Define Horizontal Buoyancy ?

It is well known that the static-pressure-coefficient values obtained from tests of axisymmetric bodies in, for example, a large water tunnel indicate the existence of tunnel wall interference. This interference is due to blockage experienced by the body operating within the boundaries of the test section walls and to the skin friction on both the tunnel walls and the surface of the body, which, in turn, causes finite-thickness boundary layers to develop. These factors result in a decrease in static pressure along the test section, which leads to a spurious horizontal buoyancy on the body.

SET of TWO MARK QUESTIONS

TWO MARK QUESTIONS

1. DEFINE MACH NUMBER?

 It’s defined as the square root of the ratio of the inertia force of a flowing fluid

to the elastic force.

Also it is defined as the ratio between Velocity of the Body and the Velocity of Sound.

2. DEFINE REYNOLDS NO?

It’s defined as the ratio of an inertia force of the flowing fluid and the viscous force of the fluid

3. DEFINE EULER’S NO?

It’s defined as the square root of the ratio of the inertia force of a flowing fluid to the pressure force

4. DEFINE WEBER’S NO?

It’s defined as the square root of the ratio of the inertia force of a flowing fluid to surface tension force

5. DEFINE FROUDE’S NO?

It’s defined as the square root of the ratio of the inertia force of a flowing fluid to the gravity force

6. DEFINE BUCKINGHAMS Л THEOREM?

If there are n variables in a physical phenomenon and if this variables contain m fundamental dimension (M, L, T) then the variables are arranged to (n-m) dimensionless terms, each term is called л-term

7. WHAT IS MEANT BY SIMILARITIES?

Model and prototype have similar properties or model and prototype are similar

8. WRITE DOWN THE TYPES OF SIMILARITIES?

 Geometric similarity

 Kinematic similarity

 Dynamic similarity 

9. WHAT ARE THE CLASSIFICATIONS OF WIND TUNNEL?

 Low speed wind tunnel,

 High speed wind tunnel,

 Special type tunnel.

10. WHAT IS FUNCTION OF EFFUSER?

It converts available pressure energy into kinetic energy and its located upstream of the test section.

11. WHAT IS FUNCTION OF DIFFUSER?

It converts the kinetic energy to pressure energy and it’s downstream of the test section.

12. WHAT IS BREATHER?

It is attached with an one way valve so that it take the air and by propeller suction, flow is maintained inside the return type.

13. WHAT ARE THE MERITS AND DEMERITS OF OPEN CIRCUIT?

 Construction cost less,

 It no surging problem is open to the free atm.

 DEMERITS.

 Tunnels much noisy,

 Make cost environment problem.

14. DEFINE ENERGY RATIO.

It is defined as the ratio between the total kinetic energy of the flow to the energy loss.

ER = 1/K₀

15. WHAT ARE THE ADVANTAGES OF BLOW DOWN TYPE WIND TUNNEL?

 This is the simplest among the supersonic tunnel and most economic to build.

 Constant blowing press can be maintaining for considerable running by throttle valve.

16. WHAT ARE THE APPLICATION OF VERTICAL WIND TUNNEL?

 It is used to study the spinning motion of the aircraft,

 Ejection of pilots from seats,

 Parachute flying,

 Helicopter operation.

17. WHAT ARE THE LOSSES IN SUPERSONIC TUNNEL?

 Friction losses,

 Expansion losses,

 Losses in the contraction cone and test section,

 Losses in guide vanes,

 Losses in cooling system.

 18. WHAT ARE THE TYPES OF FLOW ANGULARITIES?

 Sphere type yaw meter,

 Claw type yaw meter.

19. WHAT ARE THE TURBULENT MEASUREMENTS?

 Turbulence sphere,

 Pressure sphere,

 Hot wire anemometer.

20. WHAT ARE THE METHODS TO REDUCE TURBULENCE INSIDE W/T.

 Using max. no of fan blades,

 Using a very long and gradual nacelle,

 Anti swirl vanes,

 Providing max. Possible distance between propellers and test section.

21. PRINCIPLE OF HOT WIRE ANEMOMETER.

The rate of heat from an electrical heated wire and placed in an airstream is

proportional to the velocity.

22. WHAT ARE THE TYPES OF FLOW ANGULARITIES?

 Sphere type yaw meter,

 Claw type yaw meter.

