Some of the spatial plots that were obtained are shown below. Figure 2: Diagrams showing the Mean vortex (a and b) of the azimuthal velocity profiles at different positions along the tunnel length.

Phillips and Graham (1984) carried out a study on how to measure Reynolds-stress in a turbulent trailing vortex. The work described into detail how the measurement of turbulent trailing vortex in a condition of zero pressure is carried out. The measurements included the mean velocities as well as all components of the all important Reynolds-stress tensor. These measurements were made via linearized hot wires at different chordlengths (45,78 as well as 109) downstream of the aircraft wing.Axisymmetric wakes and jets were then added in a coaxial fashion to the vortex while holding the total calculation costant.Their effects were then studied. The study reveled that just as indicated in the work of Mason & Marchman and Poppleton, the act of increasing the force of the flow effectively increase the rate of radial dispersion of vorticity. This is noted to be concurrent and consistent with higher levels Reynolds shear stress and turbulence intensities. This the flow of excess axial momentum being effectively zero, a condition which approximated a turbulent line vortex, no considerable change was observed in regard to the velocity field while very little change was seen in the turbulent field. The authors also presented and discussed a balance of terms in regard to the mean-momentum equations. This work indicated that despite the fact that the amount of radial velocity is smaller (numerically) than the axial velocity, the terms that contains it like axial as well as tangential-momentum equations can never be ignored unless the magnitude of the force of flow is much smaller than the level of total circulation squared.

Zhang, Lee and Ligrani (2004) also presented a study that discussed the effects of surface roughness as well as free-stream turbulence on the structure of wake turbulence of a symmetric airfoil. This work studied the effects of...

The specific effects of these parameters on the levels of wake distribution of mean velocity, turbulence length, trubulence intensity as well as power spectral density profiles were studied. Also studied was the effects of the parameters on the vortex shedding frequencies. These were however quantified into one chord length on the foils downstream.
Katz and Bueno (1989) on the other hand discussed the effect of roughness of the rollup of tip vortices on a rectangular piece of aerofoil. Their experiment focused on the development of a tip vortex on an NACA-66 foil (rectangular in shape).The experiment was performed inside a towing tank. The study consisted of a flow visualization process by means of laser sheet illumination as well as by the distribution of fluorescing dye inside the water. The experiment also included the measurement of surface pressure in areas around the tip. The experiment was conducted on a smooth surface as well as on a rough one that was created by the application of a layer of roughness elements through painting of particles of between 250-500 microns. The observation effectively demonstrated that the physical dimensions of a given tip vortex are significantly affected by the nature and level of surface roughness. The pressure (under the tip vortex) is however noted to undergo or exhibit significant changes. This observation led to the conclusion that a general increase in the level of surface roughness significantly reduces the strength of a given tip vortex. The use of flow visualization models and experiments also indicates that for all sizes of roughness, the dimensions of tip vortex increased with the vortex's incidence angle while a decrease was observed a with an increase in velocity. Additionally, the study also indicated that the flow is largely unsteady and the region of the tip is dominated by several secondary vortex structures. The…