Boundary Layer and Drag Questions and Answers

Assertion (A): Displacement thickness feeds back into the outer inviscid solution.

Reason (R): It measures how far the external streamlines are pushed out by the velocity deficit.

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Assertion (A): Boundary-layer suction is widely used on production transport aircraft.

Reason (R): Suction removes the low-momentum fluid and leaves a fuller velocity profile.

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Assertion (A): The drag crisis causes a sphere's drag to fall even though its skin friction rises.

Reason (R): The transition to turbulence delays separation and narrows the wake.

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Consider the following statements about a turbulent boundary layer:

1. Its skin friction is higher than that of a laminar layer at the same Reynolds number.

2. It resists separation better than a laminar layer.

3. It grows more slowly along the surface than a laminar layer.

Which of the statements given above is or are correct?

Consider the following statements about boundary-layer separation:

1. It occurs where the velocity gradient at the wall falls to zero.

2. It requires an adverse pressure gradient.

3. It always occurs wherever the pressure gradient is adverse.

Which of the statements given above is or are correct?

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?

Consider the following statements about the drag crisis on a sphere:

1. It is caused by transition of the boundary layer to turbulence.

2. The drag coefficient falls sharply as it occurs.

3. The skin friction falls as it occurs.

Which of the statements given above is or are correct?

Consider the following statements about laminar-flow aerofoils:

1. They hold a favourable pressure gradient over much of the chord.

2. Their low-drag advantage is confined to a band of lift coefficient.

3. Their advantage is robust to surface contamination.

Which of the statements given above is or are correct?

Consider the following statements about drag on a bluff body:

1. Pressure drag dominates over skin friction.

2. Streamlining reduces the total drag despite increasing the wetted area.

3. The drag coefficient is independent of Reynolds number at all speeds.

Which of the statements given above is or are correct?

Consider the following statements about boundary-layer control:

1. Suction removes low-momentum fluid from near the wall.

2. Vortex generators mix high-momentum fluid down towards the wall.

3. Both techniques work by making the boundary layer laminar.

Which of the statements given above is or are correct?