Streak-Based Volumetric Flow Reconstruction

Christina Tsalicoglou and Thomas Rösgen

Abstract

Volumetric velocimetry techniques for fluid flows, such as 3D Particle Tracking Velocimetry (3D-PTV) and Tomographic Particle Image Velocimetry (Tomo-PIV), use the projected image of tracer particles in two or more camera views to reconstruct the tracers’ positions in 3D space, measure their displacement over time and retrieve the flow velocity. The 3D-reconstruction step can result in so-called “ghost particles,” which are reconstructed tracer particles for which it remains uncertain whether they are real or artifacts of the reconstruction process. While 3D-PTV relies on the reconstruction of points that represent “frozen” particles, a related but less commonly used technique, 3D Particle Streak Velocimetry (3D-PSV), uses long-exposure images where the signature of each particle is its pathline over the exposure time, a “streak.”
This work leverages the topological information provided by long-exposure imaging in 3D-PSV to reduce the number of ambiguous particle reconstructions. The use of information about the pathline shape allows the reduction of the required camera views and frame rate, while it can be beneficial in analyzing flows with large dynamic velocity range.

Streak reconstruction

We consider the signature of the imaged particles as points, lines, or curved segments and assess the number of ambiguous reconstructions generated in each case. We analyze theoretically why using the streak shape in the 3D reconstruction step results in fewer ambiguities and develop methods for streak identification, matching, and reconstruction.

Enlarged view: Condition for ghost streak generation — Condition for ghost streak generation: more than one streaks must be connecting two epipolar planes. Here, from two streaks per image four reconstructions are possible (red and blue streaks in 3D), of which all, three, or only two, e.g. the blue ones, might be real 3D streaks.

Results

Our simulation results agree well with our model and the number of ambiguities is clearly reduced when reconstructing streaks instead of points (figure below). Different displacement scenarios result in a different number of ghost streaks, which however remain consistently below the number of ghost particles. Considering the shape of the streaks further reduces the number of ambiguities.

Example Case: Vortex Ring

The case of a vortex ring is used to demonstrate the capabilities of 3D-PSV in ambiguity reduction. Three cameras record the seeding particles, and the different reconstruction methods are employed to assess their effectiveness in ghost streak reduction. Our streak processing and reconstruction methods are applied to the acquired images, providing a complete pipeline for 3D-PSV using curve reconstruction.

References

external page Tsalicoglou, C. and Rösgen, T., Ghost particle reduction in particle streak velocimetry, 14th International Symposium on Particle Image Velocimetry, 2021.