| Vortices are a common phenomenon in fluid flows that arise as kinetic energy dissipates into heat via viscous interaction. They arise naturally at large scales in the form of dust devils, tornadoes, and as a counter-rotating vortex pair in the wake of aircraft. It is important to understand the conditions leading to their formation, their duration, and their dissipation in order to forecast or prevent undesirable effects. Among these deleterious effects is a decrease in safety of aircraft operations in the wake of other aircraft, an extremely common situation at airports around the world. A large number of mathematical models and experimental data sets exists to help explain various aspects of axial wake vortex behavior, but current models fail to explain why many vortices remain tightly wound with slowly decaying azimuthal velocities about their cores the length of time for which they have been observed. The current study builds upon the theoretical work of Ash, Zardadkhan and Zuckerwar [Ash et al., 2011], and tests specific attributes of a turbulent axial vortex for agreement with non-equilibrium pressure relaxation theory. This theory provides an exact solution to a modified version of the Navier-Stokes equations for an axial vortex, with a resulting velocity model that agrees with leading empirical models. In the present investigation, axial wake vortices were created with a bi-wing vortex generator in a low speed wind tunnel, at free stream velocities between 15 and 33 m/s. Stereo particle image velocimetry was employed to map three dimensional velocity vectors at positions between 5.4 and 10 chord lengths downstream of the vortex generator, and at a sampling rate of 1Hz for 200 seconds. A Reynolds time averaging approach was employed to express instantaneous velocity measurements as localized mean and fluctuating components and to study turbulent structures within the vortices. Periodicity in turbulent energy and Reynolds stress structures was observed by comparing vortex velocity fields normalized by age, based on free stream velocity and downstream distance. The cores of these vortices appeared to periodically ingest turbulent energy and compress it into approximately one half of local core radii. The cyclical ingestion of turbulence was shown to have the effect of tightening the core radius in the wake of the vortex generator center body. If this phenomenon persists for the life of the vortex, it could provide an explanation for the longevity of the azimuthal velocity component, as observed in natural wake vortices. |
