Boundary Layer Theory MCQ with answer and solution

Concept

Boundary Layer Separation Mechanics

Formula

Adverse pressure gradient: dp/dx > 0 Separation condition: (du/dy) at y=0 = 0

Step-by-step solution

1. Statement 1 is correct: Separation is initiated by an adverse pressure gradient (dp/dx > 0) where fluid particles slow down due to increasing pressure. 2. Statement 2 is correct: Separated flow forms turbulent eddies and a broad wake, which greatly increases form drag and head loss. 3. Statement 3 is correct: Delaying boundary layer separation over an airfoil maintains attached flow, reducing drag and increasing lift. 4. Statement 4 is incorrect: Flow separation occurs in a region where pressure increases along the flow direction (adverse pressure gradient), not decreases (favorable pressure gradient).

Explanation

Boundary layer separation occurs when the fluid encounters an adverse pressure gradient (dp/dx > 0), causing the kinetic energy of fluid particles near the wall to deplete. Separation results in large wake formation, significantly increasing drag and head losses. Delaying separation on an airfoil increases lift and delays aerodynamic stalling, thereby improving performance.

Shortcut

Remember: Separation requires an *adverse* pressure gradient (pressure increases along flow, i.e., dp/dx > 0).

Common mistake

Confusing favorable (dp/dx < 0, pressure decreases) and adverse (dp/dx > 0, pressure increases) pressure gradients.