Experiments in Fluids最新文献

The study of heat and mass transfer in thermal fluids relies on simultaneous temperature and velocity measurement techniques, and temperature-sensitive phosphor-particle-based velocimetry is an effective tool. Among the available methods, the lifetime method performs well in simplicity and precision. However, it faces challenges when capturing the position and phosphorescence intensity of a particle as it moves and decays. A precise phosphorescent-decay particle tracking (PDPT) method based on Gaussian intensity optimization was proposed in this study for phosphorescent particles whose center positions are coupled with the phosphorescence intensity. The PDPT method precisely obtains the center positions and phosphorescence intensities of moving phosphor particles that occupy only a few pixels in a single image, thereby enhancing the accuracy of thermometry and velocimetry. Tests with numerically synthesized particles and experimental measurements were employed to validate the proposed method, which achieved a relative trajectory error of < 0.25% at a particle velocity of 200 pixel/s and a relative temperature error of < 0.05% at 473 K. Compared with existing approaches, the PDPT method showed significant improvements in the tracking ability of the center positions and intensities of individual particles, representing a notable enhancement in the simultaneous temperature and velocity measurement of thermal fluids.

Graphical abstract

This study experimentally investigates the flow around a near-wall rectangular cylinder at a Reynolds number of (Re_D = 1000), focusing on the influence of gap ratios ((G/D = 0.5, 1.0, 2.0)) and aspect ratios ((L/D = 3, 6, 9)). The results demonstrate that both parameters profoundly impact vortex dynamics and turbulence characteristics. At small (G/D), near-wall effect suppresses lower leading edge vortex formation and leads to asymmetric recirculation. As (G/D) increases, the flow becomes more symmetric, and vortex shedding from both the upper and lower sides intensifies, forming Kármán vortex streets at suitable (L/D). The behavior of secondary vortices and their interaction with primary wake vortices vary significantly with geometry, influencing their development into coherent boundary layer structures or their entrainment into the wake. The fluctuations grow with increasing (G/D) and are especially strong at (L/D = 3) due to enhanced wake oscillations. Wall-normal integrated velocity fluctuations reveal that leading edge and trailing edge vortex shedding contributes comparably to turbulence production, particularly at larger gap ratios, where clear bimodal distributions are observed.

Dust accumulation on solar photovoltaic (SPV) panels is a pestering issue in SPV technology. Therefore, panel-cleaning has become increasingly significant to improve the system performance. Traditional panel-cleaning methods rely heavily on water-based cleansing, which entail large water footprints. Self-cleaning of these surfaces through optimal usage of water is a promising alternative, which relies on developing super-hydrophobic (SHPB) panels on which a measured quantity of water is sprayed to achieve the maximum possible liquid–solid contact area. Droplets generated from such spray would impact on the inclined  SHPB surface, where they spread and slide due to the combined action of droplet momentum and gravity, and pick-up the surface-dust before rebounding from the surface. Herein, we experimentally analyze this attribute in-depth, surpassing what is existing in the literature, particularly in the context of self-cleaning of inclined SHPB surfaces. We augment the traditional definition of maximum spreading factor by introducing a new droplet-sweeping parameter—integrated sweeping factor—based on the total liquid–solid contact area arising out of the simultaneous spreading and sliding of the droplet. Effects of the impact Weber number and surface inclination on instantaneous spreading and integrated sweeping behaviour of the droplet are characterized to identify the extent of self-cleaning. In addition, shape of the droplet-surface contact area along with their lateral and longitudinal spreads, and the contact time are characterized. Suitable correlations are developed based on regression analysis of the experimental data. Findings from the study are identified to be relevant for designing a nozzle array system for effective self-cleaning.

