Laser speckle image velocimetry by Fast Fourier Transform technique

In this paper, laser image velocimetry, a unique optical method for the velocity measurement of fluid flow has been validated. A laser sheet is illuminated on microscopic seeded particles to produce the speckle pattern at the recording plane. Double frame-single exposure speckle images are captured in such a way that the second speckle image is shifted exactly in a known direction. The auto-correlation method has an ambiguity to identify the direction of flow. To sort out this problem, the partial shift of the second image has been premeditated. Cross-correlation of sub interrogation areas of the speckle patterns is attained by the Fast Fourier Transform technique. Segmented four sub-windows are processed to acquire the velocity information with a vector map precisely.