Numerical Investigation of Laminar Separation Control Using Vortex Generator Jets

Abstract

Direct numerical simulations (DNS) are employed to investigate laminar boundary layer separation and its control by vortex generator jets (VGJs), i.e. by injecting fluid into the flow through an array of small holes. Particular focus is directed towards identifying some of the relevant physical mechanisms associated with VGJ control of low Reynolds number separation, as encountered in low-pressure turbine applications. In a comparison of selected controlled cases, pulsed VGJs are shown to be much more effective than steady VGJs, when the same momentum coefficient is used for the actuation. The formation and the dynamics of steady as well as unsteady flow structures are subsequently investigated in more detail. For steady VGJs, up to a certain "threshold" amplitude, angled injection is shown to be more effective than vertical injection, which is attributed to the fact that the generated longitudinal vortices remain closer to the wall while penetrating deeper into the boundary layer (in the spanwise direction). Beyond this "threshold" amplitude, however, vertical VGJ injection "suddenly" yields fully attached flow along the entire surface. This change in the global flow dynamics is explained by the formation of symmetric horseshoe-type vortices which are shown to augment the entrainment of high-momentum fluid from the free stream. For pulsed VGJs, the increased control effectiveness is attributed to the fact that hydrodynamic instabilities of the underlying flow can be exploited. When pulsing with frequencies to which the separated shear layer is naturally unstable, instability modes are shown to develop into large scale, spanwise coherent structures. These structures provide the necessary entrainment of high-momentum fluid to reattach the flow. In a series of additional simulations, the effects of varying the frequency as well as the duty cycle are investigated. While deviations from the "optimal" pulsing frequency are shown to result in increased separation losses, changes in the duty cycle have only a minor influence on the effectiveness of the control.