Tomographic PIV calibration procedure in confined optical engine geometry

Tomographic PIV measurements require calibration accuracies of a fraction of a voxel throughout the entire investigation volume in order to provide reliable 3D data. This can be achieved today in a variety of applications by combining multiple planar recordings of a calibration target across the volume with the established self-calibration technique. However, when the considered application faces high confinement and severe optical deformations, obtaining an accurate calibration becomes difficult to achieve. A novel ex-situ calibration procedure is developed to overcome confinement and compensate optical deformation. The proposed approach establishes the correspondence between an originally confined investigation volume and its mapping function computed in the absence of confinement. The described ex-situ process is evaluated for the in-cylinder aerodynamics of an internal combustion engine. A particular pyramidal configuration of the tomographic recording system is employed. The in-cylinder flow is first computed via a classical in-situ calibration procedure obtained from an additional adjacent setup allowing free access into the engine's combustion chamber. Results are then discussed for multiple investigation volume thicknesses which is a critical parameter for 3D measurements. Finally, velocity fields obtained from the ex-situ procedure are compared to those ensuing from the in-situ calibration showing that the proposed approach is able to provide faithful camera models for volumetric reconstruction of confined flows.