Unsteady flow phenomena in a highly bend engine intake with passive and active flow control

Authors

Philipp Max  - University of the Bundeswehr Munich, Germany
Marcel Stößel - University of the Bundeswehr Munich, Germany
Dragan Kozulovic - University of the Bundeswehr Munich, Germany
Michael Krummenauer - Bundeswehr Technical Centre for Aircraft and Aeronautical Equipment

Abstract

The design of present-day civil, as well as military aircrafts is more and more driven by fully integrated concepts whereby the propulsion system and its intake form a functional unit with the airframe. In addition, particular military requirements such as stealth applications can be considered in a better way. Since the rotating fan or compressor of the engine increases the vehicle’s radar visibility, highly bent intakes are used to suppress a direct line of sight to the rotating components. Such an intake, however, can cause significant secondary flow systems and is delivering air to the engine with a highly non-uniform total pressure distribution. These inhomogeneities in the compressor inflow cause a reduction in efficiency as well as a decrease in available surge margin. Different unsteady flow phenomena are known to develop in enclosed flow systems, like in pipe elbows, but in curved engine intakes, as well. They have an additional negative impact on the performance and stability of the compressor system since they cause not only local but also temporal inhomogeneity on the total pressure and velocity field. The instabilities tend to roll up the shear layer periodically into vortices, which are transported downstream. Swirl switching refers to the phenomenon that the detachment area of flow never exactly aligns with any symmetry plane of such an intake but fluctuates on either side. As a result, one of the two Dean vortices is intensified and the other weakened in an unsteady manner. The unsteady character of the flow becomes particularly complex when the intake system is highly contoured and the shape of the cross-sectional areas changes along the duct. In order to improve the knowledge on unsteady flow characteristics in highly bent engine intakes, in this study four Kulites were installed on a double s-bent engine intake. The Kulite sensors are located in pairs on symmetrical positions on the longitudinal axis of the duct and on the transversal axis in the area of the main flow separation zone. This allows an analysis of the phase shift of the distorted flow within the duct and thus conclusions can be drawn on particular unsteady flow phenomena present in S-shaped intakes, which can be expected from results published in open literature. Furthermore, analysis of the relevant Strouhal numbers for different rotational speeds allow for conclusions on the most dominant effects, their intensity, as well as their dependence on the rotational speed. Initially, the s-duct was investigated without any flow-stabilizing measures (baseline configuration). Further investigations with vortex generators and an air injection configuration for active flow stabilization were performed afterwards in different setups. The steady-state measurement results show that the flow-stabilizing measures strongly reduce the separation bubble and the flow distortion in the intake. Unsteady pressure measurements indicate accordingly that dominant Strouhal numbers, associated with the Dean vortices, are weaker or not present at all in case of flow control. These unsteady phenomena are further analysed in terms of their respective Strouhal numbers versus the power spectral density and compared to the baseline configuration.