strategy to partially regain the power loss related to its presence.
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