Copyright © A. Filippone (1996-2001). All Rights Reserved.

Aerodynamic Drag

Vortex Drag


Vortex drag is due to some form of separation (tip flow separation, separation from a bluff body, etc.). For this reason it is also called form drag or pressure drag.

Separation and related effects cannot be avoided on bluff bodies and particular situations of streamlined bodies (most road vehicles, aircraft after bodies). Separation is generally associated with adverse pressure gradients.

The pressure field in the separated areas is lower than it would be in presence of a boundary layer. The low pressure field at the base of the body is the origin of the base drag. Specific vortex drag reduction techniques are listed below.

The technology presented below is intended for subsonic speeds. Vortex drag reduction at supersonic speeds are far less successful.

Splitter Plates

The mechanism by which the device works is the movement of the separation vortex downstream, away from the body. Fig. 1. below shows the artrangement of a 3D bluff body. Splitter plates have also been applied to airfoils and wings.

Splitter plate
Figure 1: Splitter plate behind a bluff body

Ventilated cavities

These are thin surfaces mounted at the edge of the base. Regular slots are cut through, that allow for ventilation of the low pressure separated field. Horizontal vented cavities are sometimes applied to passenger cars.

Vented Cavity
Figure 2: Vertical and horizontal vented cavities

Tangential slots

As shown in Fig. 4, they are used to accelerate slow air flow behind a corner (they are used on commercial vehicles of all sizes.)

Tangential Slot

Figure 3: Corner slot for vortex drag reduction; velocity field shown.

The acceleration of the slot flow serves to push flow that has been slowed down by the abrupt change of direction.


The use of fences on after-bodies is sometimes justified by the need to redirect the flow streamlines. The effect is to remove the flow separation. An example is shown in the figure below, that is an aircraft after-body.

Fences on C-17 afterbody

Figure 4: Fences to reduce after-body drag on C-17.

Fences are also used on the main wing to redirect the boundary layer flow. These devices can be found in most of the 1st generation of commercial jets (for example, Vickers VC-10, BAC 1-11, Trident) and in some early military aircraft (MiG-17). Fig. 5 below shows the large fence on the wing of the Hawker-Siddeley Trident 2.

Fences on Hawker-Siddeley Trident 2

Figure 5: Wing fences on Hawker-Siddeley Trident 2 (Duxford Air Museum, England).

Boat-tailed afterbodies

Such afterbodies are streamlined and designed for optimal shape.

Base drag reduction rates of 50 % (at subsonic speeds) can be achieved with the devices listed above.

For lifting wings some of the devices commonly designed are the following:

  • Vortex Generators
  • Wall Suction
  • Wall jets

Concave Surface Cavities

As dimples on sports ball, used to promote turbulent transition, which shifts the drag crisis of the bluff body to a lower speed. There is an amount of research available on this particular topic (for ex. Metha, 1985).

Related Material

Selected References

  • Green, Sheldon I (editor). Fluid Vortices, Kluwer Academic Press, 1995.

  • Lachmann GV(editor). Boundary Layer and Flow Control, Pergamon Press, 1961.

  • Hoerner SF. Fluid Dynamic Drag, Hoerner Fluid Dynamics, 1965.

  • Tanner M. “Reduction of Base Drag”, in Progress in Aerospace Sciences, Vol. 16, No. 4, 1975.

  • Chang PK. Separation of Flow, Pergamon Press, 1966.

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Copyright © A. Filippone (1996-2001). All Rights Reserved.