23. WHAT ARE THE TURBULENT MEASUREMENTS?

 Turbulence sphere,

 Pressure sphere,

 Hot wire anemometer.

24. WHAT ARE THE METHODS TO REDUCE TURBULENCE INSIDE W/T.

 Using max. no of fan blades,

 Using a very long and gradual nacelle,

 Anti swirl vanes,

 Providing max. Possible distance between propellers and test section.

25. DEFINE WIND TUNNEL BALANCE

Wind tunnel balance is a device to measure the actual forces &moments acting on a model placed in the test section stream

26. WHAT ARE THE CLASSIFICATIONS OF WIND TUNNEL BALANCE?

 Wire type balance

 Strut type balance

 Platform type balance

 Yoke type balance

 Strain gauge type balance

27. WHAT DO YOU MEAN BY WIRE BALANCE?

In wire type wind tunnel balances only wires are used to support the model. All the load components are transmitted to the measuring device by these wires.

28. WHAT ARE THE CLASSIFICATIONS OF STRUT TYPE BALANCES?

 Yoke type

 Platform type

 Pyramid type

29. WHAT ARE THE TYPES OF STRAIN GAUGE BALANCE?

 Internal balance

 Semi internal balance

 External balance

30. WHAT IS THE PRINCIPLE OF LIQUID MANOMETER?

The principle is that the pressure is balanced by the weight of a liquid column.

31. WHAT ARE THE TYPES OF BAROMETERS?

 Syphon barometer

 Fortin barometer

 Aneroid barometer

32. GIVE SOME DISADVANTAGES OF DIAL TYPE PRESSURE GAUGE?

 They must be calibrated periodically to ensure that they continue to read correctly

 The manometers are less expensive when there is a large number of pressures to be read

 Like manometers, they cannot be easily read electronically

33. WHAT ARE THE TYPES OF PRESSURE TRANSDUCERS?

They are classified as mechanical, electrical & optical type

34. LIST OUT SOME ADVANTAGES OF PRESSURE TRANSDUCERS?

 They provide signal proportional to the applied pressure which can be automatically recorded by acquisition system

 They are relatively low volume devices & consequently respond more rapidly to

pressure changes

 They are small enough to be mounted inside wind tunnel models

35. STATE THE PRINCIPLE OF LDA?

The principle is that a moving particle illuminated by a light beam scatters light at a frequency different from that of the original beam. This difference in frequency is known as Doppler shift & it’s proportional to the velocity of the particle.

36. WHAT ARE THE SCATTERING SUBSTANCES USED FOR LDA?

 Micro polythene spheres

 Diluted milk droplets

 Diluted smoke particles

 Aerosol

 Fine alumina powder

37. WHAT ARE THE ADVANTAGES OF LDA?

 It has high frequency response

 It has negligible probe interference

 It is applicable to the wide range of application

 The measurement with LDA is absolute, linear with velocity and require no pre calibration.

38. GIVE SHORT NOTES ON REFERENCE BEAM SYSTEM?

In reference beam system the scattered beam of light is optically mixed with original  beam & the difference is obtained as the Doppler shift in frequency. This technique is known as heterodyning & it’s the characteristic of a photo multiplier.

39. WHAT IS THE ADVANTAGE OF VORTEX SHEDDING TECHNIQUE?

It is capable of measuring low speeds of air which cannot be measured accurately with a

conventional manometer.

40. DEFINE PATHLINE?

The path of a point or particle convected with the flow is called a path line. If we could

release a tracer particle at any selected point and record its subsequent path, this would be

a path line.

41. DEFINE STREAKLINE?

It is a curve which represents the instantaneous motion of the fluid particle from the

given point.

42. DEFINE STREAMLINE?

A streamline through a point at an instant in time is the line whose tangent is the velocity

at that point and that follows a path through the fluid such that the tangent at every point

is the local instantaneous velocity.

43. DEFINE TIMELINE?

A timeline is generated by simultaneously emitting a short burst of tracers along a line

perpendicular to the local flow. This marks a line of elements that are in a straight line at

the initial time.

44. HOW CAN WE CLASSIFY FLOW VISUALISATION?

The flow visualization can be broadly classified into two, they are

 surface flow visualization

 flow field visualization

 45. WHAT ARE THE KEY ASPECTS OF SURFACE FLOW THAT CAN BE

INVESTIGATED FROM SURFACE FLOW VISUALISATION?

