Abstract
The flow over a mushroom-shaped microscale coating was experimentally inspected over a diverging channel that followed the pressure side of a wind turbine blade (S835). High-resolution particle image velocimetry was used to obtain in-plane velocity measurements in a refractive-index-matching flume at Reynolds number Reθ ≈ 1200 based on the momentum thickness. The results show that the evolution of the boundary layer thickness, displacement thickness, and shape factor change with the coating, contrary to the expected behavior of an adverse pressure gradient boundary layer over a canonical rough surface. Comparison of the flow with that over a smooth wall revealed that the turbulence production exhibited similar levels in both cases, suggesting that the coating does not behave like a typical rough wall, which increases the Reynolds stresses. Proper orthogonal decomposition was used to decompose the velocity field to investigate the possible structural changes introduced by the wall region. It suggests that large-scale motions in the wall region lead to high-momentum flow over the coated case compared to the smooth counterpart. This unique behavior of this surface coating can be useful in wind-turbine applications, with great potential to increase the power production.
Highlights
Flow separation plays a significant role in the drag experienced by terrestrial, marine, and aerial vehicles, among others.[1]
Due to the flow separation close to the wall, positive velocity difference is the result of stronger reverse flow regions experienced by the smooth surface compared with the coated surface
The coating led to a reduction of the reverse flow and higher streamwise and vertical velocities over most of the inner region and portion of the outer flow, which resulted in a reduction of the separation bubble
Summary
Flow separation plays a significant role in the drag experienced by terrestrial, marine, and aerial vehicles, among others.[1]. Flow separation occurs on wind turbine blades due to a large pressure gradient, which causes power losses and unsteady loading. The performance of wind turbines is sensitive to the surface characteristics of the blade surfaces. Ice accretion increases the surface roughness and decreases lift-to-drag ratio;[2] the roughness induced by insect contamination, at the leading edge, facilitates flow separation at normal operating conditions.[3] The phenomenon of ‘double stall’ is attributed to the roughness effects created by insect contamination.[4] deposition of dust or erosion due to sand blasting results in performance reduction[5]. Flow separation owing to natural causes should be mitigated to achieve designed performance of wind turbines
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