Skin friction drag

Skin friction drag or viscous drag is a type of aerodynamic or hydrodynamic drag, which is resistant force exerted on an object moving in a fluid. Skin friction drag is caused by the viscosity of fluids and is developed from laminar drag to turbulent drag as a fluid moves on the surface of an object. Skin friction drag is generally expressed in terms of the Reynolds number, which is the ratio between inertial force and viscous force.

Total drag can be decomposed into a skin friction drag component and a pressure drag component, where pressure drag includes all other sources of drag including lift-induced drag. In this conceptualisation, lift-induced drag is an artificial abstraction, part of the horizontal component of the aerodynamic reaction force. Alternatively, total drag can be decomposed into a parasitic drag component and a lift-induced drag component, where parasitic drag is all components of drag except lift-induced drag. In this conceptualisation, skin friction drag is a component of parasitic drag.

Laminar flow over a body occurs when layers of the fluid move smoothly past each other in parallel lines. In nature, this kind of flow is rare. As the fluid flows over an object, it applies frictional forces to the surface of the object which works to impede forward movement of the object; the result is called skin friction drag. Skin friction drag is often the major component of parasitic drag on objects in a flow.

The flow over a body may begin as laminar. As a fluid flows over a surface shear stresses within the fluid slow additional fluid particles causing the boundary layer to grow in thickness. At some point along the flow direction, the flow becomes unstable and becomes turbulent. Turbulent flow has a fluctuating and irregular pattern of flow which is made obvious by the formation of vortices. Turbulent flow causes higher skin drag than laminar flow. This is usually undesirable, for example in pipes or ducts, or on aircraft wings. However, in some cases, turbulent flow can reduce net drag. For example, the dimples on a golf ball cause turbulence, which increases skin drag, but which also reduces pressure drag. This effect occurs because turbulent flow remains attached to the surface of the ball for longer than laminar flow, and thus delays flow separation. This results in a narrower wake behind the ball, which reduces pressure drag. Overall drag is reduced, which enables the ball to fly farther.

The skin friction coefficient is defined as:

where:

The skin friction coefficient is a dimensionless skin shear stress which is nondimensionalized by the dynamic pressure of the free stream. The skin friction coefficient is defined at any point of a surface that is subjected to the free stream. It will vary at different positions. A fundamental fact in aerodynamics states that ${\displaystyle ({\tau _{w}})_{laminar}<({\tau _{w}})_{turbulent}}$. This immediately implies that laminar skin friction drag is smaller than turbulent skin friction drag, for the same inflow.

The skin friction coefficient is a strong function of the Reynolds number ${\displaystyle Re}$, as ${\displaystyle Re}$ increases ${\displaystyle c_{f}}$ decreases.