AERODYNAMICS -II

TWO MARKS QUESTION AND ANSWER

1. What is meant by Normal Shock?

When a shock wave appears normal to the flow direction of the free stream is called

Normal shock. The Mach number behind the shock is always subsonic. The flow process

through the normal shock is highly irreversible and adiabatic.

2. How does speed of the sound varies?

The speed of sound is directly proportional to the square root of the absolute temperature.

3. Write the shock relation of the perfect gas.

a¿=V 1 V 2 or M 2¿= 1

M 1¿

4. Write the Hugoniot equation and explain each terms involved in it.

or

e2 ,e1 = Internal energy

P1 & P2 = Static pressure in station 1 & 2

5. Write the Prandtl-Glauret Rule. .

The rule giving the relation between incompressible and compressible flow coefficient is

known as Prandtl- Glauret Rule. It is the similarity rule which relates incompressible flow

over a given two dimensional profile to subsonic compressible flow over a same profile.

C p=Cpincomp

√1−M 2

C p=coefficient of pressure of compressible flow

M – Mach number

6. Write the Geothert’s Rule.

C p=Cp

'

√1−M 2

C p' =coefficient of pressure

M – Mach number

7. Define Mach waves.

A Mach wave is the weak limit of an oblique shock wave (a normal shock is the other

limit). They propagate across the flow at the Mach angle μ .

8. State Supersonic area rule.

The area rule of supersonic condition states that the cross sectional area distribution of an

airplane, including fuselage, wing tail should have a smooth distribution along the axis of

a plane.

9. Define Critical Mach number.

The free stream Mach number at which sonic speed is first achieved somewhere on the

surface of a body it is called critical Mach number.

10. Distinguish between Lower Critical Mach number and Upper Critical Mach number.

The free stream Mach number at which the entire flow around the surface is in subsonic

flow is called as Lower critical Mach number.

The free stream Mach number at which the entire flow around the surface is in supersonic

flow is called as upper critical Mach number.

11. What are the principles of model testing?

Model size is determined from the rhombus as shown in figure . The model must be

accommodated inside the rhombus formed by the incident and reflected shocks, for

proper measurements.

12. What are the salient features of hypersonic wind tunnels?

supply of high temperatures and pressures for times long enough to perform a

measurement

reproduction of equilibrium conditions

structural damage produced by overheating

fast instrumentation

power requirements to run the tunnel

13. What is a wind tunnel?

Wind tunnel is a device used to measure forces, moments, pressure, shear stress, heat

transfer and flow field (Velocity, pressure, Vorticity, and temperature)

14. Classify the wind tunnels.

Wind tunnel are classified into two types namely

i. Open circuit tunnel

ii. Closed circuit or continuous tunnel

iii. Intermittent blow down tunnel

iv. Indraft tunnel

15. What instruments are used to measure the fluctuating pressures on the models?

The instruments used to measure the fluctuating pressures are

i. Turbulence sphere

ii. Yaw meter

iii. Pressure transducer or scanivalve

16. What is meant by Supersonic flow?

When the fluid free stream velocity is greater than speed of the sound (M >1.2), then the

fluid is said to be highly compressible flow this is known as supersonic flow

17. Differentiate between compressible and incompressible flow

S.No. Compressible flow Incompressible flow

1. Fluid density varies Fluid density remains constant

18. What is shock tube?

The shock tube is a device to produce high speed flow with high temperatures, by

traversing normal shock waves which are generated by the rupture of a diaphragm

separating a high pressure gas from a low pressure gas.

19. What is meant by Blow down tunnel?

Blow down tunnels are normally used from high subsonic to high supersonic flow

conditions. The air is pumped into a closed high pressure chamber upstream of the

plenum. At the same time, air is pumped out of a closed low pressure chamber

downstream of the test section.

20. Distinguish between intermittent blow down and induction tunnel

S.No. Intermittent Tunnel Induction Tunnel

i. Intermittent type energy stored in

the form of pressure in a reservoir

Induction type energy stored in the form

of Vacuum in a reservoir.

ii. It allowed to drive the tunnel only a

few seconds out of each pump hour

No oil contamination in air since pump is

at the downstream end.

21. What is meant by Shock induced separation?

As the free stream Mach number increases above critical Mach number the shock wave

become stronger and moves rearward towards trailing edge. As all the real flows are

viscous, boundary layer develops on the airfoil and due to shock boundary layer

interaction resulting in steep adverse pressure gradient, the boundary layer separates and

wake is formed is known as Shock induced separation or shock stall.

