Date

2016

Author

Koçak, Göktuğ

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Flight condition of helicopters proximity to the ground, so-called “Ground Effect”, is among one of the most recent research areas since the aerodynamic performance of these vehicles significantly vary due to the flow dynamics associated with the interaction between the rotor and the surface. Thus, a comprehensive understanding of the rotor wake during in and out of ground effect is needed to improve the flight performance. Helicopters operate in ground effect in many different ways including full ground, partial ground and inclined ground, which correspond to the standard landing or takeoff condition, ship deck operation, and steep valley operations, respectively. All these flight conditions cause complex, unsteady, and turbulent flow fields, which must be understood thoroughly. In the present study, ground effect of a scaled model rotor at hover condition in the absence of forward and lateral movement was investigated experimentally and numerically. First, an experimental set up was designed and constructed to investigate different ground effect scenarios including full, partial and inclined conditions, compared to out of ground condition, for a scaled model rotor in a confined vi environment. Then, laser illuminated smoke visualization and Laser Doppler Anemometry (LDA) measurements were conducted to characterize the flow field qualitatively and quantitatively. In addition, generic numeric models, which were applicable to rotor wake fields, were developed and examined. The results of the present study indicate that presence of the ground beneath the rotor as a solid boundary for oncoming induced velocity significantly alters the flow field both qualitatively and quantitatively. Particularly, the presence of the ground affects the flow regime near the blade tip by changing the vortex core size and its path. After impingement of the wake to the ground, highly unsteady and turbulent wake is seen. As expected, the ground effect reduces the induced velocity overall. Both the mean and the fluctuation components of the induced velocity increase toward the blade tip. In line with this, the spectral power of the dominant frequency in the velocity fluctuations significantly increases toward the blade tip. All these observations are witnessed in all ground effect conditions.

Subject Keywords

Rotors (Helicopters)., Ground-cushion phenomenon., Unsteady flow (Aerodynamics).

URI

http://etd.lib.metu.edu.tr/upload/12620347/index.pdf
https://hdl.handle.net/11511/25890

Collections

Graduate School of Natural and Applied Sciences, Thesis