Seeing the unseeable in a rotor wake

Watching slow turning turbines on a prairie wind farm may convey a sense of serenity. But there is more going on than meets the eye. Some of that is revealed in the stunning images of air flow and turbulence generated by Postdoctoral Scholar Raymond Chow and Professor C.P. van Dam at UC Davis in California.

The color image was generated by computational fluid dynamic (fluid flow) software to reveal vortices, the red circles (below) at the top and bottom, generated by the rotor tips. The “contours of velocity magnitude” were induced by a simulated three-blade wind turbine. The colors provide an intensity scale for the vortices. The 3D monotone image shows an iso-vorticity surface, providing a more complete picture of the complex 3D nature of the wake structure.

“At the center of the flow, a complex evolution in the structure occurs as well,” says Chow. The three root vortices shed by the three blades merge to form an initial large root vortex in the middle. This root vortex becomes unstable (at least for these flow conditions) and breaks down into pockets of turbulent eddies as it advects downstream and interacts with the tip vortex flow.

The large and detailed model that produced the images calls for trillions of calculations, about two weeks on a 64 processor computer. “But this can greatly increase depending on the complexity of fluid dynamics, with very unsteady flows potentially increasing this by a factor of four or five,” says Chow.

For the CFD software, Chow and van Dam use OVERFLOW-2, a NASA research code that has been enhanced and refined over the past two decades. The code, developed and maintained by Dr. Pieter G. Buning at NASA Langley, is used throughout NASA and the aerospace industry for simulating various applications from rockets, returning hypersonic space vehicles, and aircraft. Chow says his research group at Davis, led by van Dam, has been a leader in taking these tools from the aerospace industry over to the field of wind energy. WPE

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