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.
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.
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.
Person in charge:
Publications:
- Rochlitz H, Scholz P, Fuchs T (2014) The flow field in a high aspect ratio cooling duct with and without one heated wall. Experiments in Fluids 56:208
- Schosser C, Fuchs T, Hain R, Kähler CJ (2016) Non-intrusive calibration for three-dimensional particle imaging. Experiments in Fluids 57:69
- Fuchs T, Hain R, Kähler CJ (2016) Uncertainty quantification of three-dimensional velocimetry techniques for small measurement depths. Experiments in Fluids 57:73
- Fuchs T, Hain R, Kähler CJ (2016) Double-frame 3D-PTV using a tomographic predictor. Experiments in Fluids 57:174
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.
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.
Persons in charge:
Publications:
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Cierpka C, Segura R, Hain R, Kähler CJ (2010) A simple single camera 3C3D velocity measurement technique without errors due to depth of correlation and spatial averaging for microfluidics. Measurement Science and Technology 21:045401
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Fuchs T, Hain R, Kähler CJ (2014) Three-dimensional location of micrometer-sized particles in macroscopic domains using astigmatic aberrations. Optics Letters 39:1298-1301
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Rossi M, Kähler CJ (2014) Optimization of astigmatic particle tracking velocimeters. Experiments in Fluids 55:1809
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Barnkob R, Kähler CJ, Rossi M (2015) General defocusing particle tracking. Lab on a Chip 15:3556-3560
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Fuchs T, Hain R, Kähler CJ (2016) In situ calibrated defocusing PTV for wall-bounded measurement volumes. Measurement Science and Technology 27:084005