A slip line downstream of a shock intersection is a surface across which
Answer: C
Mechanical equilibrium requires the pressure and the flow direction to be continuous across the surface, but there is no requirement for the speed, temperature or entropy to match, and they generally do not. The result is a vortex sheet trailing downstream from the intersection point.
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Assertion (A): A shock standing in the divergent section of a nozzle moves downstream as the back pressure is lowered.
Reason (R): A lower back pressure requires a weaker compression to match the exit condition.
Select the correct answer from the options below.
Answer: D
Both are true and the second explains the first. Further downstream the flow has accelerated to a higher Mach number, but the shock must deliver less total compression, and the balance works out so that the shock settles further from the throat. At the design back pressure it passes out of the nozzle entirely.
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Assertion (A): Shock-induced boundary layer separation is a serious problem on transonic wings.
Reason (R): The shock imposes a sharp adverse pressure gradient on already slow near-wall fluid.
Select the correct answer from the options below.
Answer: A
Both are true and the second is the reason for the first. The boundary layer has little momentum to spend against a steep rise in pressure, so it thickens and often separates, causing buffet, a loss of lift and a sharp rise in drag. Supercritical sections are shaped largely to keep that shock weak.
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Consider the following statements about a shock in the divergent section of a nozzle:
1. The flow is supersonic immediately upstream of the shock.
2. The flow is subsonic immediately downstream of the shock.
3. The stagnation pressure downstream of the shock equals that in the reservoir.
Which of the statements given above is or are correct?
Answer: B
The first two are correct and follow from the shock being normal. The third is false: the shock destroys stagnation pressure, so the downstream value is below the reservoir value, and it is that deficit which determines where the shock has to sit for a given back pressure.
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Consider the following statements about shock-boundary layer interaction:
1. It can cause the boundary layer to separate.
2. It smears the shock near the wall into a lambda foot.
3. It has no effect on the drag of a transonic wing.
Which of the statements given above is or are correct?
Answer: C
The first two are correct. The third is plainly false: shock-induced separation is among the largest contributors to transonic drag rise and to buffet, and controlling it is a principal aim of supercritical aerofoil design.
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Match each nozzle back-pressure condition in List I with the resulting flow in List II.
List I
A. Back pressure equal to the design value
B. Back pressure slightly above the design value
C. Back pressure just below the choking value
D. Back pressure above the choking value
List II
1. Fully supersonic exit, no shock in the nozzle
2. Normal shock standing in the divergent section
3. Subsonic flow throughout, throat just sonic
4. Subsonic flow throughout, throat not sonic
Select the correct match.
Answer: D
Lowering the back pressure takes the nozzle through a well-defined sequence: subsonic throughout, then just choked, then a shock appearing at the throat and moving downstream, and finally the design condition with the shock swept out of the exit. Below that come the underexpanded cases with expansion fans outside.
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The critical or Crocco point on a shock polar marks the deflection at which
Answer: A
The sonic point sits very slightly below the maximum deflection on the polar. Between the two the flow is subsonic downstream even on the weak branch, which is a narrow but real range. Detachment corresponds to the maximum deflection itself, a distinct point a little further round the curve.
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Compared with a normal shock at the same upstream Mach number, an oblique shock produces
Answer: B
Only the normal component drives the shock, and it is smaller than the full velocity, so the shock is weaker in every respect: less compression, less heating and less entropy generated. Getting a required compression therefore costs less if it is done obliquely and in stages.
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The principal aerodynamic reason for giving a supersonic aircraft a slender, pointed nose is to
Answer: C
A slender nose asks for a small flow deflection, which an attached, weak oblique shock can supply, and the wave drag that follows is small. A blunt nose forces a detached bow shock with a strong nearly normal portion and a large drag penalty, which is acceptable only where heating governs, as on a re-entry capsule.
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Which best explains why re-entry capsules are blunt rather than pointed?
Answer: D
The design trade is reversed at re-entry: drag is wanted, not avoided, and the real problem is heat. A detached bow shock stands off from the body and dumps most of the energy into the gas between shock and surface rather than into the structure, so bluntness is a deliberate thermal-protection choice.
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