Large-scale volumetric velocimetry for aeronautics

Abstract

Volumetric Particle Image Velocimetry (PIV) is a state of the art technique for quantitative flow diagnostics. Its ability to measure the velocity field around the typically complex objects, as needed in the field of aeronautics, makes it a valuable tool for designers and engineers. Wind tunnel experiments making use of PIV as a diagnostic tool are used to gain physical insight into the flow field organization, generate data for validation of computational methods, and perform optimization in various domains, most notably in the field of aerospace and wind engineering.

Despite continuous advancements in the measurement technique, performing a PIV experiment in industrial wind tunnel environments remains challenging, making its use rather limited to niche applications within aeronautics or the automotive industry. In this thesis the recent advancements in helium-filled soap bubbles (HFSB) technology and 3D particle tracking algorithms are synthesized, to demonstrate the impact of PIV in industrial environments and extend its utility to a wider range of aeronautical applications.

Two limitations currently preventing the broad adoption of PIV are addressed: i) the limited spatial coverage of full-field volumetric velocimetry due to shadows and blocked optical access; ii) the limited measurement accuracy and the accessible velocity ranges by conventional two-pulse PIV systems.

Both these problems are introduced and treated in Part I of this thesis. Multi-directional illumination and imaging systems with redundancy are introduced for the study of volumetric flows around complex geometries. A volumetric loss parameter is defined, that can be used as a guideline in the phase of experimental setup design of these systems. Additionally the logics of the combinations of the multiple cameras that work in a single system are examined and the results of the different combinations are presented.

The technical limitations in hardware technology of the cameras’ frame rates have inspired the revisiting of original PIV methods of multi-exposure imaging. This is investigated as a way to increase the dynamic velocity ranges of more common and practical two-pulse systems used in industrial testing. Methodology and initial results are presented for the workings of a novel concept.

Part II introduces specific application experiments in two fields. The first is part of integrated propulsion, the study of the flow around a thrust-reverser at- tached to a complete aircraft; and the second, the flow around the top side of the superstructure of a ship with helicopter and drone landing capabilities in the deck, investigated across a spectrum of incoming wind directions.

The study around the thrust reverser has demonstrated the feasibility and added value of PIV even in challenging complex industrial wind tunnel experiments, by providing insight into the mechanisms of jet reversal and re-ingestion, as well as a rich database that is in good agreement with, and complementary to, more traditional wind tunnel re-ingestion measurement methods.

The study of the redundant multi-illumination and camera systems has proven to increase the spatial coverage of measurements with these setups, providing more complete data for numerical low-fidelity turbulence models validation, as well as practically proving to increase the robustness of PIV systems against reflections. This has been demonstrated in the final chapter of the thesis.