Reduction in size and unsteadiness of VTOL ground vortices by ground fences

A ground vortex, produced when a jet impinges on the ground in the presence of crossflow, is encountered by V/STOL aircraft hovering near the ground, and is known to be hazardous to the aircraft. The objective of this research was to identify a ground-based technique by which both the mean size and fluctuation in size of the ground vortex could be reduced. A simple passive method has been identified and examined in the laboratory. Specifically, one or two fine wire mesh screens (ground fences), bent in a horseshoe shape, located on the ground in front of the jet impingement point, proved to be very effective. The ground fences work by decreasing the momentum of the upstream-traveling wall jet, causing an effectively higher freestream-to-jet velocity ratio V_x/V_j, and therefore, a ground vortex smaller in size and unsteadiness. At V_x/V_j = 0.15, the addition of a single ground fence resulted in a 70% reduction in mean size of the ground vortex. With two ground fences, the mean size decreased by about 85%. Fluctuations in size decreased nearly in proportion to the mean size, for both the single and double fence configurations. These results were consistent over a wide range of jet Reynolds number (10⁴ < Re_jct < 10⁵); further development and full-scale Reynolds number testing are required, however, to determine if this technique can be made practical for the case of actual VTOL aircraft.