Literature Review on "Stability of Wing Tip Vortex Over Rough"

Literature Review 8 pages (2189 words) Sources: 12

[EXCERPT] . . . .

Extant literature has been dedicated to the study of the stability of wing tip vortex over rough wing.The work of Beninati and Marshall (2005) for instance was an experiment which was dedicated to the study of the effects of the phenomenon of free-stream turbulence on a trailing vortex. In this study the duo investigated the evolution of turbulence as well as the spectra that lie within as well as outside of the core of the trailing performed. The vortex was generated by a special vortex generator that consisted of four blades that were orthogonally positioned to each other with similar angles of attacks. The experiment was performed inside a lowspeed wing tunnel. A grid of appropriate size was then placed upstream of the generator in order to produce the turbulence that effectively wrapped around as well as interacted with the formed columnar vortex. The experiment involved the instantaneous measurement of 3 velocity components by means of a miniaturized 4-wire sensor hot-wire probe. The work of Beninati and Marshall (2005) therefore focused on the distribution of the turbulence energy as well as Reynolds stress. Other spectral components of this particular flow at various positions across the core of the vertex and across the channel were also measured. The effect of the background grid turbulence on the vortex's spectral energy distribution was also examined and compared to the vortex alone.

Beninati and Marshall (2005) noted that the evolution of turbulence near a rather large scale vortex structure is a common problem that affects most high cases of Reynolds number fluid flow. Irrespective of whether the presence of the vertex structures location in mean flows like aircraft trailing vortices, jet engine intake vortices, delta-wing nose vertices or vortices that are highly coherent in structures in a given turbulent flow, the problem of how larges scale vortices effectively interact with the smaller scale vortex structures is usually a key to comprehending the breakup of the vortices as well as the regime changes of a particular flow.

Bradshaw (1969) noted that small-scale turbulence within the core of a vortex tends to get dampened by the rotational nature of fluid motion. On the other hand, the turbulence that ocures immediately outside of the core's lateral surface usually tend to end up forming very strong coherent structures. The external coherent structures therefore end up buffeting the core of the vertex and in the long run leads to the subsequent breakup of the vertex as noted by Belanin et al. (1978) and Liu (1992).

Despite the obvious significance of this problem, not much literature has been dedicated towards the nature of vortex interaction with the external turbulence (Marshall and Beninati 2000). Various flow virtualization studies that were carried out by Sarpkaya and Daly (1987) and Liu (1992) on the concept of vortex pair interactions having background turbulence indicated intermittent "bursts" at the vortex with the turbulence structures that are responsible for these burst not being clear.The works of Uberoi (1976) and Phillips and Graham (1984) reported wind tunnel experiments that had no background turbulence while examining the effect of axial vortex flow on the development of turbulence.

Bandyopadhyay et al. (1991) on the other hand, while examining the effect of various screens as well as perforated plates on the development of a vortex in a wind tunnel study that involves a vortex which was immersed in an internal turbulence. The author employed a combination of pitot-probe velocimetry and hot-wires anemometry. These were in addition to visualization of the flow by means of smoke injection into the vicinity of the trailing vortex core that was generated by means of a pair of aerofoil that was joined via the central hub. The author then made turbulence measurements at a single position (axial) by means of miniaturized x-wire probes for the characterization of the external turbulence. However, a 7-hole probe was used in the measurement of the mean velocity field. This was done by traversing in a vertical fashion through the vortex core. Bandyopadhyay et al. (1991) further described a process in which a set of organized vortex structures that surrounds the vortex core effectively caused an intermittent ejection of high-vorticity, low-momentum core fluid to flow outwards into the area surrounding the vortex core as well as the entrainment of external fluid of high-turbulence into the vortex core. After which the fluid in the core gets relaminarized by the rotation of the core. The exchange of momentum that takes place between the vortex core and the outer more turbulent region is created by an organized set of motions.

The vorticity exchange process that takes place between the vortex core and the turbulence at the exterior section has been effectively confirmed and even improved in computational studies. A study by Melander and Hussain (1993) investigated the phenomenon using direct numerical computations (DNS) of a homogenous turbulence that initially surrounded a columnar vortex. The turbulence was observed by the author to appear to wrap around the formed columnar vortex and thereby forming loops that were almost axisymmetric around the core of the vortex. In this near-axisymmetrical state, the small-scale vortices from the homogenous turbulence at the exterior were noted to merge with one another in a process that led to the formation of turbulence structures having length scales ranging from the size of the vortex core diameter to about 1/5 th of the core diameter. Melander and Hussain (1993) effectively referred to these structures as the "polarized" structures of the vortex.

A number of investigators have also reported extensions of the abovementioned computations with Miyazaki and Hunt (2000) reporting the application of the rapid-distortion approximation method in the estimation of the wave motions that are induced on the core of the vortex by the external turbulence having a weak force. Marshall (1997) on the other had examined a simple mode; of the axisymmetric external vortex structures as they interacted with column vortex. Marshall (1997) reported the formation of special forms of standing waves on the core of the vortex for the weak external vortex structures as well as the stripping of elements of vorticity from the columnar vortex for the strong external vortex structures. Ragab and Sreedhar (1995) reported a study that employed both the large eddy-simulations as well as directnumerical simulations on a largely unstable q vortex. In their study, they reported the existence of turbulence within the vortex core. This turbulence was reported to grow as the level of instability increased.

This happened as the corresponding increase in the magnitude of the positive and the negative azimuthal vorticity. This process was then followed by the subsequent decay of the vortex under the influence of the dampening effects of vortex core rotation. The work of Beninati

(2004) examined the effect of external turbulence of the wavelengths and strengths on a columnar vortex having and not having axial vortex flow.In this study, Beninati

(2004) employed a multi-step initiation process/sequence in order to obtain the initial conditions that represented the entrained external turbulence surrounding the vortex core with varying level of strengths in the part of the external turbulence. The work of Marshall and Beninati

(2003) employed DNS cxomputation of the external turbulence of a columnar vortex in the examination of the various methods for a priori testing of various subgrid-scale closures employed for a large-eddy simulation process. Risso et al. (1997) reported a study that entailed a turbulence that surrounded a vortex pair while Devenport et al. (1997), reported a similar process. This illustrated the evolution of turbulence as a consequence of the stretching of the external turbulence structures that lies between the two vortices of the given pair. The work of Devenport et al. (1996) which involved an experimental study of the tip of a vortex effectively provided useful data from a single wing. This experimental study indicated that even the small-scale -- amplitude vortex wandering can have a major effect on the Reynolds stress data for the case of turbulent flows. The authors are noted to argue that even the vortex wondering alone can in most cases be responsible for a large portion of Reynolds stresses that are measured in the vortex flows that are located downstream of a given wing. It is necessary to note that in the vortex turbulence studies, there is a need to design a vortex generate properly so as to avoid cases of vortex wandering and also top be able to remove most or all of the effects of vortex wandering. This is accomplished by the examination and filtering of the vortex turbulence's spectrum.

The work of Beninati and Marshall (2005) was an experimental study of a trailing vortex which was immersed in a turbulence field which was external grid-generated.The study mainly focused on the distribution of the wave energy and turbulent energy within a spectral space so as to determine the influence of the various length-scale oscillations on the Reynolds shree stress and turbulent kinetic energy. The experimental setup was a shown below.

Using a combination of high level spectral filtering, an examination of the… READ MORE

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