This set of Gas Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Reflection and Intersection of Shocks and Expansion Waves”.

1. What properties are equal for a reflected and incident shock which is weak?

a) Flow deflection angle and shock strength

b) Static pressure and temperature

c) Static temperature and wave angle

d) Static density and pressure ratios

View Answer

Explanation: For any shock reflected from the surface, flow deflection angle before the incident shock and after the reflected shock doesn’t change. In weak shocks, shock strength is assumed to be equal since the change in pressure is very small.

2. What properties are equal for an incident and reflected shock which is not weak?

a) Flow deflection angle and shock strength

b) Pressure ratios and shock strength

c) Flow Mach number and shock wave angle

d) Only the deflection angle

View Answer

Explanation: When a strong shock is reflected from a surface, the flow directions before and after the shocks are parallel and hence deflection angle is the same. The pressure ratios for each shock is different and the overall strength is the product of these two pressure ratios.

3. What is true about the shock wave angle when a shock wave is reflected from a surface?

a) Shock wave angles of incident and reflected shock waves are equal

b) Shock wave angle of reflected shock is always greater than the incident wave angle

c) Reflected shock wave angle is greater than the incident shock wave angle when the Mach number of the flow is high

d) Shock wave angles of incident and reflected shock waves are not equal

View Answer

Explanation: When a shock wave is reflected from a rigid surface, the waves angles are not equal. For low Mach numbers, the reflected shock wave angle is greater than the incident shock wave angle. For high Mach numbers, the shock wave angle of reflected shock is always greater than the incident wave angle.

4. What is true about the interaction of two shock waves of the opposite family but same strength?

a) Flow direction before and after the shock interaction is same and there is no bent in the flow throughout the shock system

b) There is a slight bend in the flow inside the shock system but flow direction doesn’t change before and after the interaction

c) Flow downstream is deflected more towards the positive direction family of the shock interaction

d) Flow direction downstream of the shock system is deflected towards the negative direction family of the shock system

View Answer

Explanation: When two shock waves of the opposite family but same strength interact, the flow direction upstream and downstream of the shock system doesn’t change. The flow bents a little between the reflected shocks by some angle but again is bent back to the previous direction.

5. What is true about the interaction of two shock waves of the opposite family but unequal strength?

a) The two portions downstream, below and above the slipstream of the shock system, have equal pressure

b) The downstream flow deflection angle of the portion above the slipstream is not equal to the portion below the slipstream

c) The downstream flow direction is equal to freestream flow direction

d) The flow is always parallel to the ground due to multiple shock formation

View Answer

Explanation: The interaction of two shock waves of opposite sign and unequal strength leads to the division of the flow into two portions by a slipstream. The downstream flow deflection angle and pressure are equal in these two portions. The downstream flow direction is may not be always equal to the freestream flow direction.

6. In literature, the contact surface has different names. Which of these names doesn’t refer to contact surface?

a) Entropy boundary

b) Slipstream

c) Vortex sheet

d) Tangential discontinuity

View Answer

Explanation: In literature, the contact surface is referred to as material boundary, entropy discontinuity, slipstream or slip surface, vortex sheet, and tangential discontinuity. The contact surface is a surface of discontinuity. It can be stationary or moving.

7. What is true about the interaction of shock waves of the same family?

a) The shocks will coalesce and form one single stronger shock

b) The shocks will pass through and create multiple shock systems

c) Slipstream line will be created perpendicular to the flow direction

d) Shock waves will coalesce and form into many stronger shocks

View Answer

Explanation: When two shocks of the same family interact, they coalesce and form one bigger shock wave. The slipstream line is parallel to the flow downstream of the shock and at the point of intersection of all the waves.

8. What is the shock wave angle of the reflected shock with respect to the horizontal, when a shock is reflected from the surface with shock wave angle given as β=40 and flow Mach number is 2?

a) 37.9°

b) 39.7°

c) 45.6°

d) 50.2°

View Answer

Explanation: From θ-β-M graph, for M

_{1}=2 and β

_{1}=40, θ=10.5°. Now from oblique shock relation tables, we can find M

_{2}=1.63. Since the flow direction is unchanged after crossing the reflected wave, from the θ-β-M graph, for M

_{2}=2 and θ=10.5°, β

_{2}=50.2°. So the reflected shock wave angle w.r.t the ground is β

_{r}=β

_{2}-θ=50.2-10.5=39.7°.

9. Given that shock strength of two shocks, incident and reflected shocks are, \(\frac {p_2}{p_1}\)=5.226 and \(\frac {p_2}{p_3}\)=0.3120. What is the overall strength of the shock system?

a) 16.75

b) 0.1675

c) 0.0597

d) 0.423

View Answer

Explanation: Overall shock strength for the given shock system is given by, \(\frac {p_3}{p_1}=\frac {p_2}{p_1}\times \frac {p_3}{p_2}\) = 5.226 × \(\frac {1}{0.3120}\) = 16.75. Since the pressure increases, the overall pressure ratio can never be less than unity.

10. When the slipstream divides the flow into two portions, which parameter is not equal in those two portions?

a) Temperature

b) Pressure

c) Flow direction

d) Flow Mach number

View Answer

Explanation: The contact surface can be idealized as a surface of discontinuity. It separates flow into two flow fields, with different temperature and density. Pressure, flow direction, and Mach number are the same.

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