Interaction Effects on the Drag of Bluff Bodies in Tandem

  • A. Roshko
  • K. Koenig


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 primary 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 forebody system is a minimum. In the most optimum configuration, d1/d2 = 0.75, g*/d2 = 0.375, and the corresponding forebody drag coefficient is 0.02, a remarkable reduction from the value of 0.75 for the cylinder alone. For each value of d1/d2, the minimum drag configuration, g*/d2, appears to correspond to a minimum dissipation condition in which the separation stream surface just matches (joins tangentially onto) the rearbody. Support for this idea is furnished by comparison with some 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 CDmin 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.

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


Shear Layer Drag Coefficient Ring Vortex Bluff Body Cavity Flow 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


A1, A2

frontal area of frontbody and rear body, respectively


drag coefficient of forebody system based on A2 and freestream dynamic pressure


minimum drag coefficient for fixed Al/A2


drag coefficient of frontbody based on Al


drag coefficient of frontbody face based on Al


drag coefficient of rearbody face based on A2


local rearbody face pressure coefficient


constant pressure surface or free-streamline pressure coefficient


average cavity pressure coefficient at optimum gap

dl, d2

diameter of frontbody and rearbody, respectively


frontbody to rearbody diameter ratio at critical g*/D2


gap between frontbody and face of rearbody


optimum gap for a given d1/d2


optimum gap ratio of critical geometry


freestream dynamic pressure


radius of corner on rearbody face


radial position of the separation surface


Reynolds number based on q and d2


shear stress on separation surface


flow velocity outside separation surface


freestream velocity


coordinate parallel to freestream velocity


radial location on rearbody face


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Copyright information

© Plenum Press, New York 1978

Authors and Affiliations

  • A. Roshko
    • 1
  • K. Koenig
    • 1
  1. 1.California Institute of TechnologyPasadenaUSA

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