Skin friction is a type of frictional force exerted by a fluid on a surface. The skin friction coefficient (Cf), a dimensionless parameter, is often used to represent skin friction. However, skin friction itself has units of force per unit area, specifically Pascals (Pa) in the SI system or pounds per square foot (psf) in the imperial system.
Here's a breakdown:
- Skin Friction: The tangential force exerted by a fluid (liquid or gas) on a surface due to viscous shear stresses.
- Shear Stress: Force per unit area. This is essentially what skin friction represents directly.
Since skin friction is a type of shear stress, it shares the same units. Shear stress is defined as:
τ = F/A
Where:
- τ = shear stress (skin friction)
- F = force
- A = area
Therefore, the unit of skin friction is force per unit area.
Units:
| System | Unit of Force | Unit of Area | Unit of Skin Friction (Force/Area) |
|---|---|---|---|
| SI | Newton (N) | m² | N/m² = Pascal (Pa) |
| Imperial | Pound (lb) | ft² | lb/ft² = psf |
Important Distinction: Skin Friction Coefficient (Cf)
It's crucial to distinguish skin friction from the skin friction coefficient. The skin friction coefficient (Cf) is a dimensionless quantity that relates the skin friction (shear stress) to the dynamic pressure of the flow:
Cf = τ / (0.5 ρ V²)
Where:
- Cf = Skin friction coefficient (dimensionless)
- τ = Skin friction (shear stress) in Pa or psf
- ρ = Fluid density (kg/m³ or slugs/ft³)
- V = Fluid velocity (m/s or ft/s)
Because Cf is dimensionless, it doesn't have any units. It represents the ratio of the skin friction to the dynamic pressure.
In summary, while the coefficient is dimensionless, skin friction itself represents a shear stress and is measured in Pascals (Pa) or pounds per square foot (psf).
