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).@en

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.@en

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.

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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.@en