A wind tunnel study of the impact of upstream obstructions on the performance of vertical axis wind turbines in low-speed wind

School of Mechanical, Industrial and Aeronautical Engineering, University of Witwatersrand, Johannesburg, South Africa

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 14 January 2026

Abstract

Vertical-axis wind turbines (VAWTs) are promising for urban renewable energy, but their performance in low-wind, complex-flow conditions is poorly predicted. The quantified effects of obstacle geometry, distance, and height on VAWT start speed and power output remain unknown. This study provides empirical data to close this gap and support optimised siting in marginal wind conditions. A controlled wind-tunnel experiment was performed using a commercial 5-blade drag-based VAWT. Rectangular obstacles (500 × 340 mm) at heights of 600 mm and 750 mm were placed 1, 2 and 3 m upstream. Freestream velocities of 3, 6, 9 and 11 m/s were tested. For each configuration, the electrical power output and rotational speed were measured. The power coefficient (Cp) and start speed were calculated and benchmarked against an unobstructed base case. The study found that the presence of an upstream obstacle significantly enhanced VAWT performance compared to the unobstructed base case. A 43.5% increase in peak power coefficient was achieved with the taller obstacle at 2 m. Furthermore, the turbine start speed was reduced by over 50%, from 2.5 m/s to approximately 1.1 m/s. Performance enhancement was non-monotonic, with optimal distance dependent on obstacle height (1 m for 600 mm; 2 m for 750 mm). This is attributed to a beneficial interaction with the obstacle’s wake, in which flow acceleration and turbulent structures simultaneously increase the impulse on the driving blades and reduce the resistance on the returning blades. The study concludes that siting a VAWT behind an upstream bluff body can enhance VAWT performance. These findings are limited to controlled wind tunnel conditions, as the simplified setup does not fully replicate real-world field flows.