Experiments in Fluids最新文献

Multiphase flows where particles, bubbles, or droplets are suspended in a fluid govern critical processes in biology, medicine, materials processing, and geophysics. However, observing their microscale dynamics in opaque systems has remained a fundamental challenge. We present Synchrotron X-ray Multi-Projection Imaging (XMPI), a novel approach enabling four-dimensional (3D + time) tracking of microparticles in visibly opaque suspension flows without requiring sample rotation. By capturing simultaneous projections from multiple angles using beam-split X-rays at synchrotron facilities, we resolve instantaneous particle positions and trajectories in opaque fluids such as blood. We demonstrate the potential of XMPI through individual particle tracking velocimetry (3D PTV) in dilute conditions, as well as multi-projection image velocimetry in dense suspensions. The methodology provides otherwise inaccessible experimental validation for particle-resolved computational fluid dynamics models and allows, e.g., observation of inertial focusing effects and microstructural dynamics relevant to suspension rheology and biomedical flows. This work paves the way for high-resolution, time-resolved 4D imaging of complex multiphase flows across a range of scientific and industrial applications. Combining XMPI with recent AI-supported 4D reconstruction algorithms opens a new spatiotemporal frontier for high-speed, rotation-free microtomography.

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A new algorithm for two-pulse particle tracking velocimetry (PTV) is introduced, which makes use of recent advances in 3D particle reconstruction, matching and position optimization. Contrary to most current state-of-the-art approaches, e.g. Godbersen et al. (2024), which progressively retrieve the true particles while maintaining a small proportion of outliers, we choose a symmetric path, operating as much as possible at a high rate of found true particles, and focusing most of our algorithmic choices on progressively eliminating outliers. First guesses of particles positions at the two instants are obtained introducing a modified, large-scale version of the Particle Volume Reconstruction (PVR) of Champagnat et al. (2014), which discretizes the image formation process on a fine voxel grid. Positions and intensities are then globally optimized, using an L-BFGS optimizer. Upon discarding particles with a low reconstructed intensity, clouds that recover a large fraction of the correct particles are obtained, although with a large amount of noise (i.e. ghost particles). The F-VFC algorithm introduced in Le Bris et al. (2025) is then used to match the particles from both instants while also removing a large amount of ghost particles from the particle clouds. This full process is repeated on residual images until convergence. For synthetic images, the obtained approach, named VICCTOR (Voxel-grid Initialized reConstruction with Consensus Tracking and Outlier Removal) nearly recovers all particles up to a seeding density of 0.2 ppp and with very little noise, while maintaining a very low error on particle positions. The approach also shows its excellent performance on experimental data from a Giant Von Kármán set-up, as evidenced by comparisons of its output with respect to results provided by the shake-the-box algorithm applied on full time-resolved sequences.

High-speed flow over an open cavity is associated with dynamically complex unsteady events, which warrant the need to better understand the flow in a reduced-order manner. The current work performs a modified version of Spectral Analysis Modal Method (Zhang et al. 2020) on supersonic cavity flows which enables significant computational speed-up and a more optimal way of encoding the large-scale structures. Synchronized measurements of non-time-resolved velocity field using particle image velocimetry and time-resolved single-point pressure measurements were conducted on an open cavity at a nominal free-stream Mach number of 1.4. The test section comprises a rectangular cavity of length-to-depth ratio of 6 and width-to-depth ratio of 6, which spans the full width of the wind tunnel. The datasets are analyzed using a combination of Proper Orthogonal Decomposition and stochastic estimation techniques to obtain the reduced-order modes. Reduced-order reconstructions at high-energy Rossiter frequencies show large-scale unsteady vortical structures indicative of Kelvin–Helmholtz shear layer instabilities convecting down the cavity. The coherence-based reconstruction enhances the extraction of Kelvin–Helmholtz instability dynamics by reducing contamination from incoherent turbulent motions. Furthermore, these interactions highlight significant shear layer–aft wall interactions along with flow ejection and in-sweep events, emphasizing the aft wall interactions of supersonic flow over an open cavity.

