The total drag on a body is attributed to form drag and skin friction drag. In some flow configurations, the contribution of form drag becomes significant.
In order to reduce the form drag, the boundary layer separation should be prevented or delayed so that better pressure recovery takes place and the form drag is reduced considerably. There are some popular methods for this purpose which are stated as follows.
By giving the profile of the body a streamlined shape( as shown in Fig. 31.2).
This has an elongated shape in the rear part to reduce the magnitude of the pressure gradient.
The optimum contour for a streamlined body is the one for which the wake zone is very narrow and the form drag is minimum.
Fig. 31.2 Reduction of drag coefficient (CD) by giving the profile a streamlined shape
The injection of fluid through porous wall can also control the boundary layer separation. This is generally accomplished by blowing high energy fluid particles tangentially from the location where separation would have taken place otherwise. This is shown in Fig. 31.3.
The injection of fluid promotes turbulence
This increases skin friction. But the form drag is reduced considerably due to suppression of flow separation
The reduction in form drag is quite significant and increase in skin friction drag can be ignored.
