Digital in-line holography for the extraction of 3D trajectories of small particles in a 2-D Benard-von Karman flow
Résumé
Digital In-line Holography is widely used to visualize fluid flows seeded with small particles. Such holograms record directly the far-field diffraction patterns of particles on a CCD camera. From the successive reconstruction planes, the three-dimensional location of the particles can be determined. This imaging system doesn't need focusing. The principle is based on the direct analysis of the diffraction patterns by mean of space-frequency operators such as Wavelet Transformation or Fractional Fourier Transformation. This method, already tested in our laboratory, leads to a better resolution than classical holography for the estimation of 3D particle locations (50 mu m instead of 0.5mm in depth). In the case of moving particles, it is interesting to illuminate the sample volume by several laser pulses. This can be easily realized by controlling the input current of a modulated laser diode. Then, the CCD camera cumulates the sum of in-line particle holograms recorded at different times. By searching for the best focus plane of each particle image, the 3D coordinate of each particle can be extracted at a given time. This technique is applied to deter-mine trajectories of small particles in a well-controlled 2D Benard-von Karman street allowing a Lagrangian approach. Preliminary results are presented.