- 2074 Asoj · 5 marks
Pressure drag, friction drag, streamlined aerofoil
Short answer
Pressure drag is caused by the front-rear pressure difference from a separated wake, acting normal to the surface. Friction drag arises from viscous shear tangential to the surface. An aerofoil's streamlined shape delays separation, yielding a narrow wake, so pressure drag is very small and total drag is mostly low skin friction, giving high lift-to-drag ratio.
Distinguish between pressure and friction drags. Explain with sketches, why the aerofoil is designed as streamlined body.
Total drag on a body in a fluid is the sum of pressure (form) drag and friction (skin) drag.
Pressure drag vs friction drag
| Point | Pressure (form) drag | Friction (skin) drag |
|---|---|---|
| Cause | Pressure difference between front and rear, due to a wake after separation | Viscous shear stress on the surface |
| Acts | Normal to the surface | Tangential to the surface |
| Depends on | Shape of body, position of separation | Surface area, roughness, boundary layer type |
| Large for | Bluff bodies (disc, cylinder, sphere) | Long streamlined bodies, plates parallel to flow |
| Reduced by | Streamlining | Smooth surface, keeping BL laminar |
Why an aerofoil is a streamlined body
flow -> bluff body streamlined body
___ ______
| | wide wake / \____
----> | | ~~~~~~ ( > narrow wake
|___| \______/----
- A bluff body causes the boundary layer to separate early (the adverse pressure gradient on the rear is steep). This gives a wide, low-pressure wake and large pressure drag.
- An aerofoil has a rounded nose and a long, gradually tapering tail. The rear pressure rises slowly, so the adverse pressure gradient is mild.
- The boundary layer stays attached almost to the trailing edge, the wake is very narrow, and the pressure drag becomes very small.
- The total drag is then mostly friction drag, which is small, so the aerofoil gives high lift with low drag ( is large).
✓ Answer checked by Claude Opus 5.5 (high effort)
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