A Tomo‐PIV study of the effects of freestream turbulence on stall delay of the blade of a horizontal‐axis wind turbine

Abstract

Volumetric velocity fields were measured using tomographic particle image velocimetry on a model of the blade of a 5 kW horizontal‐axis wind turbine to study the effects of freestream turbulence levels (FTLs) at 0.4%, 4% and 13% on stall delay phenomenon at two different global tip speed ratios of 3 and 5 with Reynolds number (Re) ࣈ 5000. Static pressures were measured, and results illustrated that FTL has stronger effect on the surface pressures of the static airfoil. Magnitudes of the absolute velocities within the separated flows above the static airfoil's suction surface increase significantly with higher FTL, while the changes of these velocities above the rotating blade's surface are less obvious. Radial flows from rotating blade's root to tip were also observed with strong spanwise velocity component located in the vicinities of the vortices. At the root and middle sections of the rotating blade, the flows with strong radial velocity component, w, become wider with higher FTL near to the rotating blade's leading edge when the angles of attack (AOAs) are large. At large AOAs, the strength and size of the vortices shed from the rotating blade's leading and trailing edges decrease significantly with higher FTL. However, at small AOAs, the size and coherence of the vortices near the rotating blade's trailing edge increase significantly with higher FTL. Surface streamlines of the rotating blade illustrated that at the rotating blade's root region and at large AOAs, the streamlines tend to lean toward the rotating blade's trailing edge at higher FTL. Copyright © 2014 John Wiley & Sons, Ltd.

An experimental study of fluid dynamics of stall delay on the blade of a horizontal axis wind turbine

“…The iso-contours represent the vorticity. There is a large vortex behind the rotating blade and the flow started to partially separate, which is different with the case for the static airfoil at the AOA of 30 degrees [14,16] where the flow is fully separated; while the flow at TSR=5 is still attached, which is the same as the previous results [14] . Note that a strong reversed flow from the trailing edge to leading edge at near the suction surface of the blade at a higher Reynolds number (Re = 8683) has been observed at TSR=3, which perhaps shows the Reynolds number effects on the stall delay.…”

“…They, for the first time, measured the detailed threedimensional and volumetric velocity fields using Tomographic PIV on a model of the rotating blade of a small-scale HAWT to study the physics of stall delay at two different global tip speed ratios (TSRs) [The tip-speed ratio or TSR for wind turbines is the ratio between the tangential speed of the tip of a blade and the actual speed of the wind.] at about Re=5000 [14][15][16][17][18] . Their results showed that rather than the recirculation separation bubbles with strong reversed flows for the static airfoil at deep stall, the attached flows were observed on the suction surface of the rotating blade [14,15,17] .…”

“…It, therefore, delayed the stall. In addition, they observed that although the frees-stream turbulence levels affected the separated flows for the static airfoil, they have much less effects on the rotating blade [16] . Furthermore, in the near wake region of a airfoil in deep stall, the turbulence structures involved the large-scale Karman vortex and interactions with the small-scale shear layer vortices [18] .…”

Drag Reduction of a Turbulent Boundary Layer over an Oscillating Wall and Its Variation with Reynolds Number

“…Further validation of our numerical results has been planned to use state-of-the-art three-dimensional PIV measurements [38][39][40][41].…”

A Review on Numerical Development of Tidal Stream Turbine Performance and Wake Prediction

“…Difficulties arise, however, when the rotor operates in an unsteady state. The unsteady-flow condition leads to cyclic turbine operation [32], dynamic stall [33], aerodynamic effects [34] and overall reduction of power in wind turbines [35], whereas in tidal stream turbines, its effects are little understood. In recent years, various scaled turbines are being studied in test laboratories comprising different flow conditions: current with low and high turbulence alone, and current combined with waves [36].…”