The mean drag coefficient (CDF) for a laminar boundary layer along a flat plate was found to be 0.015. If all other flow factors remain the same and the length of the plate is decreased to ¼ of its original value, then the drag coefficient CDF would be equal to
- A0.015
- B0.060
- C0.030Correct
- D0.0075
Solution
Concept
Laminar flat plate drag
Formula
CDF = 1.328/√ReL
Step-by-step solution
1. ReL ∝ L, so CDF ∝ L−1/2. 2. L reduced to ¼ ⇒ CDF doubles: 0.015 × 2 = 0.030.
Explanation
The mean drag coefficient of a laminar boundary layer increases as the plate gets shorter.
Why the other options are wrong
0.060 would need CDF ∝ 1/L; 0.0075 is the inverse.
Shortcut
CDF ∝ 1/√L.
Common mistake
Using CDF ∝ 1/L.
Correct answer: C) 0.030