Experiments in Fluids最新文献

This work presents a novel approach for pressure field reconstruction from image velocimetry data using SIREN (Sinusoidal Representation Network), emphasizing its effectiveness as an implicit neural representation in noisy environments and its mesh-free nature. While we briefly assess two recently proposed methods—one-shot matrix-omnidirectional integration (OS-MODI) and Green’s function integral (GFI)—the primary focus is on the advantages of the SIREN approach. The OS-MODI technique performs well in noise-free conditions and with structured meshes but struggles when applied to unstructured meshes with high aspect ratio. Similarly, the GFI method encounters difficulties due to singularities inherent from the Newtonian kernel. In contrast, the proposed SIREN approach is a mesh-free method that directly reconstructs the pressure field, bypassing the need for an intrinsic grid connectivity and, hence, avoiding the challenges associated with ill-conditioned cells and unstructured meshes. This provides a distinct advantage over traditional mesh-based methods. Moreover, it is shown that changes in the architecture of the SIREN can be used to filter out inherent noise from velocimetry data. This work positions SIREN as a robust and versatile solution for pressure reconstruction, particularly in noisy environments characterized by the absence of mesh structure, opening new avenues for innovative applications in this field.

Bedload transport, the coarser component of sediment transport moving in contact with the bed in stream channels, has major implications for public safety, water resources, and environmental sustainability. Size segregation is largely responsible for our limited ability to predict sediment flux and river morphology, particularly in mountains where steep slopes drive an intense transport of a wide range of grain sizes. Two-size experiments were carried out in a dedicated 10% steep flume to study vertical segregation at the grain scale. Particle tracking was used to obtain trajectories of high concentration bedload mixtures of spherical particles, but it fails to correctly retrieve long trajectories due to strong grain–grain interactions. In this paper, we propose a new particle tracking algorithm using a global optimization scheme based on a Continuous Energy function and a specific iterative Minimization (CEM). For the purpose of evaluating this new algorithm named CEM-ST (available at https://gitlab.univ-st-etienne.fr/labhc-iscv/cem-st), we have designed two typical experimental reference sequences with corresponding full trajectory ground truths, made available to the community. Compared to online algorithms, which consider only previous time steps, this new CEM-ST algorithm is less sensitive to the quality of the detections and performs better both globally and in the details of the trajectories and the depth profiles of concentration, particle velocity and sediment transport rate. Application of CEM-ST has allowed us to gain a better insight into the influence of the fine particle rate on segregation, in particular unraveling the role of clusters in the bedload dynamics.

Measurement of three-dimensional velocity field of an opaque material (foamed gypsum slurry) flowing under a roller is explored using a PIV surface-tracking technique employoing two types of software. The foamed slurry was deposited on a moving belt pulling it under a rotating roller. The case of the water-to-stucco ratio (WSR) of 75 was studied at 0.19 wt%, and 1.86 wt% of foam added. The cases of roller co-rotation with the belt, no rotation, and counter-rotation were explored. The effect of the added foam on widening of the slurry layer on a roller was also studied. Particle Image Velocimetry (PIV) was used to measure the surface velocity flow field in both top and side views. A significant rejection flow of slurry before the roller was observed in some cases, with its severity varying with the roller’s rotating direction, its angular speed, as well as the foam content. One of the main aims of the present work is in the comparison of two PIV software: PIVlab (Matlab source, Germany) and PIVware (provided by TUDelft).

The structure and scaling of flow separation in the adverse pressure gradient at the inlet of a diverging channel are investigated experimentally. The channel flow is diverted from an adjacent uniform flow over a surface through a surface opening, and the separation forms a fluidic constriction across the inlet that severely limits the fraction of the diverted flow. The cross-stream and streamwise scales of the separation domain are progressively diminished by forced streamwise attachment that is effected using a spanwise array of fluidically oscillating control jets placed across the inlet from the main flow. Variable momentum coefficient enables efficient regulation of the diverted fraction of the flow through the diverging channel. The evolution of the flow at separation and within the separation domain is measured using planar PIV and characterized using conditional averaging, spectral analysis, and decomposition methods in the absence and presence of fluidic actuation. Although the streamwise migration of the separation point in the presence of actuation results in changes in the characteristic cross-stream scale of the wall-tangential velocity distributions at separation, the time-averaged velocity distributions in the absence and presence of actuation collapse when scaled by the local vorticity thickness and velocity deficit of the separating shear layer. Proper orthogonal decomposition analysis indicates that despite the energy shift across the flow scales in the presence of actuation, local vorticity modes in the base and controlled flows about separation are remarkably similar, as the uncontrolled flow modes primarily undergo tilting and stretching as separation migrates downstream in the presence of actuation. The global effectiveness of the actuation is assessed by the increased fraction of the diverted flow from the main stream and the accompanying reduction in total pressure losses.

