Fluid Dynamics MCQ with answer and solution

Concept

Skin Friction Drag in Streamlined Bodies

Formula

Yes, the friction drag force is calculated using the formula: Fd = 0.5 × Cf × ρ × v² × A Where: - Fd = Friction drag force - Cf = Skin friction drag coefficient - ρ = Fluid density - v = Flow velocity - A = Surface area of the body

Step-by-step solution

1. Streamlining reduces flow separation, which minimizes pressure (form) drag. 2. Since pressure drag is negligible, the total drag is primarily dominated by surface friction drag. 3. Therefore, the correct answer is Friction drag (Option B).

Explanation

For a streamlined body, the shape is specially designed so that fluid flows smoothly around it without separating or creating a large wake behind it. Because this smooth shape minimizes pressure drag (form drag), the total resistance left is mostly friction drag (skin friction), which is caused by the fluid rubbing against the surface of the object as it moves.

Shortcut

To easily remember this type of question, use this contrast rule: - Streamlined body = Friction drag (Skin friction) -\> Smooth shape reduces pressure wake, so surface friction dominates. - Bluff/Blunt body = Pressure drag (Form drag) -\> Broad shape creates a large wake, so pressure difference dominates.Mnemonic: "Streamlined = Skin friction, Bluff = Pressure puff"

Common mistake

Students often confuse a streamlined body with a bluff (blunt) body and mistakenly select Pressure drag or Form drag (Option A or C), assuming pressure difference is always the primary source of drag.