Matteo Novara

Non-uniform optical transfer functions in particle imaging: calibration and application to tomographic reconstruction

A new approach to the weighting function, which describes particle imaging in tomographic reconstruction, is introduced. Instead of assuming a spatially homogeneous mapping function of voxels to the images, a variable optical transfer function (OTF) is applied. By this method, the negative effects of optical distortions on the reconstruction can be reduced considerably. The effects of these improvements in reconstruction quality on the methods of tomographic particle imaging velocimetry, as well as 3D particle tracking are investigated. A method to calibrate the OTF to experimental circumstances is proposed as an additional step to the volume self-calibration. It is shown that this kind of calibration is able to capture the predominant particle imaging both for simulated as well as experimental data. The most common distortions of particle imaging are blurring due to a small depth of field and astigmatism due to imaging optics. The effects of both of these distortions on reconstruction and correlation quality are investigated via simulated data. In both cases, a strong influence on relevant parameters can be seen. Reconstructions using a spatially varying OTF, calibrated to the imaging conditions, show a significant improvement in reconstruction quality and the accuracy of the particle peak position, as well as in the accuracy of the gained displacement vector field when using two time steps. Evaluation of experimental data by PTV methods shows a reduction in ghost particle intensity and improvements in peak position accuracy. A computationally efficient method of applying the OTF to tomographic reconstruction is introduced.

Adaptive interrogation for 3D-PIV

A method to adapt the shape and orientation of interrogation volumes for 3D-PIV motion analysis is introduced, aimed to increase the local spatial resolution. The main application of this approach is the detailed analysis of complex 3D and vortex-dominated flows that exhibit high vorticity in confined regions like shear layers and vortex filaments. The adaptive criterion is based on the analysis of the components of the local velocity gradient tensor, which returns the level of anisotropy of velocity spatial fluctuations. The principle to increase the local spatial resolution is based on the deformation of spherical isotropic interrogation regions, obtained by means of Gaussian weighting, into ellipsoids, with free choice of the principal axes and their directions. The interrogation region is contracted in the direction of the maximum velocity variation and elongated in the minimum one in order to maintain a constant interrogation volume. The adaptivity technique for three-dimensional PIV data takes advantage of the 3D topology of the flow, allowing increasing the spatial resolution not only in the case of shear layers, but also for vortex filaments, which is not possible for two-dimensional measurement in the plane normal to the vortex axis. The definition of the ellipsoidal interrogation region semi-axes is based on the singular values and singular directions of the local velocity gradient tensor as obtained by the singular values decomposition technique (SVD). The working principle is verified making use of numerical simulations of a shear layer and of a vortex filament. The application of the technique to data from a Tomo-PIV experiment conducted on a round jet, shows that the resolution of the shear layer at the jet exit can be considerably improved and an increase of about 25% in the vorticity peak is attained when the adaptive approach is applied. On the other hand, the peak vorticity description in the core of vortex rings is only slightly improved with approximately a 5% increase in the peak value. Results are in good agreement with the values obtained by means of the numerical simulations. From experiments conducted on a turbulent boundary layer it is observed that the strong curvature of the mean profile is better captured with adapted ellipsoidal interrogation and the amplitude of the spurious oscillation in the vorticity profile observed in the proximity of the wall (ringing) is reduced by nearly a factor 2 after the adaptive interrogation is applied.

Multi-stereo PIV measurement of propeller wake flow in industrial facility

The flow in the wake of an 11-bladed propeller is investigated by means of phase-locked multi-stereo Particle Image Velocimetry. The use of multiple stereo-PIV systems and the multi-pulse acquisition strategy allow for the reduction of the measurement error and for the enhancement of the velocity dynamic range. The application of the measurement technique in the Airbus Low Speed Wind Tunnel industrial facility (LSWT, Bremen, Germany) poses challenges mainly due to the vibrations of the imaging system and acquisition phase jitter, ultimately affecting the spatial resolution of the measurement. A correction scheme based on the analysis of velocity fields from different stereo-PIV systems is used to largely compensate these effects, providing velocity fields suitable to be used for validation of computational aeroacoustics codes. Results show that a lower measurement error can be achieved when velocity fields from several stereo-PIV systems are combined. Furthermore, the correction scheme appears to reduce the signal modulation introduced by vibrations in the wake region.