22. What is meant by Shock Stall?

As the free stream Mach number increases above critical Mach number the shock wave

become stronger and moves rearward towards trailing edge. As all the real flows are

viscous, boundary layer develops on the airfoil and due to shock boundary layer

interaction resulting in steep adverse pressure gradient, the boundary layer separates and

wake is formed is known as shock stall

23. List out the characteristics of swept wings.

The causes of pitch up on a swept back planform are

(a) Swept wings are used to increase the critical Mach number

(b) Changed down wash over the tail plane.

(c) Washout due to wing flexure.

24. How is flow over a cone different from flow over a wedge?

25. Write down the critical pressure coefficient from the Prandtl-Glauret rule.

C p=Cpincomp

√1−M 2

C p=coefficient of pressure of compressible flow

M – Mach number

26. What are the assumptions of small perturbation potential theory?

a) Small perturbation is assumed in flow velocity asu' , v ' , w '.

b) The body is streamlined and kept at low angle of attack.

27. Define Oblique shock.

An oblique shock wave, unlike a normal shock, is inclined with respect to the incident

upstream flow direction.

28. Differentiate between shock wave and expansion wave.

S.No. Shock Wave Expansion Wave

1 Velocity decreases Velocity increases

2 Total Pressure decreases Total Pressure remains Constant

3 Static pressure increases Static pressure increases

29. What is meant by Mach angle?

When the object is moving in the flow field with the supersonic speed, it acts as a source

of the pressure disturbance

A Mach wave is the weak limit of an oblique shock wave (a normal shock is the other

limit). They propagate across the flow at the Mach angle μ .

30. What are the methods to prevent boundary layer separation?

Methods of preventing wing tips stalling on swept back wings are

(a) Boundary layer fences.

(b) Leading edge slots.

(c) Boundary layer suction.

(d) Boundary layer blowing.

(e) Vortex generators.

(f) Leading edge extension.

31. Draw the flow around a wedge for subsonic and supersonic flow.

32. Define (i) Zone of action (ii) Zone of silence (iii) Mach Waves (or) Mach lines.

i.The region inside the Mach cone is called as Zone of Action.

ii. The region outside the Mach cone is called as Zone of Silence.

iii. The line at which the pressure disturbance is concentrated and which generate the

cone is called Mach lines or Mach waves

33. What is meant by Transonic and supersonic flow?

The flow region within this Mach number is known as transonic flow (0.8 < M < 1.2 )

The flow region within this Mach number is known as Supersonic flow (1.2 < M < 5)

34. Define (i) Adiabatic process (ii) Reversible process (iii) Isentropic process

(i) Adiabatic process: A process in which no heat is added to or taken away from the

system.

(ii) Reversible process: A process which can be reversed without leaving any trace on the

surrounding, i.e., both the system and the surroundings are returned to their initial states

at the end of the reverse process.

(iii) Isentropic process: A process which is adiabatic and reversible.

35. What is meant by De Laval Nozzle?

A convergent- divergent nozzle used to generate supersonoic flow is sometimes called as

De Laval Nozzle. A Nozzle which does not have an expanding portion can never produce

supersonic flow.

36. Draw the flow pattern of supersonic flow over concave corners.

37. What is meant by Shock Polar?

The locus of all points for turning angle θ values from 0 to θmax representing all possible

velocities behine the shock given in hodograph plane (Vx & Vy ) is called Shock Polar

38. What is meant by linearised two-dimensional supersonic theory?

In linearized supersonic flow, information is propagated along Mach lines where the

Mach angle μ = sin-1 (1/ M∞). Since these Mach lines are all based on M∞, they are

straight, parallel lines which propagate away from and downstream of a body. For this

reason, disturbances cannot propagate upstream in a steady supersonic flow.

39. What are the assumptions used in linearised two-dimensional supersonic theory?

a) Flow is isentropic.

b) Small perturbation is assumed in flow velocity asu' , v ' , w '.

c) The body is streamlined and kept at low angle of attack.

40. What are the practical application of Prandtl-Glauert Rule?

i) It is applied for supersonic flow to calculate coefficient of pressure.

ii) It is applicable for two dimensional bodies.

41. Define Aspect ratio.

It is the ratio of Span of the wing section to the chord length.

42. What is the effect of aspect ratio over the performance of wings?

If the aspect ratio increased for a wing causes increase in lift vale in meantime which is

also increases induce drag generated by wing tip vortices.

