Abstract
A combined experimental and numerical approach to the analysis of the secondary stability of realistic swept-wing boundary layers is presented. Global linear stability theory is applied to experimentally measured base flows. These base flows are three-dimensional laminar boundary layers subject to spanwise distortion due to the presence of primary stationary crossflow vortices. A full three-dimensional description of these flows is accessed through the use of tomographic particle image velocimetry (PIV). The stability analysis solves for the secondary high-frequency modes of type I and type II, ultimately responsible for turbulent breakdown. Several pertinent parameters arising from the application of the proposed methodology are investigated, including the mean flow ensemble size and the measurement domain extent. Extensive use is made of the decomposition of the eigensolutions into the terms of the Reynolds–Orr equation, allowing insight into the production and/or destruction of perturbations from various base flow features. Stability results demonstrate satisfactory convergence with respect to the mean flow ensemble size and are independent of the handling of the exterior of the measurement domain. The Reynolds–Orr analysis reveals a close relationship between the type I and type II instability modes with spanwise and wall-normal gradients of the base flow, respectively. The structural role of the in-plane velocity components in the perturbation growth, topology and sensitivity is identified. Using the developed framework, further insight is gained into the linear growth mechanisms and later stages of transition via the primary and secondary crossflow instabilities. Furthermore, the proposed methodology enables the extension and enhancement of the experimental measurement data to the pertinent instability eigenmodes. The present work is the first demonstration of the use of a measured base flow for stability analysis applied to the swept-wing boundary layer, directly avoiding the modelling of the primary vortices receptivity processes.
Highlights
Swept-wing crossflow-dominated boundary layers subject to low free-stream turbulence are well known to develop stationary streamwise-oriented crossflow vorticesK
Despite the small amplitude of the primary vortices, they result in a mean flow distortion giving rise to high-frequency secondary instabilities, which breakdown to turbulence, see Reed, Saric & Arnal (1996), Saric, Reed & White (2003), White & Saric (2005) and Bonfigli & Kloker (2007)
The main effect of the primary stationary crossflow vortices on the laminar flow is to redistribute momentum across the boundary layer by advection about their vortical axes
Summary
Swept-wing crossflow-dominated boundary layers subject to low free-stream turbulence are well known to develop stationary streamwise-oriented crossflow vortices. Despite the small amplitude of the primary vortices, they result in a mean flow distortion giving rise to high-frequency secondary instabilities, which breakdown to turbulence, see Reed, Saric & Arnal (1996), Saric, Reed & White (2003), White & Saric (2005) and Bonfigli & Kloker (2007). The main effect of the primary stationary crossflow vortices on the laminar flow is to redistribute momentum across the boundary layer by advection about their vortical axes. Downwash and upwash events on the respective sides of the crossflow vortex result in a spanwise modulation of the, otherwise spanwise homogeneous, boundary layer in the form of a series of highand low-speed regions. The resulting instantaneous and mean flow present elevated shear stress components in two spatial directions
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