1. Optical Measurement Techniques
K. Muralidhar
Department of Mechanical Engineering
Indian Institute of Technology Kanpur
Kanpur India

2. Three dimensional reconstruction Unsteady fields
Keywords
Laser measurements
Three dimensional reconstruction
Unsteady fields
Examples and applications

3. Radiation Measurement Techniques
Field-scale measurement
Excellent temporal and spatial response
Expensive
Examples of radiation sources:
lasers
white light
X-rays
microwaves
infrared/thermal

4. Optical Techniques Non-intrusive in nature. Inertia free.
Source of field information.
Availability of lasers and high speed computers make optical techniques an effective tool for quantitative measurements and not merely as visualization tool

5. PROPERTIES OF LASERS High intensity Monochromatic Directional
Point source
Coherent

6. Classification Transmission (refractive index technique)
Reflection based
Scattering

7. REFRACTIVE INDEX METHODS
Lorentz-Lorenz relationship for transparent media:
For gases:

8. Optical techniques: a comparison
Color Schlieren:
Interferometry:
Relatively easier to align
Requires fabrication and
calibration of color filter
Avoids camera saturation
Difficult to align
Requires a reference chamber
Requires laser as light source
Easy quantitative analysis
Monochrome Schlieren:
Shadowgraph:
Relatively easier to align
Highly sensitive to external disturbances
Requires knife-edge
Quantitative analysis relatively difficult
Very easy to align.
Less sensitive to disturbances
Data analysis is difficult.

9. Interferometry, schlieren, shadowgraph

10. Mach-Zehnder Interferometer

11. Principle of Fringe Formation
Works on the principle of formation of dark and bright fringes, due to constructive/ destructive interference of the test and reference beams.
Infinite fringe setting
Candle flame image

12. Interference

13. Interferograms (infinite and wedge)

15. Z-type schlieren set-up

16. Principle of operation
Works on the principle of deflection of ray of light when it passes through a medium in which there is a component of gradient of refractive index normal to the ray.
Knife-Edge horizontal
Two-dimensional thermal field in (x-y) plane
Laser beam in z-direction
Vertical shift of the light beam above the knife-edge

17. Candle Flame
Initial Condition
Near Wake
Far Wake

18. Knife-edge horizontal
Schlieren Analysis
Knife-edge horizontal
For a two-dimensional field:
The above equation can be evaluated to get the values of the
refractive index and then related to density, temperature and
concentration information.

20. Shadowgraph Technique

21. Image records the second derivative of refractive index field.
Shadowgraph system measures the displacement and the angular deflection of the emerging light ray.
Image records the second derivative of refractive index field.
Initial setting
Candle flame image

24. Experimental set-up for benchmarking experiment

25. Experimental images for higher Ra

26. Numerical
Experimental
Numerical
Experimental
Interferometry
Schlieren

27. Comparison of experimental results with numerical simulation
Ra = 58,000
Ra =29,000
Ra = 14,500

28. Two wavelength interferometry

29. Color schlieren set-up

30. Development of color filter

31. 2-D axisymmetric filter
Typical Color Filters
1-D filter
2-D axisymmetric filter

33. Typical color schlieren Images
Melting ice cube in water

34. after Kline et al., Optics and Lasers in Engineering, 2005

35. Case studies Transient heat conduction Salt dissolving in water
Eccentric annulus
Flow past a heated cylinder
Crystal growth
Non-destructive testing

36. Transient heat conduction

37. Salt dissolving in water

38. Interferograms in an eccentric annulus

39. Oil floating over water – natural convection

40. Eccentric annulus filled with high viscosity silicone oil

41. Vertical flow facility at IIT Kanpur

42. Mixed convection from a circular cylinder

43. Effect of heating
and inclination on
vortex shedding from
a square cylinder.

44. Flow past an oscillating square cylinder
No oscillation
Fundamental
frequency
First harmonic
AR= Amplitude ratio
Reynolds number=114

45. Crystal Growth and Buoyancy-Driven Convection
In the solution, the dots are randomly mixed but next to the crystal, the dots are mostly black.
Solute nearest the crystal attaches to the surface, leaving behind less dense water which is buoyant and rises.
Black dot – Water Molecules
White dot – Solute Dissolved

46. Experimental apparatus

47. Effect of Crystal Rotation (Ramp rate= 0.05oC/h)
1 hour
10 hours
1 hour
5 hours
hours
40 hours
90 hours
45 hours
No rotation
With rotation

48. Path integrals Refractive index methods yield path integrated data.
Integration is along the path of light beam.
Recovering local information from the integral is called tomography.

