# Aerodynamics Questions and Answers – Hypersonic Flow

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Hypersonic Flow”.

1. For which of these Mach numbers is flow considered to be hypersonic?
a) M < 1
b) M = 1
c) M > 5
d) 1 < M < 5

Explanation: Flow with Mach number less than 1 is known as subsonic flow. Mach number greater than 1 is considered to be supersonic flow with sonic flow at Mach = 1. After Mach greater than 5, the flow properties change drastically and is known as hypersonic flow.

2. How is the shock layer in case of hypersonic flow?
a) Thin
b) Thick
c) Non existent
d) Increases with increasing Mach number

Explanation: Shock layer is defined as the distance between the surface body and the shock wave. For hypersonic flow, this distance is very less thus making the shock layer thin. The shock waves usually lies very close to the surface.

3. In hypersonic flow, the shock waves often merge with the viscous boundary layer.
a) True
b) False

Explanation: In case of hypersonic flow, the shock layer is very thin and the shock lies close to the surface of the body. This often leads to shock waves merging with the viscous boundary layer which is present at the surface.

4. How is the shock over a blunt body at hypersonic speed?
a) Conical
b) Curved
c) Diamond
d) Oblique

Explanation: When a space shuttle having a blunt nose enter the earth’s atmosphere, it is at the hypersonic speed. In this case the shock wave produced is very thin which is a characteristic of hypersonic flow but is slightly detached from the nose at some distance ‘d’. The nose region has a highly curves shock wave present.

5. What is the entropy gradient at the nose region of a slender body at hypersonic flow?
a) Very high
b) Very low
c) Negligible
d) Infinity

Explanation: At the nose region of the slender body, the shock wave is considered to be a normal shock. The entropy change across a strong shock wave is high. Thus the entropy gradient in the nose region is extremely high because of strong normal shock.

6. What is viscous dissipation?
a) Loss of kinetic energy due to viscous effect
b) Loss of potential energy due to viscous effect
c) Increase in kinetic energy due to increase in temperature
d) Frictional drag

Explanation: When a very high velocity flow is over a body, the hypersonic flow has large amounts of kinetic energy. This is often dissipated and converted in the form of internal energy die to viscous effects within the boundary layer. This effect is known as the viscous effect.

7. How does viscous dissipation affect temperature inside the boundary layer?
a) Increases
b) Decreases
c) No change
d) First increases, then decreases

Explanation: Due to viscous dissipation, kinetic energy of the gas gets converted to the internal energy leading to rise in temperature inside the boundary layer. With increase in temperature, the viscosity coefficient also increases.

8. Hypersonic boundary layers grow more rapidly compared to subsonic and supersonic boundary layers.
a) True
b) False

Explanation: The pressure inside the boundary layer remains constant (in the normal direction), but the temperature increases due to viscous dissipation. Thus, from equation of state, there is a decrease in density making it important to have a larger boundary layer thickness to pass the mass flow through the boundary layer. Due to this reason, the hypersonic boundary layer thickness grows much more rapidly compared to other flow regimes.

9. What is viscous interaction?
a) Interaction between viscous flow and boundary layer
b) Interaction between inviscid flow and boundary layer
c) Interaction between shock wave and inviscid flow
d) Interaction between shock wave and viscous flow inside boundary layer

Explanation: The interaction between the outer inviscid flow and the boundary layer in hypersonic flow is known as viscous interaction. It played an essential role in surface pressure distribution in turn affecting drag, lift, moments, etc. over the object.

10. Which of these surfaces is used for reentry vehicles at hypersonic speed?
a) Aluminum
b) Carbon
c) Ablative surface
d) Copper

Explanation: The re – entry vehicles at hypersonic speed undergo viscous dissipation. This leads to rise in temperature within the boundary layer leading to excitation of the molecules causing dissociation or ionization. The surface of these vehicles are usually coated with ablative surfaces because of its high melting point and inert nature.

11. What is the detrimental effect of high temperature hypersonic flow over a vehicle?
a) Communication blackout
b) Attitude problem
c) Altitude problem
d) Friction causing depletion of ablation

Explanation: At certain altitude and Mach number in hypersonic flow, the vehicles are unable to communicate by transmitting or receiving radio waves. This is due to the high temperature flow causing ionization of the chemically reactive flow which produces free electrons that absorb these radio waves. This is known as ‘communication blackout’.

Sanfoundry Global Education & Learning Series – Aerodynamics.

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