Technical report | Flow Visualisation Around a Hemispherical Protuberance in the DST Group Water Tunnel

Abstract

The flow structures around a hemispherical protuberance were studied using dye-injection flow visualisation in the DST Group Water Tunnel. A 100 mm diameter hemisphere was placed in a laminar oncoming boundary layer, and video imaging of the dye streamlines were captured at diameter-based Reynolds numbers of 6:91 x 10^3 and 1:15 x 10^4. The observations generally agree well with previous studies, while analysis of the video footage has provided additional insight into the evolution of the unsteady flow field.

Executive Summary

Aircraft are often fitted with external sensors housed in hemispherical shrouds, which alter the flow-field characteristics in their proximity. The impact on the local flow field could negatively affect the flight envelope and performance characteristics of the aircraft, yet there are relatively few qualitative experimental studies with clear, documented imagery of the flow structures around hemispherical protuberances. This is primarily due to the fact that most previous flow visualisation is quite dated and was subject to imaging and hardware limitations, thus sharp images were difficult to reproduce. This work is therefore done to provide more detailed imagery and updated observations on these hemisphere induced flow fields.

This study was conducted in the Defence Science and Technology (DST) Group Water Tunnel and involved dye injection around a 100 mm diameter hemispherical protuberance mounted on a flat plate ground plane, at diameter based Reynolds numbers of 6:91 x 10^3 and 1:15 x 10^4. An oncoming laminar boundary layer was allowed to naturally develop, partially immersing the hemisphere. Video imagery were obtained from two orthogonal perspectives that were spatially and temporally aligned. This allowed for clearer interpretation of the three-dimensional flow structures from the two orthogonal views.

Interpretation of the video footage has revealed additional detail regarding previously described phenomena in literature. This includes the formation and evolution of 'hairpin' vortices from the hemisphere lee-side as well as the periodic oscillation of the 'horseshoe' vortex system upstream of the hemisphere. Large-scale oscillations in the wake were also observed to occur for both Reynolds numbers, which indicated a possible mix of von Kármán vortex shedding and shear layer roll up over the hemisphere. The formation Strouhal number of the highlighted flow structures were estimated using the video footage; it was found that as Reynolds number increased, the Strouhal number of shear layer roll up increased, while the large-scale wake oscillations showed a small reduction in Strouhal number.