This study presents an experimental investigation into the transition from vortex-induced vibration (VIV) to galloping for a flexible square prism in a cross-flow. Experiments were conducted in a rotating water channel over a wide range of reduced velocities. Several square prisms were studied, and their dynamic responses were characterized by three distinct regimes. For the primary square prism that is the main focus of this study, at low reduced velocities, the prism underwent a stable, low-amplitude VIV, where the oscillation frequency locked in with the vortex shedding frequency. At high reduced velocities, the system entered a pure galloping regime, featuring large-amplitude oscillations at a frequency near the prism’s natural frequency, which was decoupled from the vortex shedding. The key finding of this work is the discovery of a transitional regime characterized by intermittent switching between VIV-dominant and galloping-dominant episodes. During these transitions, the response was observed to switch between stable, low-amplitude VIV and large-amplitude, galloping oscillations that exhibited a complex dual-frequency response. The beam was under almost no externally imposed tension, leading to a response dominated by the beam’s flexural rigidity. As a result, the second-mode natural frequency of the beam was at nearly four times the beam’s first natural frequency. In the primary beam in this study, only the first mode was excited. In other beams that we considered, second-mode VIV response was also observed at higher reduced velocities. This work suggests that in low-tension, bending-dominated flexible square prism, the transition from VIV to galloping occurs through an intermittency mechanism.

This study proposes a method for determining wall shear stress and the skin-friction coefficient from the internal velocity distribution of an oil film using astigmatism particle tracking velocimetry (APTV). The method is based on measuring the velocity gradient within the liquid film and evaluating the wall shear stress without explicitly identifying the wall position. The concept was first validated using a rotating-plate system generating Couette flow. The three-dimensional velocity distribution measured by APTV was used to determine the wall shear stress, and the results showed good agreement with theoretical predictions. The method was then applied to velocity measurements within an oil film formed on the wall of a flow channel and driven by airflow. The results demonstrated that the skin-friction coefficient can be evaluated from the internal velocity distribution within the oil film and that its dependence on Reynolds number agrees with the trend predicted by the Moody diagram. An uncertainty analysis was also conducted to evaluate the reliability of the wall shear stress estimation. The results indicated that uncertainties in velocity and particle position measurements dominate the overall uncertainty. These results suggest the potential for determining wall shear stress and skin friction from internal velocity measurements in a high-viscosity liquid film.

Optimizing convective heat transfer in compact thermal management systems require improved understanding of how vortex dynamics, confinement, and near-wall transport interact in air-based internal forced convection. This study develops a non-intrusive experimental framework that combines two-color laser-induced phosphorescence (2cLIP) with particle image velocimetry (PIV) to obtain simultaneous, high-resolution velocity and temperature fields in a confined square-duct flow with a thermally active bottom wall. Experiments were performed with and without cylindrical and square vortex generators to investigate vortex-induced mixing and heat transfer mechanisms. The coupled diagnostics enable direct evaluation of correlations between velocity and temperature fluctuations through second-order statistics. For the cylindrical vortex generator, the average heat transfer enhancement over the flat plate was approximately 25% and 13% at a Reynolds number of (text {Re}_{D_text {h}}=883) and 1100, for input power to the bottom wall at (P_text {plate}=6.67) W. Increased streamwise and wall-normal velocity fluctuations intensified wake mixing, while the wall-normal turbulent heat-flux component, (overline{v'T'}) revealed a coherent and growing region of enhanced convective transport within (0.5 le (x/H_text {duct}) le 1.8). This structure was more spatially concentrated in the near wake of the vortex generator with greater influence into the duct core for the cylindrical geometry, and more diffusely distributed downstream for the square geometry, with limited influence into the duct core.

