Forward-scattering particle-image velocimetry (FSPIV) is a technique for measuring velocities in microscopic flow fields as thin liquid films. As in other particle-image-velocimetry (PIV) techniques, the velocity of a flow is computed from temporally changing images of suitably illuminated, neutrally buoyant particles entrained in the flow. Unlike other PIV techniques, (1) particles are back-illuminated with coherent or partially coherent light, (2) a microscope is used to resolve forward scattering of light from the particles, and (3) all three components of velocity can be measured from only one line of sight, without readjustment of optics.
Figure 1 illustrates a laboratory setup for FSPIV. Illumination is provided by a microscope-illumination source equipped with a wavelength-selective filter. Partial coherence is obtained by closing the aperture on the illumination source. Microscopic spherical seed particles of known diameter are suspended in the liquid. A transmitted-light microscope is aimed through the cell back toward the source and is focused to a suitable depth within the cell. Forward-scattered light from a particle in the field of view of the microscope is brought to focus on the microscope image plane to obtain a magnified image of the particle. A charge-coupled device (CCD) is mounted at the image plane to acquire the image. The CCD output is digitized, then processed as described below to extract information about the three-dimensional velocity of the particle.
Coherent illumination is needed because it makes the forward scattering of light (including such effects as diffraction from particles and phase changes in microscope lenses) amenable to analysis. Highly coherent illumination like that provided by a laser results in speckle noise; partially coherent illumination is preferred in this application because it can yield diffraction patterns adequate for analysis while generating much less speckle noise.
Figure 2 schematically illustrates how images are processed to extract velocities. First, a frame of image data is acquired at a known sampling time. Next, the image of each particle is separated from other particles. The scattering pattern for each particle is analyzed to determine the distance of the particle, relative to the focal plane, along the line of sight. This aspect of the analysis involves comparisons of the observed scattering pattern with scattering patterns that have been computed theoretically and/or recorded experimentally for known positions relative to the focal plane. The comparisons and the interpolations between known positions can be performed by neural-network software that has been trained on the known scattering patterns. Then the component of velocity of each particle along the line of sight is calculated as the distance between two positions determined in this way for two sampling instants, divided by the time between the instants.
The components of velocity in the plane perpendicular to the line of sight are determined by tracking particle-image centroids as they move across the field of view between successive frames. The tracking of particle-image centroids as a function of time is an established PIV practice; techniques for tracking particle-image centroids as functions of time have been reported in a number of previous articles in NASA Tech Briefs. An added benefit of FSPIV is that the scattering is centro-symmetric so that the centroid is found with high accuracy.
This work was done by Ben Ovryn of NYMA, Inc., and John D. Khaydarov of Ohio Aerospace Institute for Glenn Research Center.
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