A body has a friction drag of \( 80 \) N and a pressure drag of \( 320 \) N. The proportion of its total drag that is pressure drag is
Answer: A
The total is \( 80 + 320 = 400 \) N, so pressure drag is \( 320/400 = 0.80 \), that is eighty per cent. A split this heavily weighted towards pressure drag is characteristic of a bluff body with a large separated wake.
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Assertion (A): A turbulent boundary layer resists separation better than a laminar one.
Reason (R): Turbulent mixing carries high-momentum fluid down towards the wall.
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Answer: B
Both are true and the second explains the first. The extra momentum near the surface lets the layer push further into a rising pressure before the wall shear vanishes. The same mechanism raises the velocity gradient at the wall and so costs more skin friction, which is the standing trade of the subject.
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Assertion (A): Dimpling a golf ball reduces its drag.
Reason (R): The dimples trip the boundary layer turbulent, so separation moves aft and the wake narrows.
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Answer: C
Both statements are true and the second is the reason for the first. Deliberately bringing on the drag crisis at a lower Reynolds number collapses the pressure drag, which dominates for a bluff body. The friction penalty is trivial beside the saving, and the ball flies about twice as far.
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Assertion (A): Streamlining a body increases its wetted area yet reduces its total drag.
Reason (R): Streamlining delays separation and so cuts the pressure drag sharply.
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Answer: D
Both are true and the second explains the first. The gentler rear taper keeps the flow attached and leaves a narrow wake, and the pressure drag saved far exceeds the extra friction the longer surface costs. On a bluff body the pressure term is the one worth attacking.
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Assertion (A): Prandtl's boundary-layer hypothesis made practical aerodynamics possible.
Reason (R): It confines viscous effects to a thin layer, leaving the outer flow inviscid.
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Answer: A
Both statements are true and the second explains the first. Splitting the problem in two allowed potential-flow theory to be used where it works while viscosity was handled only where it matters. Before 1904 the two descriptions could not be reconciled, and d'Alembert's paradox stood unresolved.
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Assertion (A): Laminar-flow aerofoils are difficult to exploit in service.
Reason (R): Their favourable pressure gradient survives only over a narrow band of lift coefficient.
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Answer: B
Both are true and the second is the reason for the first. Outside the drag bucket the stagnation point moves and transition jumps forward, and in practice insects, rain and manufacturing waviness destroy the laminar run in any case. The measured benefit is often far below the wind-tunnel promise.
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Assertion (A): Surface roughness always increases skin-friction drag.
Reason (R): Roughness elements buried within the laminar sublayer never meet the turbulent flow.
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Answer: C
The assertion is false and the reason is true, and the reason is what refutes it. So long as the roughness is smaller than the sublayer the surface is hydraulically smooth and no drag penalty appears. Only once the elements protrude does the friction climb.
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Assertion (A): A boundary layer separates wherever the pressure gradient is adverse.
Reason (R): An adverse gradient decelerates the low-momentum fluid nearest the wall.
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Answer: D
The assertion is too strong while the reason is correct. An adverse gradient is necessary for separation but not sufficient: a turbulent layer routinely negotiates a mild rise without separating. Separation requires the gradient to be steep enough to drive the wall shear to zero.
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Assertion (A): Vortex generators are often fitted as a retrofit to cure buffet or restore control authority.
Reason (R): They mix high-momentum air into the boundary layer, delaying separation.
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Answer: A
Both statements are true and the second explains the first. Because they are small, cheap and external, they can be added after flight testing reveals a separation problem, which is exactly how many transport aircraft acquired them. The cost is a small permanent drag increment.
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Assertion (A): Skin friction is higher on a turbulent boundary layer than on a laminar one.
Reason (R): The turbulent velocity profile is fuller, giving a steeper gradient at the wall.
Select the correct answer from the options below.
Answer: B
Both are true and the second is the reason for the first. Shear stress at the wall is the viscosity times the velocity gradient there, and the fuller turbulent profile makes that gradient much steeper. On a long streamlined body the difference can be a factor of five or more.
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