Anamorphic optical processing of multiple-exposure speckle photographs

Area of Science:

• Fluid dynamics
• Optical metrology

Background:

• Traditional speckle photography analysis methods include point-by-point velocity determination via 2D Fourier analysis and spatial filtering for constant velocity contours.
• These methods have limitations in providing comprehensive velocity data across a flow section.

Purpose of the Study:

• To introduce and demonstrate a new method for analyzing double-exposed speckle photographs using an anamorphic optical system.
• To measure a single velocity component along a specific line within a flow field.

Main Methods:

• Utilizing a laser sheet, thin in the x-direction and extended in the y-direction, to illuminate a line in the flow.
• Employing an anamorphic optical system to perform a 1D Fourier transform in the x-direction and image the speckle pattern in the y-direction.
• Analyzing the resulting curved fringes, where local spacing is inversely proportional to the x-velocity component.

Main Results:

• The anamorphic optical system successfully measures the x-velocity component along a probed line.
• The method generates curved fringes whose spacing directly correlates with the velocity component.
• This approach allows for velocity measurement along a selected line, differing from contour-based methods.

Conclusions:

• The demonstrated anamorphic optical method offers a viable alternative for analyzing fluid velocity from speckle photographs.
• It enables the measurement of a velocity component across a section of flow, providing localized data.
• This technique enhances the capabilities of speckle photography for fluid flow analysis.