Density Field Measurement of Thermal Convection by Digital Laser Speckle Photography

摘要

Laser speckle photography is a well-established optical method for quantitative measurements in solid and fluid flow with wide dynamic range. In conventional technique of this method, an analogue processing based on auto-correlation evaluation has been applied to reconstruct deformation, velocity or density gradient by using optical Fourier transformation of a double-exposed laser speckle pattern recorded on a photographic film. This technique can be improved in the spatial resolution, in the dynamic range and in the efficiency of image processing by applying the digital cross-correlation evaluation between the reference and the object speckle patterns, which are separately recorded. It is called digital laser speckle photography. In practical procedure of this method, both of the reference and the object speckle pattern are recorded by using a digital still camera or CCD camera. The same algorithm with cross-correlation evaluation in PIV is applied to reconstruct the distribution of the displacement of local speckle patter from the digital images of speckle patterns. The accuracy and the dynamic range in practical measurements can be evaluated for practical devices used and experimental conditions. This method is applied to density field measurements of thermal convection constrained acoustically in a horizontal duct, and the effect of sound field on thermal convection is discussed.