43. What are the components on the Transonic wind tunnel?

i. movable plate

ii. Storage tank or reservoir

iii. Air drier

iv. Settling Chamber

44. List out the calibration carried out in the Transonic wind tunnel.

a. Changing the drive pressure ratio

b. Changing the amount of air pumped out of the plenum chamber surrounding the

test section through the use of auxiliary pumping

c. Changing the area of a second throat, i.e., using a 'choke'

d. Changing the contour of a flexible nozzle, or changing fixed supersonic nozzle

blocks.

45. What are the various problems in opertion of transonic tunnel?

a. The pressure to be measured are usually to large to be measured instead of water

or alcohol. The pressure transducers are used.

b. At the higher end of near sonic range, model or probe blockage becomes very

severe problem.

c. Mach number setting and holding are too difficult.

46. What are the components on the supersonic wind tunnel?

i. Compressor

ii. Drier

iii. Reservoir

iv. Quick valve & Throtlling valve

v. Silencer

47. What are the components on the hypersonic wind tunnel?

i. Vaccum feed pump

ii. Vaccum storage tank

iii. Air drier

iv. Settling chamber

v. Nozzle

48. What is meant by helium tunnels?

Helium is an ideal or noble gas. It is definitely far removed from air, which is the

medium bathing aircraft and reentering spacecraft. Helium is monatomic, not diatomic.

Its basic thermodynamic properties differ radically from those of air. The simulation of

upper atmosphere realities was poor.

49. What is meant by Transonic area rule?

Transonic are rule is a simple statement that the cross sectional area of the body should

have a smooth variation along the longitudinal distance of the body. That is, there should

be no rapid or discontiuous changes in the cross sectional area distribution.

50. What is meant by Sonic barrier?

The massive increase in drag encountered at the drag-divergence Mach number is the

technical base of the “sound barrier”.

51. Why does wing tip vortices appears over the wing ?

Wing tip vortices would form only when the wing is producing lift and would

disappear when the wing is not producing lift.

52. List out the practical application of linearised two-dimensional supersonic theory.

a. All these equations are valide for slender bodies like rockets and missiles

b. The small perturbtion equations for subsonic and supersonic flows are linear but

for transonic flows the eqution is nonlinear.

c. These equations are applied for streamline bodies, but not to bluff shapes like

circular cylinder

53. Explain how lift and drag produced in supersonic profiles.

The lift and drag produced by the supersonic profile varies with respect to angle of attack

of the profile

C l=4 αo

√M 12−1

Cd=4 α o

2

√ M 12−1

Α – Angle of attack

Cl – Coefficient of lift

Cd – Coefficient of drag

54. Define center of pressure.

Centre of pressure is a point on the aerofoil chord line through which the resultant

aerodynamic force acts. The movement of centre of pressure with changes in angle of

attack is greater if the camber of the aerofoil is greater.

55. Define Sonic circle.

When plotted in terms of M* the shock polar becomes compact. The circle radius M* = 1

is called the sonic circle.

56. What is meant by Detached shock?

Considering together with the oblique shock elations given by equations thorough, we

can see, for example, that for a fixed Mach number, as is increased,

increase while M2 decreases. However, if increases beyond max the shock wave

becomes detached. Alternatively, for a fixed , as M1 increases from unity, the shock

wave is first detached, then becomes attached when M1 equals that value for which

57. Define Mach Reflection.

Intersection of normal shock and the right running oblique shock gives rise to a reflected left running oblique shock in order to bring the flow into the original direction are called Mach reflection.

58. Write the Prandtl relation of Normal shock wave.

M 2¿ M 1

¿=1

This means

M 2¿= 1

M 1¿

M 1¿ is always greater than unity. Hence M 2

¿ is subsonic this shows that flow

behind the normal shock is always subsonic.

59. Write the Area Mach number relation.

A - cross sectional area of a duct

A* - cross sectional area of a throat section

M – Mach number

60. Write the Area velocity relation?

dAA

= ( M 2−1 ) duu

A – Area of cross section

M- Mach number

U –velocity

61. Explain the function of Area Velocity relation?

i. To accelerate a subsonic flow, the area must decrease.

ii. To accelerate a supersonic flow, the area must increase.

iii. The sonic flow occur at only throat, the minimum cross sectional area of the flow.

62. Define Pressure Coefficient.

The pressure coefficient, based on linearized theory, on a surface inclined at a small angle

e to the free stream is

If the surface is inclined into the free stream, Cl is positive; if the surface is inclined away

from the free stream, Cp is negative.

