Question

The drag of a rough cylinder compared to a smooth cylinder of the same size is

• A. Always Higher
• B. Always Lower
• C. First higher then lower
• D. Unaffected by surface roughness

Hint:

Surface roughness can delay flow separation by promoting a turbulent boundary layer. Hence, \[ \boxed{ \text{Smaller wake} \Longrightarrow \text{Lower pressure drag}. } \]

Answer 1

The Correct Option is B

Step 1: Recall the effect of surface roughness. Surface roughness promotes an early transition of the boundary layer from laminar to turbulent. A turbulent boundary layer possesses greater momentum near the surface.

Step 2: Determine its effect on drag. The delayed flow separation caused by the turbulent boundary layer reduces the wake behind the cylinder. Consequently, the pressure drag decreases. Therefore, \[ \boxed{ \text{The drag of a rough cylinder is lower than that of a smooth cylinder.} } \] Hence, \[ \boxed{(B)} \] is the correct answer.