Three-dimensional velocity field measurements

In order to measure three-dimensional velocity fields, the fluid is seeded with particles which are then illuminated by lasers or diodes. The light emitted by the particles is recorded by one or more cameras. From these images, the three-dimensional particle positions in space are determined, see below for further details.
In a further step, the assignment of corresponding particles to two or more points in time must be carried out in order to determine shifts or velocities. This step is called tracking and is shown here in more detail.

Multi-camera systems (3D-PTV/Tomo-PTV)

Complex flows require knowledge of the 3D velocity field for a comprehensive understanding of the flow topology. Multi-camera systems allow the measurement at high seeding densities and thus high information density.
The research at the institute in this area focuses on volumetric particle tracking methods as they achieve a high spatial resolution. This allows the resolution of small-scale flow structures and high velocity gradients.
In addition to the 3D-PTV method, which only allows relatively low seeding densities, the so-called tomographic predictor method was developed at the institute. Initially, a tomographic reconstruction of the measurement volume is performed. The reconstructed 3D particle positions are then projected onto the sensor and only those positions that can be clearly assigned to a particle image are considered valid particle positions. This effectively suppresses ambiguities in reconstruction. By resolving ambiguities, particle image densities of up to 0.05 particles per pixel can be achieved.
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Single camera systems (APTV/Defocusing PTV)

Multi-camera systems must observe the measurement volume from different angles, which requires sufficient optical access. However, this approach is often limited, e. g. in microfluidics through microscope optics, which permits only a single viewing direction of the measuring volume. In macroscopic applications, restrictions may arise due to the installation space at the facility or due to the limited geometry of the measuring volume (e. g. gaps).
Volumetric measurements are often necessary in the aforementioned measurement environments. In microfluidics, for example, the entire measurement volume has to be illuminated, since the minimum achievable light sheet thickness is compared to the depth of the measurement volume. Two-dimensional methods would only result in an averaging of the velocity along the measurement volume depth without resolving the velocity profile.
The institute uses the astigmatism/defocussing PTV methods for measurements in microscopic and macroscopic environments. The information of the particle position along the optical axis is obtained from the geometry of the particle image.
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