Wing-tip vortices are of engineering importance because they dominate the wakes of airplanes and submarines. They are both hazardous and difficult to predict. This page and its links and references describe an experimental research program in the Aerospace and Ocean Engineering Department at Virginia Tech aimed at better understanding these flows. This work has been supported by the Officeof Naval Research (grants N00014-92-J4087 and N00014-94-1-0744) and NASA Langley (grant NAG-1-1119).
Experiments were performed primarily in the Stability Wind Tunnel on the tip vortex shed from a rectangular NACA 0012 half wing (figure 1). Making such measurements is not easy since tip vortices tend to wander (waft from side to side) and be unusually sensitive to the introduction of a probe. Much effort was therefore spent analyzing and quantifying the wandering (which was found to be small enough for corrections to be derived and applied) and investigating probe interference (which was found to be absent). Detailed mean-velocity and turbulence measurements were therefore possible using hot-wire anemometry, specifically miniature four-sensor hot-wire probes (figure 2).
These measurements show the flow structure outside the core to be dominated by the remainder of the wing wake which winds into an ever increasing spiral(figure 3). There is no large region of axisymmetric turbulence surrounding the core and little sign of turbulence generated by the rotational motion of the vortex. Turbulence stress levels vary along the wake spiral in response to the varying rates of strain imposed by the vortex. Despite this complexity, the shape of the wake spiral and its turbulent structure reach an approximately self-similar form (figure4).
Moving from the spiral wake to the core the overall level of velocity fluctuations greatly increases, but none of this increase is directly produced by turbulence (figure 5). Velocity spectra measured at the vortex center scale in a manner that implies that the core is laminar and that velocity fluctuations here are a consequence of inactive motion produced as the core is buffeted by turbulence in the surrounding spiral wake (figure 6). Mean velocity profiles through the core show evidence of a two-layered structure that dies away with distance downstream.
Detailed description of the results and their implications is given in the paper "The Structure and Development of a Wing Tip Vortex", William J.Devenport, Michael C. Rife, Stergios I. Liapis and Gordon J. Follin, Journal of Fluid Mechanics, vol. 312, pp. 67-106, 1996. Other publications on this and related work are listed under papers. Click here to access numerical data from these experiments.
