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.
Density field of Mach reflection of shock waves was investigated by a laser speckle method. A 2D wedge was mounted on the test section of shock tube. When a plane shock wave attack the wedge, shock wave is reflected by the wedge and Mach reflection takes place. Density gradient in the internal region behind the reflected shock wave was measured by this method. The gradient of density plays an important role to determine the reflected shock configuration, especially in weak Mach reflection. Not only the non-uniformity might be a reason 'von Neumann paradox', but also should be important in the case of moderate Mach number. In the present report, density field near the triple point was studied mainly. Optical arrangement is the same as that in Erbeck and Merzkirch. A shock tube and YAG laser were employed in the experiment. The reference and flow fields were recorded in a film. The speckle photographs were processed by auto-correlation analysis by a computer. Displacement of speckle pattern was convected to the density by using a deflection angle-density gradient relation. It was found that the distribution of density field has large gradient near the reflected shock wave. Experimental density fields were compared with numerical results.