Characterization of junction flow under the influence of passive flow control devices

An experimental study with large-scale tomographic PTV technique

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Abstract

Junction flow denotes the fluid phenomenon where the flow on a flat surface encounters a protuberance. This type of flow, which is highly three-dimensional, turbulent and unsteady, usually leads to the generation of a Horseshoe vortex at the obstacle's leading edge. Existing extensive experimental and numerical studies focus on the inception of such vortex, as it exhibits a bi-stable behaviour which persists downstream in the vortex legs. In a practical situation such as wing-fuselage juncture, the created vortex interacts with the flow around the aircraft. Consequently, the interference drag increases and the wake can reduce the effectiveness of the stabilizers. To tackle these effects, control devices are implemented to alleviate the strength of the vortex, being leading edge fairing the most widely used device.

In this research project the junction flow is subjected to the influence of different control devices: leading edge fairing, vortex generators and the novel antifairing are tested. The flow field is captured with the state-of-the-art large-scale tomographic PTV technique. The aim of this project is to characterize the vortical structures associated with each passive control device.

The time-averaged flow field results show the fairing is the most efficient device in reducing the presence of the Horseshoe vortex, but it is also the one which contaminates a larger wake area with the vortex wake. The vortex generators are the worst performer both in the mitigation of the vortex and the turbulence level in the wake. Nevertheless, the performance of such control devices highly depends on the location of the vortex generators. Thus, it is speculated that an optimal position exists which can yield better results. The antifairing shows minimal differences in the velocity field and the vortical structures compared with the reference case, but with a slightly lower turbulence level.

An auxiliary experiment with stereoscopic PIV is performed to the wake of the junction, in which the momentum deficit, a good indicator of the drag, is calculated for the different configurations. The leading edge fairing shows minimal drag reduction associated with the HSV of just about 1-2%. The VG once again have proven not being very effective with an increment of 4%. A reduction of more than 15% is measured for the antifairing case. Although the associated flow topology assimilates to the reference case.

This study investigates the HSV system around the junction as a volume. In comparison with most of the previous experimental studies on the subject, performed mostly with planar PIV and limited to just a few planes, the volumetric measurement can deliver more information with a single measurement. Therefore, it can better define the flow topology and vortical features and facilitate the understanding of the control devices working mechanisms.

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