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 
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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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  1. Bearman, P. W. (1965) Investigation of the flow behind a two-dimensional model with a blunt trailing edge and fitted with splitter plates. J. Fluid Mech. Vol. 21, pp 241–255.MATHCrossRefGoogle Scholar
  2. Bearman, P. W. (1967) The effect of base bleed on the flow behind a two-dimensional model with a blunt trailing edge. Aero. Quart., Vol. 18, pp 207–224.Google Scholar
  3. Rostock, B. R. (1972) Slender bodies of revolution at incidence. Ph. D. Dissertation, Lhiversity of Cambridge.Google Scholar
  4. Calvert, J. R. (1967) The separated flow behind axially symmetric bodies. Ph. D. Dissertation, University of Cambridge.Google Scholar
  5. Goodyer, M. J. (1966) Some experimental investigations into the drag effects of modifications to the blunt base of a body of revolution. Inst. of Sound and Vibration, University of Southampton, Report No. 150.Google Scholar
  6. Head, M. R. (1960) Entrainment in the turbulent boundary layer. ARC R&M 3152.Google Scholar
  7. Lock, C. N. H. & Johansen, F. C. (1933) Drag and pressure distribution experiments on two pairs of streamline bodies. ARC R&M 1452.Google Scholar
  8. Mair, W. A. (1966) STOL–some possibilities and limitations. J. Roy. Aero. Soc. Vol. 70, pp 825–833.Google Scholar
  9. Mair, W. A. (1969) Reduction of base drag by boat-tailed afterbodies in low speed flow. Aero. Quart. Vol. 20, pp 307–320.Google Scholar
  10. Maull, D. J. & Hoole, B. J. (1967) The effect of boat-tailing on the flow around a two-dimensional blunt-based aerofoil at zero incidence. J. Roy. Aero. Soc. Vol. 71, pp 854–858.Google Scholar
  11. Nash, J. F., Quincey, V. G., & Callinan J. (1966) Experiments on two-dimensional base flow at subsonic and transonic speeds. ARC R & M 3427.Google Scholar
  12. Poisson-Quinton, P. & Jousserandot, P. (1957) Influence du soufflage au voisinage du bord de fuite sur les caracteristiques aerodynamiques d’une aile aux grandes vitesses. La Recherche Aeronautique, No. 56, pp 21–32.Google Scholar
  13. Reubush, D. E. & Putnam, L. E. (1976) An experimental and analytical investigation of the effect on isolated boat-tail drag of varying Reynolds number up to 130 × 106. NASA TN D-8210.Google Scholar
  14. Roshko, A. (1954) On the drag and shedding frequency of bluff cylinders, NACA TN 3169.Google Scholar
  15. Sykes, D. M. (1969) The effect of low flow rate gas ejection and ground proximity on afterbody pressure distribution. Proc. 1st Symposium on Road Vehicle Aerodynamics, City University, London.Google Scholar
  16. Tanner, M (1965) Druckverteilungsmessungen an Kegeln, DLR FB 65–09.Google Scholar
  17. Tanner, M (1972) A method of reducing the base drag of wings with blunt trailing edges. Aero. Quart. Vol. 23, pp 15–23.Google Scholar

Copyright information

© Plenum Press, New York 1978

Authors and Affiliations

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

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