Secondary instability of crossflow vortices

Abstract

Crossflow-dominated swept-wing boundary layers are known to undergo a highly nonlinear transition process. In low-disturbance environments, the primary instability of these flows consists mainly of stationary streamwise vortices that modify the mean velocity field and hence the stability characteristics of the boundary layer. The result is amplitude saturation of the dominant stationary mode and strong spanwise modulation of the unsteady modes. Breakdown is not caused by the primary instability but instead by a high-frequency secondary instability of the shear layers of the distorted mean flow. The secondary instability has been observed in several previous experiments and several computational models for its behaviour exist. None of the experiments has been sufficiently detailed to allow either model validation or transition correlation. The present experiment conducted using a 45 • swept wing in the low-disturbance Arizona State University Unsteady Wind Tunnel addresses the secondary instability in a detailed fashion under a variety of conditions. The results reveal that this instability is active in the breakdown of all cases investigated, and furthermore, it appears to be well-described by the computational models.

Roughness Based Crossflow Transition Control: A Computational Assessment

“…As shown in Figure 7, the presence of the control mode apparently affects the Z-mode of secondary instability much more than the Y-mode. This indicates that, if indeed transition is caused by the growth of the Z-mode as some experiments seem to suggest [17,18], the DRE is a very effective means to achieve transition delay in the present case.…”

“…Of course, the receptivity characteristics of each of these modes will determine whether or not that mode gets excited with an adequately large initial amplitude in order to have a visible influence on the transition process. For example, previous experimental studies [17,18] indicate a preferential excitation of the z modes under certain conditions. Given this scenario, it seems prudent to track the N-factor envelopes for each mode family separately rather than a single envelope for all secondary instability modes.…”

“…As discussed before, experimental evidence suggests that the crossflow vortex breakdown tends to occur via the Z-mode of secondary instability [17,18]. Thus, a parametric study of the transition onset location as a function of the initial amplitude of secondary instability is carried out.…”

Three-dimensional organisation of primary and secondary crossflow instability

“…Experimental investigations on three-dimensional boundary layers were mainly carried in Germany (Deutsches Zentrum für Luft-und Raumfahrt (DLR) Göttingen) by Bippes and coworkers (Bippes 1999;Deyhle & Bippes 1996) and in United States by William Saric and his group (Arizona State University (ASU) first and Texas A&M University (TAMU) up to date) (Saric et al 2003;White & Saric 2005). These campaigns made use of very quiet wind tunnels, exhibiting a free stream turbulence level typically lower than 0.1% of the free stream velocity.…”

“…For more complete reviews the reader is instead referred to Bippes (1999); Arnal & Casalis (2000); Saric et al (2003) and to some more recent studies such as Wassermann & Kloker (2002; White & Saric (2005); Bonfigli & Kloker (2007); Downs & White (2013) and Hosseini et al (2013).…”