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The turbulent impinging jet flows associated with vertical or short take-off and landing (VTOL) aircraft hovering in ground effect can have a critical effect on aircraft performance, and they are modeled very poorly by existing models. Three flow phenomena representative of VTOL ground effects flows, the upwash fountain, the ground vortex, and the impingement zone of a round jet, are considered. Extensions to the k-ϵ model are presented which are designed to account for streamline curvature, large scale mixing, and anisotropy. The extensions significantly improve the model's ability to predict some aspects of these flows. Requirements for further model development are identified.

Enhanced turbulence in an upwash fountain and fluid/acoustic resonance of an impinging axisymmetric jet are investigated by numerical simulations of the mean flow and the largest scales of the unsteady fluid motion. In the planar upwash, the simulated shear stress and spreading rate are three times greater than in a normal jet and are in good agreement with experimental data. Reynolds-stress transport mechanisms which lead to the enhanced turbulence are discussed, and a qualitative description of the large scale turbulent motions is proposed. A model for the pressure-strain term is determined to be a major source of error in Reynolds-stress transport modeling of the upwash. In an axisymmetric impinging jet at Mj = 0.9, resonant-like behavior with elevated levels of pressure fluctuations and dominance of a single frequency of vortex generation are observed. Vortex stretching is observed to be critical to the generation of noise in the impingement zone.