Interference Effects on the Drag of Bluff Bodies in Tandem

Author: Koenig, Keith

Year: 1978

Degree: Dissertation (Ph.D.)

Advisor: Roshko, Anatol

Committee Member: Unknown, Unknown

Option: Aeronautics

DOI: 10.7907/EXEV-R625

Abstract

The objective of this study is to obtain better understanding of the flow over two tandemly positioned bluff bodies in close enough proximity to strongly interact with each other. This interaction is often beneficial in that the drag of the overall system is reduced. Prototypes for this problem come from tractor-trailer and cab-van combinations and from various add-on devices designed to reduce their drag.

The object of the present investigation is an axisymmetric configuration which seems to have first been studied by Saunders (1966). A disc of diameter d1 is coaxially placed in front of a flat faced cylinder of diameter d2. For a given ratio d1/d2, there is a value of gap ratio, g*/d2 for which the drag of the system is a minimum. In the most optimum configuration, d1/d2 = 0.75, g*/d2 = 0.375, and the corresponding drag coefficient is 0.01, a remarkable reduction from the value of 0.72 for the cylinder alone. For each value of d1/d2, the minimum drag configuration g*/d2 appears to correspond to a condition in which the separation streamsurface just matches (joins tangentially onto) the rearbody. Support for this idea is furnished by comparison with results derived from free-streamline theory and from flow visualization experiments. However, when g*/d2 exceeds a critical value of about 0.5, the value of CD, while still optimum, is almost an order of magnitude higher than for subcritical optimum gap ratios. The increase seems to be connected with the onset of cavity oscillations.

Measurements of the velocity field in the vicinity of the forebody have been made using a frequency-shifted laser-Doppler velocimeter. These measurements indicate an order of magnitude difference in the shear stress along the separation surface between optimum subcritical and supercritical geometries.

The drag characteristics of the axisymmetric forebody system are altered by modifying the shape of the component bodies. Modifications that change the conditions at separation from the frontbody, interfere with the cavity flow or effect the flow on the rearbody face can produce significant changes in the forebody drag.

For non-axisymmetric geometry (square cross-sections) the separation surface cannot exactly match the rear body and the subcritical minimum values of drag are higher than for circular cross-sections.

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