# Gas Dynamics Questions and Answers – Mach Angle

This set of Gas Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Mach Angle”.

1. What is the Mach wave angle for flow at Mach 1.43?
a) 0.774°
b) 44.37°
c) 30°
d) 45.6°

Explanation: Mach wave angle is given by, μ = sin-1⁡($$\frac {1}{M}$$) = sin-1⁡($$\frac {1}{1.43}$$) = 44.37°.

2. Consider an object moving with velocity V in a flow field. At what velocity all the wave fronts created by the disturbance coalesce and move along with the object?
a) V = a
b) V = $$\frac {a}{2}$$
c) V < a
d) V > a

Explanation: When the object velocity reaches sonic velocity all the wave fronts coalesce and move along with the object with same velocity. When the object velocity is more than the sonic velocity, the wave fronts are trailing the object. When the object velocity is less than sonic velocity the wave fronts don’t intersect and are ahead of the object.

3. What happens to the wave fronts emanating from the pressure disturbance, when the object moves with velocity greater than the speed of sound?
a) Wave fronts emanating from the disturbances will be behind the object
b) Wave fronts created by the disturbance will be ahead of the object
c) All the wave fronts and the pressure disturbances will move together
d) The wave fronts emanating from the disturbances will be stationary

Explanation: The wave fronts created by the disturbances will be trailing the object, this gives rise to a Mach cone. A Mach cone is formed by creating an imaginary line from the source object and touching all the trailing wave fronts tangentially.

4. What is the region outside the Mach cone called?
a) Zone of action
b) Zone of silence
c) Control volume
d) Zone of reaction

Explanation: The region with all the disturbances inside the Mach cone of the moving body is called the zone of action. The region outside the Mach cone is known as the zone of silence where the objects are unaware of the disturbances. A control volume is any volume in space where analysis is conducted.

5. What is true about a supersonic flow around a wedge?
a) The flow changes its direction smoothly and pressure decreases with acceleration
b) There is a sudden change in flow direction at the body and pressure increases downstream of the shock
c) The flow changes its direction abruptly and pressure decreases with acceleration
d) The flow changes its direction smoothly and pressure increases downstream with acceleration

Explanation: When the flow is supersonic around a wedge, it is observed that there is a sudden change in flow direction at the body and pressure increases downstream of the shock.

6. What will happen when the wedge angle is finite?
a) Small disturbances will be introduced and we can consider these to be identical to sound pulses
b) Streamlines over the wedge are parallel to the flow ahead of the wedge
c) The deviation of streamlines will be finite and pressure will decrease across the shock wave
d) Finite disturbances will be introduced and the wave will be called a shock wave

Explanation: When the wedge angle is finite, direction of flow changes rapidly and this deviation causes shock wave. There is pressure increase across the shock wave but total pressure reduces since velocity reduces.

7. Which is not a property of Mach Wave?
a) Isentropic
b) Non-isentropic
d) Reversible

Explanation: Flow across a Mach wave follows an isentropic process. Isentropic process is characterized by its adiabatic and reversible process. Stagnation properties remains constant across the wave.

8. Among the following, choose the irreversible process.
a) Across a Mach wave
b) Across a shock wave
c) Across a subsonic flow
d) Across an expansion wave

Explanation: Entropy increase across a shock wave makes it an irreversible process. The entropy increase can be found out as it is a function of static temperature and pressure ratios across the normal shock.

9. If the Mach number is increased keeping the same wall deflection angle, then what is going to happen to shock angle?
a) Shock angle decreases
b) Shock angle increases
c) Shock gets detached
d) Shock angle is not affected by Mach number

Explanation: In θ-β-M relation, where θ represents wall deflection angle, β represents shock angle and M represents Mach number, it shows that with increasing Mach number and constant wall deflection angle, the shock angle decreases.

10. What makes an expansion wave?
a) Collection of shock waves
b) Collection of Mach waves
c) Collection of normal shock waves
d) Collection of oblique shock waves

Explanation: Expansion waves are collection of infinite Mach waves which are diverging from a convex corner. They together form an expansion fan and the flow is isentropic across it.

11. What happens to the Mach number across a normal shock?
a) The Mach number will become supersonic
b) The Mach number will become subsonic
c) The Mach number will become transonic
d) The Mach number will become hypersonic

Explanation: Across a normal shock, the flow becomes subsonic downstream of the shock wave, as seen from the following equation, M$$_2^*=\frac {1}{M_1^*}$$. Characteristic Mach number downstream and upstream of the shock is represented by M$$_2^*$$ and M$$_1^*$$ respectively.

Sanfoundry Global Education & Learning Series – Gas Dynamics.

To practice all areas of Gas Dynamics, here is complete set of Multiple Choice Questions and Answers.

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