Experiments on secondary instability of streamwise vortices in a swept-wing boundary layer

Abstract

A detailed experimental study on the formation of crossflow vortex mode packets and their high-frequency secondary instability in a swept-wing boundary layer was carried out. Stationary vortex packets are most likely to be generated under natural flight conditions and transition to turbulence is quickest within these disturbances. In the present experiments, different methods of controlled excitation are used so that the crossflow vortex packets are generated by surface-roughness elements and by localized continuous suction. It is found that as the stationary disturbance reaches a significant amplitude, of about 10 % of the free-stream velocity, while being below the saturation level, high-frequency secondary instabilities start to grow. Influence of the crossflow vortex packet magnitude on the development of the secondary instability is investigated in detail and below its threshold the crossflow vortex packet was found to be nearly neutrally stable. By studying the unstable packets, the frequency of natural secondary perturbations was identified and the travelling disturbances were forced in a controlled manner by periodic blowingsuction applied locally under the stationary vortex. Two modes of secondary instability were found to develop and the preferred mode was dependent on the properties of the primary stationary disturbance. Additionally, the underlying physics of the process of nonlinear formation and development of the vortices in the boundary layer is clarified. It was observed that the large-amplitude co-rotating vortices may interact, thus reducing their amplitude. Also a large-scale excitation by an isolated roughness element produced two individual stationary crossflow vortex packets at its tips, each with different preferred secondary instability modes.

Three-dimensional organisation of primary and secondary crossflow instability

“…Deyhle & Bippes (1996); Högberg & Henningson (1998);Malik et al (1999); Wassermann & Kloker (2002); White & Saric (2005)). The general topological features revealed by modes Φ 9 and Φ 10 are similar to those presented in cross-cut planes, by Malik et al (1999); Högberg & Henningson (1998); Kawakami et al (1999) and Janke & Balakumar (2000) and to those described, in their three-dimensional organisation, by Wassermann & Kloker (2002; Bonfigli & Kloker (2007) and by Chernoray et al (2005).…”

“…These results offer the first three-dimensional experimental measurement of these structures under natural flow conditions offering an important comparison to the outcomes of e.g. Högberg & Henningson (1998); Malik et al (1999); Koch (2002); Wassermann & Kloker (2002 and Bonfigli & Kloker (2007) regarding the spatial organisation and the convecting nature of these modes, although the experiments performed by Kawakami et al (1999) and Chernoray et al (2005) forcing the unsteady modes gave already illuminating outcomes in this regard.…”

“…The measured mean out-ofplane velocity at this location isŪ W ≈ 0.65Ū W e . This can be directly associated with the phase velocity of the convective structure (u 6−7 c ) (Chernoray et al 2005;Bonfigli & Kloker 2007). Kawakami et al (1999) report a value of 0.47 times the local external velocity for the convecting velocity of these structures.…”

“…Similarly to Kawakami et al (1999), Chernoray et al (2005) performed phase locked hot-wire measurements on the secondary instability of streamwise vortices in a swept wing boundary layer. These vortices were not generated by crossflow instability mechanisms, but rather directly caused by large roughness elements or localised continuous suction and were subsequently subject to the crossflow.…”

Large Eddy Simulation of Crossflow Vortices on an Infinite Swept Wing

“…Chernoray et al 1 compares measured velocity profiles at 0.3x/c and results approximate to a FSC similarity solution giving Re θ = 175 at 0.3x/c. Aft of 0.5 chord a self-similar FSC boundary layer is formed.…”