DragDrag is a force that opposes motion. An aircraft flying has to overcome the drag force upon it, a ball in flight, a sailing ship and an automobile at high speed are some of the other examples. It is clear that viscosity is an agent that causes drag. We have seen that it gives raise to boundary layers on solid surfaces. There is shear stress in boundary layers that do tend to retard the motion of fluid past the solid surface. This is sketched for an aerofoil surface in Fig.6.6. This is termed Skin friction Drag.
There is another agent that can cause drag. This is the pressure difference upon the flow. This could come about due to geometrical effects that induce separation as happens with a cylinder to be discussed later. This is called Pressure Drag or Form Drag, since it is due to the body geometry. The sum of pressure drag and skin friction drag constitutes Drag about the body or Profile Drag.
The shape of the body determines the relative magnitude of the drag components. A thin body (small t/l ratio) as shown in Fig.6.7 obviously causes less pressure drag. Almost all drag comes from skin friction. A thick body (large t/l ratio) is readily prone for separation and produces considerable pressure drag. Streamlining a body to avoid separation will enable to decrease pressure drag considerably. It is obvious that a bluff body like a cylinder or a sphere or a flat plate placed normal to flow will cause separation and lead to pressure drag which may far more than the skin friction drag. In case of a flat plate placed parallel to flow (Fig.6.8), it is the skin friction drag that dominates. On the other hand, in case of a flat plate placed normal to flow, it is the pressure drag that dominates. In the latter case, the plate behaves like a blunt body and gives raise to separation behind it which contributes to pressure drag.
Subsections (c) Aerospace, Mechanical & Mechatronic Engg. 2005 University of Sydney |
