Drag-Reducing Techniques for Axi-Symmetric Bluff Bodies

  • W. A. Mair


The numerous experiments that have been made on drag-reducing devices for two-dimensional bluff bodies have been used as a guide to indicate promising lines of investigation for axi-symmetric bodies. For the latter case, experiments on splitter plates, cylindrical extensions, base bleed and ventilated cavities are reviewed. Of these devices, base bleed is the only one that gives any useful reduction of drag. Unfortunately base bleed cannot be effectively applied to road vehicles. The air flow rate available on a typical vehicle from its ventilation system is too small to give any significant effect. If a special air supply giving a larger air flow were to be provided, the intake momentum drag would be more than enough to counteract any drag reduction due to base bleed.

For a blunt-based axi-symmetric body, a boat-tailed afterbody is much more effective in reducing zero-yaw drag than any other device that has been tried. Furthermore, experiments have shown that as the yaw angle of a boat-tailed body is increased from zero, the axial force can decrease slightly up to a yaw angle of about 10 or 15 degrees, although at larger yaw angles it becomes much greater.

The mode of action of a boat-tailed afterbody is explained, and some of the factors leading to a good design are discussed. The possibility of using boundary-layer control in conjunction with a boat-tailed afterbody is considered briefly.


Drag Coefficient Bluff Body Vortex Street Splitter Plate Road Vehicle 



Base Area


Porous area of base (with base bleed)


Width of cruciform splitter plate on a cone (see Fig. 1)


Drag coefficient referred to maximum cross-sectional area


Reduction of drag coefficient


Pressure coefficient


Base-pressure coefficient


Bleed-flow coefficient, ≡ Q/UA


Axial-force coefficient referred to maximum cross-sectional area


Maximum diameter of body of revolution


Base height (two-dimensional) or base diameter (axi-symmetric)


Diameter of a sting-like cylindrical extension


Drag-reduction factor, ≡ ΔCD/0.165


Resistance coefficient at bleed-air outlet

Length of boat-tailed afterbody


Number of air changes per hour, for ventilation


Volume flow rate of base bleed


Maximum radius of boat-tailed afterbody


Local radius of boat-tailed afterbody


Maximum thickness of two-dimensional aerofoil


Stream velocity


Average bleed velocity


Internal volume of vehicle


Distance from base to re-attachment on sting or to mean position of bubble closure


Distance downstream from section A in Fig. 5


Boat-tail angle (Fig. 5.)


Boundary layer thickness


Kinematic viscosity of air


Density of air


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Copyright information

© Plenum Press, New York 1978

Authors and Affiliations

  • W. A. Mair
    • 1
  1. 1.Cambridge UniversityCambridgeEngland

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