Viscous Drag Measurement and Its Application to Base Drag Reduction

In this study two important topics have been investigated. First, a new automation procedure has been developed to detect fringe patterns in oil- film interferograms. The procedure combines a windowed Fourier transform and a cross correlation technique to determine regions where fringes are likely to reside. The cross correlation of a single cycle cosine function and the original intensity produce a filtered result that isolates the information being sought. The automation procedure is able to capture approximately 75% of the fringes in an image but additional refining of the technique could increase this percentage. Second, the effect of viscous fore-body drag on base drag has been investigated on a 2-D ramp model. Measurements made during the study include oil-film interferometry and hot-wire anemometry for viscous fore-body drag, and pressure measurements on the fore-body and base for fore-body and base drag. The results of this investigation show that an increase in viscous fore-body drag has very little effect on base drag, which contrasts previous studies where increases in viscous fore-body drag tended to decrease base drag. This means that a simple jet pumping mechanism is not the sole effect responsible for base drag reduction in this type of geometry. Hot-wire measurements in the wake of the model show that the flow in the base region is devoid of a dominant shedding frequency. It may be possible that the boundary layer somehow affects the vortex shedding mechanism but further investigations would be necessary to demonstrate that this is true.Air Force Inst of Tech Wright-Patterson AFB OH is the author of 'Viscous Drag Measurement and Its Application to Base Drag Reduction', published 2002 under ISBN 9781423510048 and ISBN 1423510046.