A method for making a flow visible in a plane and determining in-plane velocities involves the digitization and digital processing of a sequence of monochrome video images of particles entrained in the flow. The particle-laden flow passes through a sheet of laser or lamp light in the plane, which is oriented perpendicularly to the viewing direction. As described thus far, this method involves the use of techniques that have become common in the flow-visualization art and that have been described in numerous articles in NASA Tech Briefs. The unique aspects of the present method originate in the following critical digital image-processing steps:

  • In a grey-scale preprocessing stage, each image is enhanced to visually separate the particles from the background. Preprocessing is accomplished by one or more techniques that can include simple semi-thresholding, percentile semi-thresholding, and/or a top-hat morphological transformation. The choice of preprocessing technique(s) depends on the quality of the original image.
  • A different known false color is assigned to each video frame in the sequence, and the frames are superimposed into a single composite, time-lapse color video image. The information from each frame in the composite image can be distinguished by color from the information from the other frames and thereby identified by color as having been recorded at a unique observation instant in a sequence of observation (video-frame) instants.

The color sequence must be chosen so as not to give rise to aliases in a hue subset. For example, hues of green, yellow, and red could result in a yellow spot for both a particle that was stationary for the full exposure and a particle that just moved through the light sheet during the "yellow" time step. These yellow spots would be indistinguishable. The use of a color sequence from magenta to cyan obviates this "yellow" ambiguity.

This Composite Color Time-Lapse Image was made from a sequence of eight false-color versions of original video frames that show particles entrained in a flammable liquid that was stirred into a vortex by a flame (not shown) moving to the right across the top. The border of each original frame shows the color assigned to that frame in the composite image. The eight frames were recorded sequentially during a total elapsed time of 0.267 second.

In older methods of streak photography and time-lapse imaging, there is no way to determine which way a particle moved along its image streak; there is also no way to determine whether a particle remained in the light sheet during the entire observation time or, alternatively, when it entered and left the light sheet. In the present method, the color of each streak changes from one end to the other in the known sequence, giving a clear indication of the direction of the motion of the particle. If the entire sequence of colors is present in a streak, then the particle can be assumed to have remained within the light sheet during the entire sequence of frames; if any colors are missing, then the times when the particle entered and left the light sheet can be determined from the colors at the ends of its streak image. The missing-color information also provides a qualitative indication of the degree of flow in directions other than in the illuminated plane.

The average in-plane speed of a particle can be computed as the length of its image streak divided by the total elapsed time of the frame sequence. In analyzing streaks to extract velocities (speeds and directions), one might wish to reject streaks with missing colors on the ground that the corresponding particles did not spend the full elapsed time in the light sheet. Optionally, one could use these streaks, provided that the times used in the denominators for computing speeds are the partial elapsed times determined from the missing colors.

Unlike in conventional streak photography, there is no need for a priori knowledge of exposure times. Different parts of an image can be processed differently, if necessary. A composite exposure time can be chosen after a test.

By use of a digital frame recorder, images processed by this method can be rerecorded onto video tape to make digital movie sequences that can be viewed on computer workstations. These movies show moving particle streamlines (or pathlines in unsteady flows), rather than only moving particles; in so doing, the movies make it much easier to visualize the flows.

This work was done by Fletcher J. Miller of Case Western Reserve University and Mary B. Vickerman and Howard D. Ross of Lewis Research Center.


Photonics Tech Briefs Magazine

This article first appeared in the March, 1999 issue of Photonics Tech Briefs Magazine.

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