Optical Methods
When the wavelength of the radiation used is in the visible range, the measurement procedure classifies as an optical technique and the region being scanned appears on a screen as an image that is visible to the naked eye. In thermal sciences, a revival of optical techniques for temperature and velocity measurements in fluids has occurred. Possible reasons for this developments are:
- Commercially available lasers have a high degree of coherence (both spatial and temporal) and are cost-effective.
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- Optical images can be recorded conveniently through computers and can be processed as a string of numbers through numerical algorithms.
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The implications are that coherence generates stable image patterns, which truly reflect the flow behavior, and computer programs now simplify and replace very tiring microscope operations The image formation can be related to the patterns formed by solid particles suspended in the fluid, attenuation of radiation, scattering or the dependence of reflectivity and refractive index on temperature. Optical methods that utilize the dependence of refractive index of light on quantities such as density, concentration and temperature can be configured in many different ways. Three available routes are:
1. Shadowgraph, where the reduction in light intensity on beam divergence is employed,
2. Schlieren, where light deflection in a variable refractive-index field is captured, and
3. Interferometry, where the image formation is related to changes in the refractive index with respect to a reference environment.
For a wide class of applications where temperature differences are within certain bounds, interferometry appears to be a versatile tool for accurate measurement of three-dimensional unsteady temperature field, and with some modification, for velocity fields. Many examples involving free, mixed, and forcedconvective heat transfer are incleded in this category. worth mentioning is the satellite-based imaging of the planetary atmosphere using coherent optics. In this context, issues such as evaluation of model constants in weather pridiction codes, and stablizing thesecodes using images of the flow field are being addressed.
The author's experience is with a Mach-Zehnder interferometer for studying two-and three-dimensional temperatur fields in buoyancy-driven flows with air as the working fluid. Steady as well as slowly evolving fields have been considered. Flow configurations that have been addressed are:
The Mach-Zehnder interferometer for studying two-and three-dimensional temperature fields in buoyancy-driven flows with air as the working fluid. Steady as well as slowly evolving fields have been considered. Flow configurations that have been addressed are :
1. Natural convection from a discrete protruding heater mounted on a vertical wall.
2. Rayleigh-Benard convection in a two-dimensional square cavity.
3. Tomographic reconstruction of three-dimensional temperature field using interferograms.
4. Rayleigh-Benard convection in a horizontal fluid layer at high rayleigh numbers.
5. Buoyancy-driven convection in axisymmetric geometries.
The Mach-Zehnder interferometer used in the present work employs a .png) He-Ne laser and .png) (diameter) optics. Interferograms are recorded using a CCD camera with a .png) pixel resolution. The camera is interfaced with a PC through an 8-A/D card which digitize light intensity levels over a range of 0 to 255. Image acquition is at video rates .png)
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