What happens to air velocity when it passes over the fins of air-cooled engines?

What happens to air velocity when it passes over the fins of air-cooled engines?

• A. Increases
• B. Stays the same
• C. Decreases
• D. Slows down

Answer: (D)

Air flow over finned surfaces slows down because the fins act as obstacles that create friction and a growing boundary layer. The air in contact with the metal is slowed to zero at the surface (no-slip condition), and as it travels over and between the fins the boundary layer thickens and becomes more turbulent. This resistance reduces the average air velocity in the finned region relative to the free-stream flow, increasing the time the air spends in contact with the fin surfaces and improving heat transfer. Locally, there can be complex flow patterns, but the overall effect is a reduction in velocity over the fins.

Air flow over finned surfaces slows down because the fins act as obstacles that create friction and a growing boundary layer. The air in contact with the metal is slowed to zero at the surface (no-slip condition), and as it travels over and between the fins the boundary layer thickens and becomes more turbulent. This resistance reduces the average air velocity in the finned region relative to the free-stream flow, increasing the time the air spends in contact with the fin surfaces and improving heat transfer. Locally, there can be complex flow patterns, but the overall effect is a reduction in velocity over the fins.