Liquid foams have shown a significant potential in mitigating pressure waves such as acoustic, shock and blast waves. However, the variety of experimental set-ups in the literature makes it difficult to draw definitive conclusions and compare results from one study to another. This variability can often be attributed to the lack of control over foam parameters, with some, such as bubble size, being undocumented or insufficiently detailed. The present study addresses this issue by developing a set-up that precisely controls critical parameters such as bubble size, liquid fraction, wave Mach number and waveform (sustained shock or blast wave). Pressure waves are generated with a shock tube, and their interaction with foams is analysed in a specifically designed and carefully regulated test section. The versatility of this set-up allows for the exploration of a wide range of wave conditions and foam properties. Preliminary results are presented, which validate our set-up design and its ability to control the relevant parameters for studying pressure wave attenuation.

In this study, we experimentally investigated the kinematic characteristics of solitary vortex rings in polymer solutions. Two distinct experimental configurations are examined: (1) free downstream translation of the vortex ring and (2) vortex ring impingement onto a wall. Under the flow parameters in this study, the vortex ring maintains a laminar state throughout its evolution. Two-dimensional particle image velocimetry measurements of flow velocity fields were performed in the meridional plane of the vortex ring. Compared with its Newtonian fluid counterpart, the polymer solution vortex core exhibits a marked asymmetry in its vorticity distribution, with this disparity becoming more distinct at increased polymer concentrations. Nevertheless, normalized vorticity profiles demonstrate semi-self-similarity across different polymer concentrations. During its free downstream translation, both the translation speed and circulation of the vortex ring exhibit distinct power-law decay characteristics over time. These decay trends can be empirically described by separate scaling relationships. In wall impingement experiments, the azimuthal stretching of a vortex ring in a polymer solution is suppressed compared with its Newtonian counterpart when the inertial effect dominates. After colliding with the wall, a vortex ring in a polymer solution is less prone to develop secondary vortices compared to its Newtonian counterpart.

Detecting the transition from laminar to turbulent flow in particulate pipe systems remains a complex issue in fluid dynamics, often requiring sophisticated and costly experimental apparatus. This research presents an innovative streak visualization method designed to offer a simple and robust approach to identify transitional turbulent patterns in particulate pipe flows with neutrally buoyant particles. The technique employs a laser arrangement and a low-cost camera setup to capture particle-generated streaks within the fluid, enabling the capture of the temporal evolution of flow patterns. The novelty of the method lies in identifying laminar and turbulent flow patterns from the statistical properties of streak-angle distributions. Validation of the proposed method was conducted through comparison with established techniques like particle image velocimetry (PIV) and pressure drop measurements, confirming its accuracy and reliability. Experiments demonstrate the streak visualization method’s capacity to differentiate between laminar, transitional, and turbulent flow regimes by analysing the standard deviation of streak angles. The method is applicable across a wide range of particle concentrations, as long as the statistical distributions of laminar and turbulent patterns differ, making it versatile where other methods may face limitations. Furthermore, this technique enables us to identify a critical Reynolds number using the Kullback–Leibler divergence built on the statistical distribution of streak angles, which is consistent with previous studies. This streak visualization method offers potential for analysing particulate pipe flows in both laboratory environments and specific industrial scenarios, especially when the fluid is transparent and particles are either naturally occurring or added as tracers.

Research on free falling particles has predominantly focused on wake dynamics and vortex shedding of individual particles in quiescent flow. However, when these particles fall collectively, the wakes of neighboring particles alter the flow fields. To investigate how the settling and wake dynamics of particles are affected by the wakes of other settling particles, we conducted volumetric experiments using the Shake-The-Box technique. Negatively buoyant 12 mm particles of four different geometries (sphere, flat cuboid, circular, and square cylinders) were first released individually into quiescent water. Subsequently, the particles were released individually into the bulk wakes of 20 monodisperse particles. Using four high-speed cameras and LEDs, we simultaneously captured both 3D particle and fluid motions in the terminal velocity regime. The imaging domain measured 90 mm × 90 mm × 40 mm. Our results show that all trailing particles settling through the bulk wakes gain additional downward momentum from the turbulent wakes, causing them to fall faster than in quiescent flow. However, when the induced velocity of the preceding wakes is subtracted, the relative settling velocity was found to be essentially the same as the particle falling in quiescent fluid. Upstream of the particle, the vortices in the bulk wake interact with the developing shear layer along the particle. The wake downstream of the trailing particle also appears more chaotic than that in quiescent flow.