 Key aspects of surface flows that may be investigated using visualization techniques

include

 Stagnation point location

 Separation lines

 Location of boundary layer transition

 Characteristic unsteadiness

 Extent of separation zones

 Types of critical points

46. WHAT ARE THE REQUIREMENTS OF TUFTS?

The Tufts must be of light, flexible material that will align itself with the local surface

flow as a result of direct of direct aerodynamic force.

47. WHAT ARE THE MATERIALS USED FOR MAKING TUFTS?

The most commonly used material is light yarn with weights and lengths chosen

according to model size and test speed.

48. WHAT ARE THE MATERIALS USED FOR MAKING MINITUFTS?

The tuft material is monofilament nylon that has been treated with a fluorescent dye.

49. WHAT ARE THE ADVANTAGES OF USING TUFTS?

 Easily producible

 Once the tufts are installed the model can be repositioned and indications studied visually & photographed for as long as desired.

50. WHAT ARE THE METHODS OF OPTICAL FLOW VISUALISATION?

 Shadow graph

 Schlieren technique

 Interferometer

51. WHAT ARE THE APPLICATION OF SMOKE VISUALISATION?

 Flow over an aerofoil

 Study vortex motion

52. WHAT ARE THE ADVANTAGES OF OPTICAL TECHNIQUES?

 Non intrusive

 Avoiding the formation of unwanted shockwaves

 Avoid problems associated with the introduction of foreign particles

53. WHAT IS THE FUNDAMENTAL PRINCIPLE OF THE INTERFEROMETER?

From corpuscular properties of light, we know that when light travels through a gas the velocity of propagation is affected by the physical properties of the gas.

C=fλ

54. WHAT IS THE FUNDAMENTAL PRINCIPLE OF SCHLIEREN TECHNIQUE?

The speed of a wave front of light varies inversely with the index of refraction of the medium through which the light travels.

Unit III Wind Tunnel Measurements

Pressure Measurements

Introduction

• Pressure measurement is important in many fluid mechanics related applications. 
• From appropriate pressure measurements velocity, aerodynamic forces and moments can be determined. 
• Pressure is measured by the force acting on unit area. Measuring devices usually indicate differential pressure i.e. in relation with atmospheric pressure.This is called gauge pressure. 
• The measured pressure may be positive or negative with reference to the atmospheric pressure .
• A negative gauge pressure is referred to as vacuum.

Unit II Hypersonic Wind Tunnel

Unit II Hypersonic flow

SET II SHORT QUESTIONS

1. What do you mean by dimensionless numbers? Name some of it.

Dimensionless numbers are those numbers which are obtained by dividing the inertia force by viscous force or gravity force or pressure force or surface tension force or elastic force.  As this is a ratio of two forces, it will be dimensionless number.  These dimensionless numbers are also called non – dimensional parameters.

The following are the important dimension less numbers:

    1.      Reynolds numbers

2. Froude’s number
3. Euler’s number
4. Weber’s number
5. Mach’s number

2. What are the different laws on which models are designed for dynamic similarity?

Model laws or laws of similarity are the laws  on which the models are designed for dynamic similarity.  Models are designed on the basis of ratio of the force, which is dominating in the phenomenon.

The following are the model laws:

1. Reynolds model law
2. Froude model law
3. Euler model law
4. Weber model law
5. Mach model law 

3. Explain the terms: distorted models and undistorted models.  What is the use of distorted models?

Undistorted Models: Undistorted models are those models which are geometrically similar to their prototypes or in other words the scale ratio for the linear dimensions of the model and its prototype is same.  The behaviour of the prototype can be easily predicted from the results of undistorted model.

Distorted Model:

            A model is said to be distorted if it is not geometrically similar to its prototyped model. Different scale ratios for the linear dimensions are adopted.  For example, incase to rivers, harbours, reservoirs etc.  two different scale ratios, one for horizontal dimensions and other for vertical dimensions are taken. Thus the models of revivers, harbors and reservoirs will become distorted models.

The followings are the advantages of distorted models.

1. The vertical dimensions of the model can be measured accurately.
2. The cost of the model can be reduced.
3. Turbulent flow in the model can be maintained. 