Laminar boundary layer suction has significant potential for reducing aircraft drag, thereby diminishing its environmental impact. This study presents wind tunnel experiments conducted on a flat plate to examine the effectiveness of laminar boundary layer suction in delaying the transition and compares the measured data with the (e^n) method based on linear stability theory (LST). The experiments, performed over a range of freestream velocities from 15 to 50 m/s, comprised infrared thermography, pressure measurements, and hot-wire anemometry. The boundary layer suction is implemented through interchangeable suction boxes mounted on the flat plate, with two types of suction surfaces tested, featuring hole diameters of 120 and (60,upmu hbox {m}) and a constant porosity of 0.9%. The study examines the influence of various parameters on transition, as the intensity of the suction coefficient, particularly at elevated values, as well as the impact of the micro-holes diameter, the chordwise distribution of the suction velocity and the freestream Reynolds number. A discrepancy between the experimentally measured transition location and the predictions from LST is observed. To identify the origin of this deviation, boundary layer measurements are taken on the porous surface while varying both the suction coefficient and its spatial distribution. A particular flow disturbance near the porous surface, amplified by the suction intensity, is identified, leading to increased velocity fluctuations in the near-wall measurement points. The difference depends on both the suction coefficient and the suction velocity distribution. For this reason, a configuration is investigated in which only the first and last of the four suction chambers are used to aspirate the boundary layer. It is observed that the flow disturbances are significantly reduced, and the boundary layer predictions align more closely with the experimental data.

In spite of hypergolic systems common use in rocket engine ignitors, the dynamics of hypergolic reaction at fluid–fluid interfaces remain underexplored. This study investigates the ignition dynamics of hydrogen peroxide (H₂O₂) droplets impacting deep pools of a sodium borohydride (NaBH₄)-based hypergolic liquid fluid. Unlike prior studies employing confined geometries (e.g., petri dishes or test tubes), the present setup minimizes wall effects and reveals several previously unreported phenomena. Key parameters–including NaBH₄ concentration (3, 6, 9 wt%), droplet height (H = 30, 100, 300 mm), pool depth (D = 10, 20, 30 mm), and isopropyl alcohol (IPA) additives–were systematically varied. High-speed shadowgraphy (6,300 fps) captured transient events such as crater formation, mist ejection, miscible plume with “tail-chasing” decomposition, two-stage ignitions, and two distinct modes of droplet combustion (evaporative and decomposition). H = 100 mm and higher NaBH₄ concentrations improved ignition reliability, while the influence of D was weaker. IPA had minimal influence on ignition but prolonged the ensuing combustion. Ignition delay times (IDT) for surface mist ranged from 16 to 55 ms, whereas plume ignitions ranged from 39 to 130 ms. Compared to confined geometries, the deep pool setup exhibited richer fluid dynamic behaviors, and comparable IDT to petri dish tests under certain conditions.

Fourier integral microscopy (FIMic), or Fourier light-field microscopy, is the latest architecture of plenoptic (also known as light-field or integral) imagers. It has the highest demonstrated spatial resolution for integral microscopy and is equivalent to an array of micro-cameras that record full views of the scene. Thus, standard tomographic or triangulation algorithms can reconstruct the measurement volume at microscopic scales. By being compact, FIMic overcomes the physical space constraints of traditional multi-camera systems. It is demonstrated with molecular tagging velocimetry (MTV) in the near-wall region of a turbulent stagnation jet; this is the first volumetric implementation of MTV. The design rules for a FIMic system are reviewed in detail, as well as the calibration procedure. With a 0.28 numerical aperture microscope objective (10(times)), the following resolutions are achieved: (7~upmu textrm{m}) laterally and (34~upmu textrm{m}) axially (wall-normal direction) over a (1700 ~upmu textrm{m}) field of view and (440~upmu textrm{m}) depth of field; however, the MTV signal can be recovered over a depth range of (1500~upmu textrm{m}). The 3D intensity field is reconstructed using Richardson–Lucy 3D deconvolution, which is commonly employed in microscopy. From the intensity field, a (2times 3) array of MTV lines is interrogated, which, at first order, gives lateral displacements in wall-parallel slices. From the two velocity components, gradients are computed, and the wall-normal velocity component is integrated from the continuity equation. Finally, visualization of submillimeter 3D flow structures is demonstrated.

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.