The local drag coefficient (Cf) at a point on a flat plate is defined as
- AThe ratio of the local shear stress to the dynamic pressureCorrect
- BThe ratio of the total drag force to the dynamic pressure and reference area
- CThe ratio of the pressure force to the dynamic pressure and reference area
- DThe ratio of the local velocity to the free stream velocity
Solution
Concept
Local Drag Coefficient
Formula
Cf = τw / (0.5 × ρ × U²)
Explanation
The local drag coefficient measures the friction force at a specific point on a surface. It is defined as the ratio of the local wall shear stress (frictional force per unit area at that spot) to the dynamic pressure of the fluid flow.
Common mistake
Many students confuse the local drag coefficient (Cf) with the average/total drag coefficient (CD) and mistakenly choose Option B, which defines the total drag coefficient over a reference area rather than the local shear stress at a specific point.
Correct answer: A) The ratio of the local shear stress to the dynamic pressure