# Drag-Reducing Techniques for Axi-Symmetric Bluff Bodies

## 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## Notation

- A
Base Area

- A
_{o} Porous area of base (with base bleed)

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

- C
_{D} Drag coefficient referred to maximum cross-sectional area

- ΔC
_{D} Reduction of drag coefficient

- C
_{p} Pressure coefficient

- C
_{pb} Base-pressure coefficient

- C
_{q} Bleed-flow coefficient, ≡ Q/UA

- C
_{x} Axial-force coefficient referred to maximum cross-sectional area

- d
Maximum diameter of body of revolution

- d
_{B} Base height (two-dimensional) or base diameter (axi-symmetric)

- d
_{s} Diameter of a sting-like cylindrical extension

- f
Drag-reduction factor, ≡ ΔC

_{D}/0.165- k
Resistance coefficient at bleed-air outlet

*ℓ*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

- U
_{o} Average bleed velocity

- V
Internal volume of vehicle

- X
_{r} 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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