50. Tomography algorithms
Direct (convolution back projection)
Iterative (algebraic reconstruction technique)

51. Incomplete data
Tomography is an inverse calculation and is sensitive to quality of measurements.
For incomplete data, there is need for developing extrapolation techniques.

52. An example
Object
Radiograph
Sinogram

53. Example of an object with an internal flaw
After correction
Reference object
Uncorrected

54. Experimental apparatus for crystal growth

55. Mass flux distribution on selected planes
y/H = 0.05
y/H = 0.10
y/H = 0.25
y/H = 0.45
y/H = 0.60
y/H = 0.75

56. Tomographic Reconstruction of concentration gradients
Relatively high gradients near the vicinity of
the side faces as compared to the top face for
the planes near the crystal.
Gradients are concentrated predominantly
along the path of the rising plume, indicative
of the width of the plume.
2 hours
30 hours
y/H=0.05
y/H=0.45
y/H=0.75
Dark shade – low gradients
Light shade – high gradients
Rising convective plume maintains its finite width up to the bulk of the
solution with gradients relatively higher along its boundaries as compared
to the core region.

57. Difficulty with unsteady fields
Data recorded at various angles are uncorrelated with one another.
Correlation can be regained if the field is periodic.
Else, a proper orthogonal decomposition route is recommended.

58. Proper orthogonal decomposition (POD)
A function z(x,t) may be represented as
Such a representation is not unique; the functions φk(x) may be chosen in several ways such that φ’s are orthogonal and the approximation is the best possible for every M
Under these conditions the functions φ are called the Proper Orthogonal Modes and the expression called the Proper Orthogonal Decomposition (POD)

59. POD modes of reconstructed data1 3 2

60. Closure
Interferometry provides information on the refractive index (and hence) temperature field itself.
Schlieren and shadowgraph record derivatives of the refractive index fields.
When the image data is analyzed, a considerable amount of information about the flow and thermal fields can be recovered.

61. Scattering methods Liquid crystal thermography
Particle image velocimetry
Fluorescence
Mie scattering
Rayleigh scattering
Raman scattering

62. A typical LC coated heated plate
Liquid Crystals, LCs ?
A typical LC coated heated plate
Temperature o C

63. Liquid Crystal Thermography
Transient liquid crystal images
Heat transfer coefficient

64. Particle Image Velocimetry (PIV)
Light scattering particles are added to the flow.
A laser beam is formed to a light sheet and the particles are illuminated by two pulses separated by a short time interval t.
If the particle in the flow field moves x in x-direction and y in y-direction in time t, the velocity in the x- and y-direction are: u= x/t, v= y/t

65. Principle of PIV (contd.)
Light scattering particles are added to the flow.
A laser beam is formed to a light sheet and the particles are illuminated by two pulses separated by a short time interval t.
If the particle in the flow field moves x in x-direction and y in y-direction in time t, the velocity in the x- and y-direction are: u= x/ t, v= y/ t

66. Correlation Function Image at t Image at t+ t
PIV IMAGE ANALYSIS PROCEDURE

67. PIV post processing Spurious vector Raw Image 1 Correlation function
Time t
Spurious vector
Raw Image 1
Time t+Δ t
Correlation function
Raw Image 2
Interpolated
vector
Filtering

68. Various aspects of PIV measurement
Seeding
Time separation
Calibration

69. Bluff body wakes

70. Near wake particle traces at different cylinder orientations
Near wake particle traces at different cylinder orientations. AR=28, Re=410
30o
45o 0o
22.5o

71. Spanwise particle traces at different cylinder orientations
Spanwise particle traces at different cylinder orientations. AR=28 and Re=410

72. Appearance of secondary vorticity (comparison with Rockwell, 1996)

73. Time-averaged velocity vectors
Flow

74. Time-averaged streamline contours. AR=28, Re=410

75. Time-averaged vorticity contours. AR=28 & Re=410.

76. Quantum optics
Rayleigh scattering
Fluorescence

77. Raman scattering

78. Fluorescence images

79. Light (laser assisted) detection and ranging (LIDAR/LADAR)

80. Synthetic jet
fluorescence

81. Rayleigh scattering
Water drops in an over-expanded jet

82. Raman spectra

83. Closure LCT yields surface temperature data as a function of time.
PIV obtains velocity field over a plane as a function of time.
Fluorescence identifies regions of high concentration and hence high vorticity.

84. Acknowledgement Students Debasish Mishra Atul Srivastava
Sushanta Dutta
Andallib Tariq
Sunil Verma
Dhruv Singh
Susheel Bhandari Financial support
DST, BRNS, DRDO
Colleagues
P.K. Panigrahi
P. Munshi
Y.M. Joshi

85. Thank you!