Skin friction and form friction are two distinct types of fluid friction that contribute to the overall drag force experienced by an object moving through a fluid (liquid or gas) or a fluid moving through a conduit. The key difference lies in their origins: skin friction arises from the viscous shear stresses at the fluid-solid interface, while form friction arises from the pressure distribution around the object due to its shape.
Skin Friction
Skin friction, also known as viscous drag, results from the fluid's viscosity and its interaction with the surface of the object.
- Cause: Arises from the shear stress between the fluid layers and the solid surface. The fluid particles closest to the surface adhere to it (no-slip condition), creating a velocity gradient within the fluid. This gradient leads to internal friction as adjacent layers of fluid move at different speeds.
- Dependence: Directly proportional to the surface area of the object in contact with the fluid, the fluid's viscosity, and the square of the fluid velocity. It is also significantly affected by the surface roughness; a rougher surface increases the surface area and turbulence, leading to higher skin friction.
- Reduction: Can be minimized by reducing the surface area, streamlining the object to promote laminar flow, and reducing the fluid viscosity (though this is often not practical). Smoothing the surface to reduce roughness also helps.
Form Friction
Form friction, also known as pressure drag or profile drag, results from the shape of the object obstructing the flow.
- Cause: Arises from the pressure difference between the front and rear of the object. As the fluid flows around the object, it separates from the surface, creating a low-pressure zone behind the object. This pressure difference creates a net force acting against the object's motion. In essence, it is a drag force generated by the shape.
- Dependence: Highly dependent on the shape of the object. Blunt or bluff bodies (like a flat plate perpendicular to the flow) experience high form friction due to significant flow separation and a large low-pressure zone. Streamlined bodies experience lower form friction because they minimize flow separation. It is also proportional to the square of the fluid velocity and the frontal area of the object.
- Reduction: Primarily reduced by streamlining the object's shape. Streamlining ensures a smooth flow around the object, minimizing flow separation and the pressure difference between the front and rear.
Summary Table
| Feature | Skin Friction | Form Friction |
|---|---|---|
| Cause | Viscous shear stress at the surface | Pressure difference due to object's shape |
| Dependence | Surface area, viscosity, surface roughness | Object shape, frontal area |
| Primary Factor | Surface properties | Object shape |
| Reduction | Smooth surface, laminar flow | Streamlining |
| Other names | Viscous Drag | Pressure Drag, Profile Drag |
Examples
- Skin Friction: The drag on a smooth, flat plate aligned parallel to the flow is primarily due to skin friction.
- Form Friction: The drag on a parachute is primarily due to form friction, as its shape creates a large pressure difference.
- Combined: The drag on a car or airplane is a combination of both skin friction (acting on the surface) and form friction (due to its overall shape). Streamlining car designs aims to reduce form friction to improve fuel efficiency.
In many real-world scenarios, both skin friction and form friction contribute to the total drag force. Understanding the relative importance of each is crucial for designing objects that minimize drag and improve performance.