4. Write the drawbacks of analytical methods.

            The following are the disadvantages of analytical methods used for study and analysis of many problems in fluid mechanics.

1.          It involves a number of approximations and assumptions and hence application of the analytical methods are restricted.

2.                  It involves highly complicated equations which cannot be solved.

3.                  The solutions to various complex flow patterns cannot be obtained by analytical methods alone.

4.              It is impossible or impracticable in some cases to make a satisfactory and complete mathematical analysis of the problems 

5. What you mean by hydraulic similitude?

            The observations made on the performance of the model are useful to predict the performance of the prototype. Hence it is very necessary that the model should represent the prototype in every respect i.e. the model should represent the prototype should have similar properties. The similarity between a prototype and its model is called similitude.

            For absolute similitude between a model and the prototype the following types of similarities should exist.

 a)      Geometric similarity

 b)      Kinematics similarity and

 c)      Dynamic similarity 

6. What are the demerits of distorted models?

The following are the demerits of distorted models:

i)                    Due to unequal horizontal and vertical scales the pressure and velocity distribution are not truly reproduced in the model.

ii)                  The wave pattern in the model will be different from that in the prototype due to depth distortion.

iii)                Slopes, bends and earth cuts are not truly reproduced.

7. What do you mean by scale effect?

            This is a defect which occurs in certain models due to which the computed properties of the prototype from model experiments deviate much from the actual properties of the prototype.

            For example, a model cannot match with prototype if it large depths, high velocities, surface tension factor, flow conditions and force. Here the models do not have exact properties with prototype. Hence, the scale effect occurs.

8. What are the different types of forces acting in moving fluid?

Types of Forces Acting in Moving Fluid:

For the fluid flow problems, the forces acting on fluid mass may be any one, or a combination of several of the following forces :

1. Inertia Force F_i.
2. Viscous force F_v.
3. Gravity force F_g.
4. Pressure force F_p.
5. Surface tension force,
6. F_s Elastic force F_e.

1. Inertia Force (F₁) :

 It is equal to the product of mass and acceleration of the flowing fluid and acts in the direction opposite to the direction of acceleration. It is always existing in the fluid flow problems.

2. Viscous Force (F_v) :

 It is equal to the product of shear stress (t) due to viscosity and surface area of the flow. It is present in fluid flow problems where viscosity is having an important role to play.

3. Gravity Force (F_g) :

 It is equal to the product of mass and acceleration due to gravity of the flowing fluid. It is present in case of open surface flow.

4. Pressure Force (F_p)  :

 It is equal to the product of pressure intensity and cross sectional area of the flowing fluid. It is present in case pipe flow.

5. Surface Tension Force(F_s) :

 It is equal to the product of surface tension and length of surface of the flowing fluid.

6. Elastic Force (F_e) :

 It is equal to the product of elastic stress and area of the flowing fluid.

For a flowing fluid, the above – mentioned forces may not always be present. And also the forces, which are present in a fluid flow problem, are not of equal magnitude. There are always one or two forces which dominate the other forces. These dominating forces govern the flow of fluid.

UNITS and DIMENSIONS for few important terms

S.No		Quantity	Unit generally adopted	DIMENSIONS
				MLT SYSTEM	FLT SYSTEM
		Geometric
1.		Length	M	L	L
2.		Area	M2	L2	L2
3.		Volume	M3	L3	L3
Kinematic
5		Time	Sec	T	T
6		Velocity (linear)	M/sec	LT-1	LT-1
7		Velocity (angular)	Rad / sec2	T-1	T-1
8		Acceleration (linear)	M/sec	LT-2	LT-2
9		Acceleration (angular)	Rad /sec2	T2	T2
10		Discharge	Cum /sec	L3 T-1	L3T-1
12		Kinematic velocity	M/sec2	L2 T-1	L2T-1
Dynamic
13	Mass		Kg	M	FL-1 T2
14	Force		Newton	MLT-2	F
15	Weight		Newton	MLT-2	F
16	Mass density		Kg /cum	ML-3	FL-4T2
17	Specific weight		Newton/cum	ML-2T2	FL-3
18	Dynamic viscosity		Newton /cum	ML-1T-1	FL-3T
19	Surface tension		Newton/m	MT-2	FL-1
21	Pressure		Newton/m2	ML-1T2	FL-2
26	Power		Newton /sec	ML2T3	FLT-1