Short Answer:
Skin friction drag is the resistance or frictional force experienced by a body moving through a fluid due to the viscous effects within the boundary layer. It is caused by the shear stress developed between the surface of the body and the adjacent fluid layers.
This drag acts tangentially to the surface and depends on factors such as fluid viscosity, flow velocity, surface roughness, and the nature of the boundary layer (laminar or turbulent). Skin friction drag is an important consideration in the design of aircraft, ships, pipelines, and other systems where fluid flow over surfaces occurs.
Detailed Explanation:
Skin Friction Drag
When a fluid flows over a solid surface, the layer of fluid in immediate contact with the surface adheres to it due to the no-slip condition, which means the velocity of the fluid at the wall is zero. As we move away from the wall, the velocity of the fluid gradually increases until it reaches the free-stream velocity (). This velocity variation across the thin layer near the surface is known as the boundary layer.
Within this boundary layer, viscosity plays a significant role, creating shear stress between adjacent layers of the fluid. This shear stress causes a tangential resistance or drag force on the surface, known as skin friction drag (or viscous drag).
The skin friction drag is therefore the result of the viscous shear forces acting parallel to the surface due to the fluid’s resistance to motion. It is one of the two main components of total drag on a body, the other being pressure drag (or form drag), which acts due to pressure differences around the body.
Cause of Skin Friction Drag
The cause of skin friction drag lies in the viscosity of the fluid and the velocity gradient that develops in the boundary layer.
- The fluid molecules at the wall stick to the surface (zero velocity).
- The molecules just above move slightly faster due to reduced frictional effects.
- This creates a velocity gradient (change in velocity with respect to distance from the wall).
- The product of viscosity () and velocity gradient gives the shear stress at the wall ():
This wall shear stress, when integrated over the entire surface area, gives the total skin friction drag force ():
where,
= Skin friction drag force,
= Surface area of the body in contact with the fluid.
Factors Affecting Skin Friction Drag
- Fluid Viscosity:
- Higher viscosity fluids produce greater shear stress and hence more drag.
- Example: Oil produces more skin friction drag than air or water.
- Flow Velocity:
- Drag increases with increasing velocity because shear stress depends on the velocity gradient.
- Surface Roughness:
- A smooth surface offers less resistance, while a rough surface increases turbulence and enhances drag.
- Nature of Flow:
- Laminar Flow: Smooth and orderly, with less mixing and smaller velocity gradients; hence, lower drag.
- Turbulent Flow: Chaotic with intense mixing; higher momentum transfer near the wall causes higher drag.
- Flow Length:
- The longer the surface in the direction of flow, the greater the development of the boundary layer, resulting in increased drag.
- Reynolds Number:
- Skin friction drag depends on the Reynolds number , which represents the ratio of inertial to viscous forces.
- For laminar flow, friction decreases with increasing Reynolds number, while for turbulent flow, it varies differently.
Mathematical Expressions for Skin Friction Drag
The skin friction coefficient (Cf) is defined as the ratio of wall shear stress to the dynamic pressure of the fluid:
The value of depends on whether the boundary layer is laminar or turbulent.
- For Laminar Flow over a Flat Plate:
where .
- For Turbulent Flow over a Flat Plate:
Once is known, the total skin friction drag force can be found using:
This shows that skin friction drag is directly proportional to the density of the fluid, the square of the velocity, and the surface area of the body.
Difference Between Skin Friction Drag and Pressure Drag
| Aspect | Skin Friction Drag | Pressure Drag |
| Cause | Due to viscosity and shear stress along the surface | Due to pressure difference between front and rear of the body |
| Direction | Acts tangential to surface | Acts normal to surface |
| Flow Type | Dominant in streamlined bodies | Dominant in bluff (non-streamlined) bodies |
| Depends on | Surface roughness, viscosity, velocity | Shape and separation of flow |
| Example | Flow over aircraft wings, pipes | Flow around a sphere or car body |
Methods to Reduce Skin Friction Drag
- Streamlined Design:
Smooth and elongated shapes reduce boundary layer thickness and shear stress. - Surface Polishing:
Smooth surfaces minimize friction between fluid and surface. - Use of Low Viscosity Fluids:
Reduces the shear stress and frictional resistance. - Boundary Layer Control:
Techniques like suction, blowing, and vortex generators can manage boundary layer behavior. - Laminar Flow Maintenance:
Keeping the flow laminar through design and control reduces skin friction drag significantly.
Applications in Engineering
- Aeronautical Engineering: Skin friction drag affects aircraft performance; minimizing it increases lift-to-drag ratio.
- Marine Engineering: Ship hulls are designed and polished to reduce drag in water.
- Turbomachinery: Used to calculate energy losses in turbine blades, compressors, and pumps.
- Pipeline Flow: Helps estimate pressure drop and pumping power requirements.
- Automotive Engineering: Important for optimizing vehicle body designs for better aerodynamics.
Conclusion
The skin friction drag is the tangential resistance force developed due to viscosity when a fluid flows over a surface. It arises from the shear stress within the boundary layer and plays a vital role in determining total drag on bodies such as aircraft, ships, and turbines. By controlling factors like surface smoothness, flow velocity, and boundary layer behavior, engineers can minimize skin friction drag, leading to improved efficiency, reduced energy losses, and enhanced aerodynamic performance.