Discussion
Consider the following statements about the three boundary-layer thicknesses:
1. Displacement thickness measures the outward shift of the external streamlines.
2. Momentum thickness is directly related to the skin-friction drag.
3. The shape factor is the ratio of momentum thickness to displacement thickness.
Which of the statements given above is or are correct?
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: displacement thickness is the distance the external streamlines are pushed outward by the retarded flow, and momentum thickness is directly proportional to the skin-friction drag through the momentum integral. Statement 3 inverts the definition - the shape factor is the *displacement* thickness over the *momentum* thickness, running about 2.6 for a laminar layer and 1.3 to 1.4 for a turbulent one, which is what makes it a useful indicator of impending separation.
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