Jongil Han

Department of Marine, Earth and Atmospheric Sciences
North Carolina State University
Raleigh, North Carolina, USA
Tel: (919) 515-1442
E-Mail: han@rossby.meas.ncsu.edu


Education and Professional Career

Dissertation Research



ABSTRACT

This dissertation is composed of two separate papers where atmospheric turbulence effects on the behavior of aircraft wake vortices are studied using a validated large eddy simulation model. Information from this study may contribute to the development of a predictor algorithm for the National Aeronautic and Space Administration's (NASA) Aircraft Vortex Spacing System (AVOSS), which will determine safe operating spacing between arriving and departing aircraft and provide a safe reduction in separation of aircraft compared to the now-existing flight rules.

In the first paper, the development of Crow instability in a homogeneous field of turbulence is studied. The Crow instability becomes well developed in most atmospheric turbulence levels, but in strong turbulence the vortex pair deforms more irregularily due to turbulence advection. The maximum Crow instability wavelength decreases with increasing nondimensional turbulence intensity ($\eta$), while it increases with increasing turbulence integral length scale. The vortex lifespan is controlled primarily by $\eta$ and decreases with increasing $\eta$, while the effect of integral scale of turbulence on vortex lifespan is of minor importance. The lifespan of vortex in our numerical simulations is estimated to be about 40$\%$ larger than the theoretically predicted value. This larger lifespan compared with an analytic theory agrees very well with the data from laboratory experiments and is caused by slower growth in the middle of the vortex lifespan. The maximum deviation in lifespan from the average value due to ambient turbulence alone is about $7 \, \%$ of the mean for small $\eta$ and about $20 \, \%$ of the mean for large $\eta$, showing much less scatter in lifespan compared with atmospheric observations. The larger scatter of lifespans in atmospheric observations appears to result from other factors such as stratification, wind shear, and inhomogeneous ambient turbulence.

In the second paper, the vortex decay and descent in a homogeneous field of turbulence is investigated. The decay rate of the vortex circulation increases clearly with increasing ambient turbulence level but decreases with increasing radial distance, which is consistent with field observations. Simple vortex decay models are proposed as functions of $\eta$ and radial distance. A Gaussian type of vortex decay model can be applied at larger radial distances, while an exponential type of vortex decay model can be applied at smaller radial distances near the core, but the latter can be extended up to a radial distance of about half of the initial vortex separation distance (b0) for strong turbulence. The circulation averaged over radial distances from $2.5\,r_c$ (rc is the initial core radius) to $0.5\,b_0$,which is related to the rolling moment of an encountering aircraft, shows a Gaussian decay for weak and moderate turbulence and an exponential decay for strong turbulence. A model for the vortex descent based on the vortex decay model is also proposed as a function of $\eta$ and dimensionless time. The model predictions for the decay of the average circulation appear to be in reasonable agreement with observation data which have a large scatter, as long as the theoretical circulation is used as the initial value. However, the decay rates of observed circulations are much larger than those obtained from our numerical simulations, impling that besides ambient turbulence there are some additional factors enhancing the vortex decay. In particular, the effect of ambient stratification on the enhancement of the vortex decay appears to be significant.