Raf Theunissen

An adaptive sampling and windowing interrogation method in PIV

This study proposes a cross-correlation based PIV image interrogation algorithm that adapts the number of interrogation windows and their size to the image properties and to the flow conditions. The proposed methodology releases the constraint of uniform sampling rate (Cartesian mesh) and spatial resolution (uniform window size) commonly adopted in PIV interrogation. Especially in non-optimal experimental conditions where the flow seeding is inhomogeneous, this leads either to loss of robustness (too few particles per window) or measurement precision (too large or coarsely spaced interrogation windows). Two criteria are investigated, namely adaptation to the local signal content in the image and adaptation to local flow conditions. The implementation of the adaptive criteria within a recursive interrogation method is described. The location and size of the interrogation windows are locally adapted to the image signal (i.e., seeding density). Also the local window spacing (commonly set by the overlap factor) is put in relation with the spatial variation of the velocity field. The viability of the method is illustrated over two experimental cases where the limitation of a uniform interrogation approach appears clearly: a shock-wave–boundary layer interaction and an aircraft vortex wake. The examples show that the spatial sampling rate can be adapted to the actual flow features and that the interrogation window size can be arranged so as to follow the spatial distribution of seeding particle images and flow velocity fluctuations. In comparison with the uniform interrogation technique, the spatial resolution is locally enhanced while in poorly seeded regions the level of robustness of the analysis (signal-to-noise ratio) is kept almost constant.

Particle Image Velocimetry in Lung Bifurcation Models

To better understand the human pulmonary system and governing aerosol deposition mechanisms within the lung, an accurate description of the airflow in the conductive and respiratory pulmonary airways is considered to be essential. In-vivo measurements are deemed impossible due to the small scales in the lung structure. Though numerical simulations can improve the insight, validation of the results is mostly lacking, especially in the extraction of particle trajectories. Numerical and experimental studies have been performed at the von Karman Institute for Fluid Dynamics on single and multiple bifurcation models representing simplifications of the lung system. Both steady and oscillating flows have been studied for the upper lung airways, while steady flow conditions were imposed in the modeling of the alveolar zones. Further experiments consisted in the extraction of particle trajectories in the models of the respiratory airways. This chapter presents an overview of the conducted measurement campaigns complemented with the main observations and results.