Global Stability Analysis of Flow Behind an Upswept Aftbody

Wakes of aircraft and automobiles with relatively flat slanted aftbodies are often characterized by a streamwise-oriented vortex pair, whose strength affects drag and other crucial performance parameters. We examine the stability characteristics of the vortex pair emerging over an abstraction comprised of a streamwise-aligned cylinder terminated with an upswept plane. The Reynolds number is fixed at 5000 and the upsweep angle is increased from 20deg to 32deg. At 20deg, the LES yields a steady streamwise-oriented vortex pair, and the global modes are also stable. At 32deg, the LES displays unsteady flow behavior. Linear analysis of the mean flow reveals different unstable modes. The lowest oscillation frequency is an antisymmetric mode, which is attached to the entire slanted base. At the highest frequency, the mode is symmetric and has the same rotational orientation as the mean vortex pair. Its support is prominent in the rear part of the slanted base and spreads relatively rapidly downstream with prominent helical structures. A receptivity analysis of low- and high-frequency modes suggests the latter holds promise to affect the vortical flow, providing a potential starting point for a control strategy to modify the vortex pair.