Vortex Drag
Summary
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.
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.
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.
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.)
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.
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.
Figure 5: Wing fences on Hawker-Siddeley Trident 2 (Duxford Air Museum, England).
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:
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).
Splitter Plates
Figure 1: Splitter plate behind a bluff body
Ventilated cavities
Figure 2: Vertical and horizontal vented cavities
Tangential slots
Figure 3: Corner slot for vortex drag reduction; velocity field shown.
Fences
Boat-tailed afterbodies
Concave Surface Cavities
Related Material
Selected References