An experimental investigation of wing-tip vortex decay in turbulence

Abstract

Particle image velocimetry measurements were conducted for a wing-tip vortex decaying in freestream turbulence. The vortex exhibited stochastic collapse with free-stream turbulence present, with the breakdown initiating earlier for higher levels of turbulence. An increased rate of decay of the vortex tangential velocity was also observed, increasing with increasing free-stream turbulence. The decay of the vortex tangential velocity without the free-stream turbulence was well represented by viscous diffusion, resulting in an increase in the core radius and decrease in the peak tangential velocity. With the addition of free-stream turbulence, the rate of decay of the peak tangential velocity of the vortex increases, whereas the rate of increase of core radius remains unchanged. The circulation of the vortex decayed in time when immersed in free-stream turbulence, whereas it remained approximately constant when free-stream turbulence was not present. This decay in circulation was found to be almost entirely due to a decrease in circulation of the vortex core, caused by the relative decrease in the peak tangential velocity without a corresponding increase in the core radius. The scaling of the radial profiles of velocity and circulation was also examined, and it was found that, regardless of the free-stream condition, the core was scaled by the peak tangential velocity and core radius. The region outside the core did not scale with these quantities, and an alternative scaling for circulation is proposed which results in improved collapse of the profiles.

Experimental examination of vorticity stripping from a wing-tip vortex in free-stream turbulence

“…Secondary structures in the form of small-scale vortical structures have been observed forming around line vortices immersed in turbulent surroundings in both experiments [2][3][4][5][6] and simulations [7][8][9][10][11]. These secondary structures often form in pairs having alternating signs of azimuthal vorticity [7,[9][10][11] and typically form at a radial distance from the vortex axis of twice the vortex core radius.…”

“…The present work is an extension of the study presented in Ghimire and Bailey [2]. In the previous study, we examined the impact of the turbulence on the mean decay of the vortex, with corresponding observations of nondiffusive decay in the presence of free-stream turbulence, as detailed above.…”

“…1. The towing tank, wing, and grids used in this experiment are the same as those used in Ghimire and Bailey [2] with further details are provided below.…”

“…Instead, the increase in decay rate occurs only through decay of the tangential velocity within the core of the vortex, which outpaces the corresponding rate of increase of core radius [1,2]. The core radius remains unaffected by either the presence of, or the intensity of, external turbulence; its rate of growth is unchanged from that produced in a laminar free-stream through viscous diffusion.…”

“…The core radius remains unaffected by either the presence of, or the intensity of, external turbulence; its rate of growth is unchanged from that produced in a laminar free-stream through viscous diffusion. The increased rate of decay of the vortex, as measured through its circulation, is thus caused by the decrease in tangential velocity in the vortex core, which accounts for almost all the circulation loss when external turbulence is present [2]. As this enhanced decay cannot be attributed to small-scale turbulent diffusion, it must therefore be caused by a more dynamic, three-dimensional process.…”