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.
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Recording onto a single-frame multiple exposures of the tracer particles has the potential to simplify the hardware needed for 3D PTV measurements, especially when dealing with high-speed flows. The analysis of such recordings, however, is challenged by the unknown time tag of each particle exposure, alongside their unknown organization into physical trajectories (trajectory tag). Using a sequence of two or more illumination pulses with a constant time separation leads to the well-known directional ambiguity problem, whereby it is not possible to distinguish the direction of motion of the tracer particles. Instead, an irregular and asymmetric sequence of time separation for the illumination pulses allows recognizing the time tag of the unique sequence of positions in the image, composing the trace. A criterion is formulated here that recognizes unambiguously the trace pattern, based upon the principle of kinematic similarity. A combinatorial algorithm is proposed whereby a signal-to-noise ratio is introduced for every candidate trace. The approach is combined with an additional criterion that favors trace regularity (minimum velocity fluctuations). The algorithm is illustrated making use of particle motion examples. Furthermore, it is assessed using 3D experimental data produced with time-resolved analysis (single-frame, single-exposure) using the Shake-the-Box method. Traces with a three-pulse sequence yield a detection rate of 85%. The latter declines with the number of pulses. Conversely, the error rate rapidly vanishes with the samples number, which confirms the reliability of trace detection criterion when more pulses are comprised in the sequence. ... Read more

Recording multiple exposures of the tracer particles onto a single digital image has the potential to simplify the hardware needed for 3D PTV measurements. A sequence with equal time intervals leads to the well-known directional ambiguity problem. Instead, a sequence with irregular time-intervals (two or more) allows resolving the ambiguity. Two fundamental criteria are introduced that determine whether a set of particle samples corresponds to a physical trace for a given neighborhood: the kinematic similarity criterion imposes similarity (i.e. proportionality) between the length travelled by the particle and time elapsed. Additionally, the trace regularity criterion favors sets with minimum fluctuations of the velocity vector. A numerical algorithm, based on combinatorial analysis is presented, whereby all possible sets are scrutinized and the set of particles yielding the minimum disparity from the above criteria corresponds to the physical trace. The algorithm is demonstrated using 3D experimental data and results are benchmarked against the state-of-the-art time-resolved analysis Shakethe-Box (single-frame, single-exposure). ... Read more

In the past years, volumetric velocimetry measurements with helium-filled soap bubbles as tracer particles have been introduced in wind tunnel experiments and performed at large-scale, enabling the study of complex body aerodynamics. A limiting factor is identified in the field of wind engineering, where the flow around ships is frequently investigated. Considering multiple wind directions, the optical access for illumination and 3D imaging rapidly erodes the measurement regions due to shadows and incomplete triangulation. This work formalizes the concepts of volumetric losses and camera redundancy, and examines the performance of multi-directional illumination and imaging for monolithic and partitioned modes. The work is corroborated by experiments around a representative ship model. The study shows that a redundant system of cameras yields the largest measurement volume when partitioned into subsystems. The 3D measurements employing two illumination directions and seven cameras, yield the time-averaged velocity field around the ship. Regions of flow separation and recirculation are revealed, as well as sets of counter-rotating vortices in several stations from the ship bow to the flight–deck. The unsteady regime at the flight–deck is examined by proper orthogonal decomposition, indicating that the technique is suited for the analysis of large-scale unsteady flow features. ... Read more

Volumetric particle tracking velocimetry measurements are performed in a low-speed wind tunnel to study the flow around a 1:12-scale aircraft model with jet engines operating with thrust reversers. The engine jet and freestream flow velocity are varied to yield a jet to freestream velocity ratio of V_jet/V_∞ ranging from 1.5 to 6. Measurements at such scale (0.5 m³) require the use of strongly scattering helium-filled soap bubbles as flow tracers, which are introduced in both the jet and the freestream flow. The tracer’s three-dimensional motion is determined using an array of cameras and a Lagrangian particle tracking algorithm. The mean velocity field reveals the jet inner structure as well as its interaction with the freestream, the ground board, the nacelle, the fuselage, and the horizontal and vertical tails. The experiments allow detection of exhaust reingestion as well as the aerodynamic interference with control surfaces at the tail segments in a single measurement volume. The results are in good agreement with conventional temperature rake measurements while adding details of the flow topology and of the large-scale unsteady flow fluctuations. Finally, the jet reversal characteristics with varying freestreams and nozzle pressure ratios are assessed, demonstrating the feasibility and versatility of volumetric velocimetry measurements for industrial aerodynamics. ... Read more

Volumetric PIV measurements are performed to study the flow development around a 1:12 scale model of a thrust reverser in a low-speed wind tunnel. The thrust-reverser operates in a freestream flow of 3-5 m/s and with a jet to freestream velocity ratio Vjet/Vinf ranging from 1.5 to 6. Making use of sub-millimeter helium-filled soap bubbles, large-scale PIV measurements are performed that encompass a 3D domain of approximately 0.4 m3. The flow exiting the thrust-reverser features two inclined jets that interact with the wind tunnel free stream, the nacelle, fuselage and ultimately the tunnel walls. Such interactions result in highly three-dimensional patterns and jets large scale fluctuations. The jet reversal characteristics with varying freestream velocity and nozzle pressure ratio are characterized quantitatively. The work demonstrates the feasibility of quantitative inspection of the flow behavior in a three dimensional domain for industrial applications. ... Read more