63. Draw the shock polar for different Mach numbers.

64. What is meant by Reflection and Intersection of Shocks and Expansion waves?

Intersection of normal shock and the right running oblique shock gives rise to a reflected

left running oblique shock in order to bring the flow into the original direction are called

Mach reflection.

65. Define Simple and Non-simple regions.

Supersonic expansion or compression with mach lines which are straight is called simple

region.The intersetion of mach lines of different form , lies leads to non simple region

where all the mach lines are not straight but curved.

66. How pitching moment and center of pressure varies with angle of attack for a supersonic

profile?

67. How lift and drag varies with angle of attack for a supersonic profile?

Coefficient of lift and drag depends upon:

(a) Angle of attack.

(b) Shape of the wing section and planform.

(c) Reynolds Number.

(d) Speed of Sound.

68. Why drag increases drastically over sonic speed?

When the free-stream Mach number exceeds Mcr, a finite region of supersonic flow exists

on the top surface of the airfoil. At a high enough subsonic Mach number, this embedded

supersonic region will be terminated by a weak shock wave. The total pressure loss

associated with the shock will be small; however, the adverse pressure gradient induced

by the shock tends to separate the boundary layer on the top surface, causing a large

pressure drag. The net result is a dramatic increase in drag.

69. How the drag coefficient (CD) varies with Mach number (M)?

70. What is meant by tip effects?

The difference in effective downwash angle for the two tail spans is again probably

caused by an inboard shift of the lift load, for sweptback wings at high angles of attack

when the tips stall.

71. What is meant by Shock tube?

The shock tube is a device to produce high speed flow with high temperatures, by

traversing normal shock waves which are generated by the rupture of a diaphragm

separating a high pressure gas from a low pressure gas.

72. What are the applications of Shock tube?

It is very useful research tool to investigate the shock phenomenon.

It is also useful study the behaviour of materials and objects when subjected to extreme

conditions of pressure and temperature.

A body during reentry into earth atmosphere can be studied with shock tube.

The shock wave and expansion wave may be used to analyze flow conditions in the

shock tube.

73. What is meant by flow visualizations?

Flow Visualisation is a powerful tool in experimental fluid mechanics. The motion of the

fluids can be recognized by making use of techniques by which the flow is made visible

and such techniques are called flow visualisation techniques.

74. What are the data we can get from the flow visualization?

We can able to measure the shock wave angle and expansion waves for the supersonic

flows. The flow pattern over a different shapes can be visualised for the ideal and real

flows. The separation point and wake formation can be analysed due to increase in angle

of attack.

75. List out the various optical flow visualization methods.

i. Schlieren method

ii. Interferometer

iii. Shadowgraphy

76. Write down the compressible Bernoulli’s equation for isentropic flow.

γγ−1

Pρ+ V 2

2=constant

77. What do you meant by Mach reflection ?

When an oblique shock incident on a normal shock reflected shock appears this condition

leads to formation of slip stream.

M1 M2 M3

M1 M4

78. What is mean by Rayleigh flow?

Rayleigh flow is the compressible one dimensional flow through a constant area duct

with heat transfer without considering frictional effect.

79. What is meant by Rayleigh line?

The variation of entropy and temperature given in graphical form for Rayleigh flow is

called Rayleigh line.

80. What is meant by Fanno line ?

Fanno flow is a compressible flow through a constant area duct with frictional effect.

Fanno line is a graphical representation of Fanno flow which given the variation of

temperature and entropy.

81. What is Supercritical aerofoil?

Supercritical airfoils are airfoils developed to increase the increment between the critical

Mach number and the drag divergent Mach number. These airfoils have a relatively

flatter upper surface which makes the shock relatively weaker.

Incident shock Reflected shock

Slip stream

82. What is Supersonic tunnel?

A supersonic wind tunnel is a wind tunnel that produces supersonic speeds (1.2<M<5)

The Mach number and flow are determined by the nozzle geometry. The Reynolds

number is varied changing the density level (pressure in the settling chamber). Therefore

a high pressure ratio is required.

83. What is Schlieren system?

The most used optical system in wind tunnel operation is the schlieren system. This

system gives a picture or an image representative of the variation of density throughout

the complete flow field.

84. What is Shadowgraph method?

Shadowgraph is a flow visualization technique meant for high speed flows with transonic

and supersonic Mach number. This employed for field with strong shock waves.

85. What is Interferometer?

It is an optical technique to visualize high speed flows in the ranges of transonic and

supersonic Mach numbers. This gives a qualitative estimate of flow density in the field.