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Optical Measurement Techniques
K. Muralidhar Department of Mechanical Engineering Indian Institute of Technology Kanpur Kanpur India
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Three dimensional reconstruction Unsteady fields
Keywords Laser measurements Three dimensional reconstruction Unsteady fields Examples and applications
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Radiation Measurement Techniques
Field-scale measurement Excellent temporal and spatial response Expensive Examples of radiation sources: lasers white light X-rays microwaves infrared/thermal
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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
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PROPERTIES OF LASERS High intensity Monochromatic Directional
Point source Coherent
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Classification Transmission (refractive index technique)
Reflection based Scattering
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REFRACTIVE INDEX METHODS
Lorentz-Lorenz relationship for transparent media: For gases:
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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.
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Interferometry, schlieren, shadowgraph
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Mach-Zehnder Interferometer
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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
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Interference
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Interferograms (infinite and wedge)
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Z-type schlieren set-up
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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
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Candle Flame Initial Condition Near Wake Far Wake
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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.
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Shadowgraph Technique
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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
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Experimental set-up for benchmarking experiment
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Experimental images for higher Ra
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Numerical Experimental Numerical Experimental Interferometry Schlieren
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Comparison of experimental results with numerical simulation
Ra = 58,000 Ra =29,000 Ra = 14,500
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Two wavelength interferometry
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Color schlieren set-up
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Development of color filter
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2-D axisymmetric filter
Typical Color Filters 1-D filter 2-D axisymmetric filter
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Typical color schlieren Images
Melting ice cube in water
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after Kline et al., Optics and Lasers in Engineering, 2005
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Case studies Transient heat conduction Salt dissolving in water
Eccentric annulus Flow past a heated cylinder Crystal growth Non-destructive testing
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Transient heat conduction
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Salt dissolving in water
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Interferograms in an eccentric annulus
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Oil floating over water – natural convection
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Eccentric annulus filled with high viscosity silicone oil
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Vertical flow facility at IIT Kanpur
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Mixed convection from a circular cylinder
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Effect of heating and inclination on vortex shedding from a square cylinder.
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Flow past an oscillating square cylinder
No oscillation Fundamental frequency First harmonic AR= Amplitude ratio Reynolds number=114
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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
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Experimental apparatus
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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
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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.
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Tomography algorithms
Direct (convolution back projection) Iterative (algebraic reconstruction technique)
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Incomplete data Tomography is an inverse calculation and is sensitive to quality of measurements. For incomplete data, there is need for developing extrapolation techniques.
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An example Object Radiograph Sinogram
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Example of an object with an internal flaw
After correction Reference object Uncorrected
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Experimental apparatus for crystal growth
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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
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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.
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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.
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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)
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POD modes of reconstructed data
1 3 2
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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.
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Scattering methods Liquid crystal thermography
Particle image velocimetry Fluorescence Mie scattering Rayleigh scattering Raman scattering
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A typical LC coated heated plate
Liquid Crystals, LCs ? A typical LC coated heated plate Temperature o C
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Liquid Crystal Thermography
Transient liquid crystal images Heat transfer coefficient
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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
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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
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Correlation Function Image at t Image at t+ t
PIV IMAGE ANALYSIS PROCEDURE
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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
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Various aspects of PIV measurement
Seeding Time separation Calibration
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Bluff body wakes
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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
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Spanwise particle traces at different cylinder orientations
Spanwise particle traces at different cylinder orientations. AR=28 and Re=410
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Appearance of secondary vorticity (comparison with Rockwell, 1996)
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Time-averaged velocity vectors
Flow
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Time-averaged streamline contours. AR=28, Re=410
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Time-averaged vorticity contours. AR=28 & Re=410.
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Quantum optics Rayleigh scattering Fluorescence
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Raman scattering
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Fluorescence images
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Light (laser assisted) detection and ranging (LIDAR/LADAR)
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Synthetic jet fluorescence
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Rayleigh scattering Water drops in an over-expanded jet
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Raman spectra
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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.
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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
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Thank you!
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