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Drag-Reducing Techniques for Axi-Symmetric Bluff Bodies

  • W. A. Mair

Abstract

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.

Keywords

Drag Coefficient Bluff Body Vortex Street Splitter Plate Road Vehicle 
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.

Notation

A

Base Area

Ao

Porous area of base (with base bleed)

b

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

CD

Drag coefficient referred to maximum cross-sectional area

ΔCD

Reduction of drag coefficient

Cp

Pressure coefficient

Cpb

Base-pressure coefficient

Cq

Bleed-flow coefficient, ≡ Q/UA

Cx

Axial-force coefficient referred to maximum cross-sectional area

d

Maximum diameter of body of revolution

dB

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

ds

Diameter of a sting-like cylindrical extension

f

Drag-reduction factor, ≡ ΔCD/0.165

k

Resistance coefficient at bleed-air outlet

Length of boat-tailed afterbody

n

Number of air changes per hour, for ventilation

Q

Volume flow rate of base bleed

R

Maximum radius of boat-tailed afterbody

r

Local radius of boat-tailed afterbody

t

Maximum thickness of two-dimensional aerofoil

U

Stream velocity

Uo

Average bleed velocity

V

Internal volume of vehicle

Xr

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

X

Distance downstream from section A in Fig. 5

β

Boat-tail angle (Fig. 5.)

δ

Boundary layer thickness

ν

Kinematic viscosity of air

ρ

Density of air

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References

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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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