Correct Answer : Cross – sectional area
Explanation : One – dimensional flows are those flows that have a constant crossectional area and only the flow variables i.e. pressure, velocity, density, temperature etc vary wrt x. What separates Quasi one – dimensional flow from this is the variable cross – sectional area wrt x.
Correct Answer : Euler's equation
Explanation : The differential form of momentum equation for the quasi one – dimensional flow is given by:dp = – ρuduThis is known as Euler’s equation which is derived from the momentum equation.
Correct Answer : pB = pe
Explaination : Back pressure, which is the pressure downstream the nozzle’s exit is equal to the exit pressure when the flow at the exit is subsonic. This is because of a discrepancy between maintaining a steady subsonic flow in case of pressure difference.
Correct Answer : Exit pressure is same as the inlet pressure
Explanation : Choked flow is a condition where the exit pressure is lower than inlet pressure to a point where the flow after throat section becomes frozen. After this, despite lowering the value of exit pressure, inlet pressure has no effect. The mass flow remains constant despite reducing the exit pressure.
Correct Answer : 2
Explanation : The area – Mach relation which is the ratio of local area to throat area as a function of Mach number yields two solutions for a given Mach number. One is a subsonic value and the other is the corresponding supersonic value.
Correct Answer : Exit pressure < Inlet pressure
Explaination : The air in the nozzle will never flow by itself considering that the area ratio between the inlet and the throat is very high. There needs to be a pressure difference created for the air to move. This happens only when the exit pressure is less than the the inlet pressure (pe < p0)
Correct Answer : False
Explanation : In the case of subsonic flow, convergent nozzle leads of increased velocity using the area – velocity relation. Similarly, for supersonic flow the divergent nozzle leads to increased flow velocity. Thus, in order to obtain the supersonic flow we make use of convergent – divergent nozzle.
Correct Answer : Subsonic flow
Correct Answer : Increases
Explanation : Supersonic flows have M > 1 which results in M2 – 1 value in the area – velocity relation to be positive. Thus for divergent sections where cross – sectional area increases with x, the value of velocity increases.
Correct Answer : Decrease the flow velocity after test section
Explanation : Diffuser is a duct used mostly in the subsonic wind tunnel in order to slow down the high flow velocity post test section to a lower velocity at the diffuser’s exit.
Explanation : Diffusers work for both subsonic and supersonic incoming flows. Although the design and shape of diffuser in both the cases vary. In both the cases, the work of the diffuser is to lower the speeds by least total pressure reduction.
Correct Answer : Total pressure
Explanation : Isentropic supersonic diffusers have a constant entropy throughout the diffuser duct. Since the entropy is constant, the total pressure along is the duct is also constant.
Correct Answer : At2 > At1
Explaination : There are two throats in the supersonic wind tunnel. The first is that of the nozzle and the second is that of the diffuser. The nozzle’s throat region is equal to A * (sonic area) and the diffuser’s throat area is greater than the nozzle’s throat area because of the rise in entropy within the diffuser.
Correct Answer : ηD < 1
Explaination : Usually for low supersonic test section Mach numbers, diffusers perform better than the normal shock diffuser, hence the numerator is greater than the denominator in the efficiency formula. This leads to ηD being greater than 1. But for hypersonic conditions, this is not the case and ηD < 1 as the normal shock recovery is not that great.
Correct Answer : Shock wave interaction with walls
Explanation : In real life, oblique shock diffusers have viscous flow. The presence of shock waves inside the diffuser leads to interaction with the viscous boundary layer of the diffuser walls which leads to additional pressure losses. There’s also friction involved which makes oblique shock diffusers far from the ideal diffusers which have no total pressure losses.
Correct Answer : Isentropic
Explanation : In an ideal supersonic diffuser, the air is slowed to subsonic speed and then expended to the atmosphere. This is done by isentropic compression as ideally there should be no loss in total pressure.
Explanation : The goal of the diffuser is to reduce the flow velocity with small total pressure loss. If we reduce the incoming supersonic flow through a series of oblique shock followed by a weak normal shock wave, leads to lower total pressure loss compared to reducing the incoming supersonic flow to subsonic using one strong normal shock.
Correct Answer : Presence of shock waves
Explanation : Due to the presence of oblique shock waves on the convergent portion, the ideal supersonic diffuser is far from achievable. This contributes to the breaking of the isentropic flow characteristics according to which the entropy in the diffuser is constant. In fact, the flow in reality is viscous and there is an increase in entropy near the boundary layer.
Correct Answer : Adiabatic expansion
Explanation : The process of slowing down the incoming flow inside the diffuser is done using adiabatic expansion. The pressure is constant throughout, if this was not the case then reduced total pressure in front of the compressor would yield less flow velocity.
Correct Answer : Shock waves
Explanation : In case of actual supersonic diffuser, there’s a series of oblique shock formations. It’s present in the convergent and the test section of the wind tunnel. The interaction between the viscous flow and the shock waves resulting in slowing down the flow velocity.
Correct Answer : Convergent – Divergent
Explanation : Supersonic wind tunnel produces flow of Mach number above 1. This is achieved using convergent – divergent nozzle. This selection is based on area – velocity relation, according to which the subsonic flow accelerates in the convergent section to sonic speed after which the speed is further accelerated in the divergent section.
Correct Answer : Test model
Explanation : Supersonic wind tunnel mainly comprises of three components – convergent – divergent nozzle, test section which is constant area duct and a diffuser to further slow down the speed to low subsonic speed. Test model is not a part of the wind tunnel and is inserted inside to take measurements such as lift, drag.
Explanation : In case of supersonic wind tunnels, there are two throats present. One is the nozzle throat where the sonic speed is achieved and the second throat is the diffuser throat where supersonic incoming flow is isentropically compressed to Mach 1.
Correct Answer : True
Explanation : The goal of the diffuser is to reduce the flow velocity with small total pressure loss. If we reduce the incoming supersonic flow through a series of oblique shock followed by a weak normal shock wave, leads to lower total pressure loss compared to reducing the incoming supersonic flow to subsonic using one strong normal shock.True
Correct Answer : Higher total pressure loss
Explanation : In real scenario far from hypothetical nature of the diffuser, the oblique shock waves interact with the viscous boundary layer of the diffuser wall. This leads to the separation of the boundary layer thereby increasing the total pressure losses. One thing to remember is that the aim of the diffuser is to reduce the flow velocity by low total pressure losses, but the boundary layer separation prevents that from happening.
Explanation : Supersonic wind tunnel is broadly classified into two categories – Intermittent wind tunnel and continuous wind tunnel. Out of these two, the intermittent wind tunnel is more commonly used due to simpler design, faster start and more power available to start the wind tunnel.
Correct Answer : 2.36
Correct Answer : Ideal isentropic diffuser