Event-based cameras have emerged as a powerful tool for object detection and tracking in autonomous driving, robotics, and experimental physics. In particular, they facilitate the study of complex turbulent fluid flows by enabling the tracking of numerous tiny tracer particles, while benefiting from the superior temporal resolution, high dynamic range, and low data rate of the asynchronous event stream. However, exploiting the sparse event stream requires precise and efficient data processing pipelines that either accumulate events into a dense representation or process them directly by clustering algorithms. In this study, we present a modular framework for SpatioTemporal Event-based Lagrangian particLe trAcking (STELLA), which integrates detection and tracking strategies from both pipelines into a unified tracking system. To benchmark the proposed framework, we introduce demanding synthetic and experimental datasets covering the motion of numerous particles, which are made publicly available. Leveraging the rich ground truth of these datasets, established recurrent vision transformer and heat conduction-based detection architectures are trained and applied to particle tracking in fluid flows for the first time. Using STELLA, robust and reliable particle tracking is demonstrated, achieving subpixel-accurate tracks and a mean absolute error in the predicted velocity down to 1.9 % of the peak velocity. Which is the best-performing processing pipeline strongly depends on the dynamics and composition of the considered dataset. In particular, approaches based on dense representations yield accurate tracks for high-frequency periodic particle motions. Conversely, direct processing of the event stream enables simultaneous tracking of more than 900 particles in the wake of a cylinder, with uncertainties comparable to state-of-the-art particle tracking velocimetry (PTV) using a high-speed camera. Despite significant spatial and temporal velocity gradients, slow- and fast-moving particles are precisely tracked in the event stream, challenging conventional approaches using frame-based cameras. Hence, the openly available framework STELLA paves the way for a versatile and easily accessible application of event-based cameras for flow diagnostics.

A method for experimentally determining the adiabatic wall temperature and recovery factor in high-speed flows through the use of IR thermography is presented. This involves characterizing the heat flux–wall temperature relationship across multiple wind tunnel tests at varying wall temperatures. The method yields values on a per-pixel basis, making it applicable to complex flows. Consequently, this allows for the calculation of an accurate Stanton number based on the local adiabatic wall temperature. The technique is demonstrated in a Mach 2.3 indraft facility, initially focusing on a turbulent boundary layer and subsequently on a more complex swept shock–boundary layer interaction. To demonstrate the relevance of this method, the local adiabatic wall temperature approach is compared to the commonly used simplifying assumption of a constant adiabatic wall temperature based on the turbulent flat-plate boundary layer recovery factor. For the demonstration cases, a significant variation in adiabatic wall temperature was measured across the domain. It is then shown that neglecting this variation leads to substantial errors in both trend and magnitude of the Stanton number. These errors are particularly pronounced when the wall-to-recovery temperature ratio (T_w)/(T_r) is close to unity.

The two-colour laser-induced fluorescence (2c-LIF) method is capable of time-dependent, spatially resolved temperature and pH measurements, respectively. In the present study, temperature and pH are investigated in separate measurement series, using the same optical principle and dye system under different experimental conditions. Together with a still uninterrupted growth in the life science field—especially molecular biology and diagnostics—the adaptation of this method might play a key role in improving future assays. 2c-LIF is providing a new level of process verification within temperature-controlled microplate wells under realistic operating conditions, i.e. in automated laboratory workflows for both high-throughput and high-content. Specifically, within each microwell the confined liquid volumes and the imposed vertical temperature gradients lead to thermally induced flow structures that influence scalar transport and desired mixing of the liquid. Within this paper, a Fluorescein/Sulforhodamine 101 dye system in water enables combined spectroscopic and imaging-based thermometry in the temperature range 283–343 K under realistic conditions. Temperature sensitivities of 2–6%/K by using a spectrometer and 2–3.5%/K for an image-based measurement are achieved, yielding a statistical precision of approximately 0.23–0.50 K. Absorption-corrected dual-camera imaging is providing quantitative full-field temperature maps and also resolving the temporal evolution of thermally driven flow patterns during a heating ramp. In buffered solutions (pH 6–11), the fluorescence ratio exhibits a linear response between pH 6 and 9, allowing spatially resolved visualisation of biochemical mixing processes. Time-dependent isothermal droplet-injection experiments are conducted to quantify homogenisation dynamics in the absence of external mechanical stimuli. A normalised mixing index based on the coefficient of variation is introduced to characterise mixing times and to assess the relative contributions of advective and diffusive transport, which can be interpreted in terms of local Péclet numbers. Thus, it can be demonstrated that 2c-LIF is providing a quantitative diagnostic framework for investigating coupled heat and mass transfer in confined microplate systems and for analysing transport-dominated mixing processes in small-scale liquid volumes.

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