LEGO Calibration Targets For Large-FOV Particle Image Velocimetry
A. Parikh, T. Fuchs, M. Bross, C. J. Kähler
Institute of Fluid Mechanics and Aerodynamics, Universität der Bundeswehr München, Germany
Particle-image or particle-tracking velocimetry (PIV/PTV) nonintrusively provides velocity field information, and consequently, the proposition of measurements in a large plane is highly attractive from an experimental standpoint, particularly for experiments conducted in large atmospheric and flight-scale wind tunnels. Physical limits to the size of the useful field-of-view (FOV) that can be achieved with a single camera depend on striking a balance between the physical capabilities of the cameras and the physics of interest in the flow. A typical solution is to stitch together multiple smaller FOVs to achieve the large FOV of interest, presenting a number of challenges, some of which are rooted in the calibration process. For SPIV calibrations, the use of a multi-level target simplifies the calibration process. However, this is problematic for large-FOV measurements, as standardised multi-level targets are typically relatively small and expensive due to the precision-engineering required. As an alternative, LEGO bricks are extremely well-suited to the construction of large, customised multi-level targets due to their high dimensional tolerance and their stackability with high precision. In order to create a prototype two-sided multi-level SPIV target and evaluate whether the LEGO bricks can be used successfully to calibrate a large FOV, the LaVision Type 31 target was chosen as a model. In order to further augment the flexibility offered by the LEGO-based targets, the backing used to mount the baseplates was designed to be modularly reconfigurable. The resultant two-sided multi-level target has an area of approximately 380 x 1150 mm. To evaluate the target, SPIV measurements of the inflow conditions of the Atmospheric Wind Tunnel Munich (AWM) were performed; calibrations were also performed using the Type 31 target for comparison. Analysis of the datasets with both calibration targets shows good agreement in the measurement of the streamwise, out-of-plane component u. However, there is some uncertainty regarding the accuracy of the computation of in-plane components. Due to the high level of agreement in calibration parameters, out-of-plane component, and the qualitative location of flow features, this disagreement is believed to be a fixable issue. These early results indicate that with some refinement, the LEGO-based calibration target can be developed further and used for large-FOV measurements in the future.