The structure and development of a counter-rotating wing-tip vortex pair

Abstract

Experiments have been performed to examine the turbulence structure and development of a pair of counter-rotating wing-tip vortices. The vortices were generated by two rectangular NACA 0012 half wings placed tip to tip, separated by 0.25 chordlengths. Preliminary studies showed the vortices to be insensitive to the introduction of a probe and subject only to small wandering motions. Meaningful measurements could therefore be made using hot-wire probes. Three-component velocity measurements were made 10 and 30 chordlengths downstream of the wing leading edges for a chord Reynolds number of 260000.At 10 chordlengths the vortex cores are laminar. True turbulence levels within them are low and vary little with radius. The turbulence that surrounds the cores is formed by the roll-up of and interaction of the wing wakes that spiral around them. This turbulence is stretched and organized but apparently not produced by the circulating mean velocity fields of the vortices.At 30 chordlengths the vortex cores have become turbulent. True turbulence levels within them are larger and increase rapidly with radius. The turbulent region surrounding the cores has doubled in size and turbulence levels have not diminished, apparently being sustained by outward diffusion from the core regions. The distribution of the turbulence has also changed, the wake spirals having been replaced by a much more core-centred turbulence field.This change in flow structure contrasts sharply with what is seen in the equivalent isolated tip vortex, produced when one of the wings is removed. Here the vortex core remains laminar and the turbulence surrounding it decays rapidly with downstream distance. This implies that the transition to turbulence in the cores of the vortex pair is stimulated by interaction between the vortices. Spectral measurements at 10 chordlengths suggest that short-wave instability may be the cause.

Experiments on the elliptic instability in vortex pairs with axial core flow

“…Leweke & Williamson (2011) have shown a first detailed comparison with measurements made on vortex pairs without axial flow, showing good agreement. Quantitative results from other experimental studies (Sarpkaya 1983;Thomas & Auerbach 1994;Devenport, Zsoldos & Vogel 1997), as well as observations from full-scale flight tests (Scorer & Davenport 1970), mostly show coarse or only qualitative agreement with theoretical predictions. In this paper, we compare Crow's inviscid theory with experimental results, concerning the wavelength and the growth rate of the long-wave instability in a pair of counter-rotating vortices with axial flow.…”

“…The set-up is illustrated in figure 2(a). A similar set-up was used by Devenport et al (1997Devenport et al ( , 1999.…”

“…Similar observations were reported by Chen, Jacob & Savaş (1999) and Ortega, Bristol & Savaş (2003), who studied a four-vortex system generated by a flapped wing in a towing tank. The only known quantitative data were presented by Devenport et al (1997) and Devenport, Vogel & Zsoldos (1999). They investigated counter-and co-rotating vortices generated by two symmetrical wings in a wind tunnel.…”

Effect of a splitter plate on the dynamics of a vortex pair

“…The experimental setup consists of two NACA0012 wings mounted vertically in the middle of the test section. This style of setup has been used by several authors before; for example, Devenport et al 17 and Heyes et al…”

Mean flow and turbulence measurements in the near field of a fighter aircraft wing-tip vortex at high Reynolds number and transonic flow conditions

“…Although many recent studies have been devoted to this topic, most have used generic rectangular wings with symmetrical profiles (eg. Devenport et al 1,2 , Chow et al 3 ), and relatively few have been performed at high enough Reynolds numbers to reflect realistic flight conditions.…”