How High is High Enough? Horizontal Axis Wind Turbine Aerodynamics at Full-Scale Reynolds Numbers Using Laboratory Models

Achieving the high Reynolds numbers and matched tip speeds of full-scale horizontal axis wind turbines in a laboratory setting remains a major challenge for experimentalists. As a result, large-scale experiments and field tests are typically required to achieve dynamic similarity with the flow of interest. This forces most small-scale experiments on these types of systems to be performed at reduced Reynolds numbers. The underlying assumption being that lower, laboratory Reynolds numbers are still large enough to safely assume inertia effects dominate viscous ones. However, the experimental evidence for this is limited and has not been extensively confirmed. Current work at Princeton University utilizes a specialized, high-pressure wind tunnel facility to adjust the Reynolds number independent of the tip speed ratio. Results are available at Reynolds numbers based on free-stream conditions and diameter up to 14 million, a value typically encountered by field turbines of 25 meters in diameter operating at 8-10 m/s. Trends with Reynolds number are discussed and support presented for Reynolds number invariance occurring at values much larger than previously found.