Experiments in Fluids最新文献

A thorough understanding of media tightness in automotive electronics is crucial for ensuring more reliable and compact product designs, ultimately improving product quality. Concerning the fundamental characteristics of fluid leakage issues, the dynamic wetting and penetration behavior on small scales is of special interest and importance. In this work, four T-shaped microchannels with one inlet and two outlets are experimentally investigated in terms of contact angle dynamics and interface movement over time, generating novel insight into the wetting mechanisms and fluid distribution. With a main channel width of 1 mm, a crevice width of (w = {0.3},hbox {mm}, {0.4},hbox {mm}) and a rounding edge radius of (r = {0.1},hbox {mm}, {0.2},hbox {mm}), the geometrical effects on the fluid penetration depth in the crevice and the interface edge pinning effect are analyzed quantitatively using an automated image processing procedure. It is found that the measured dynamic contact angles in all parts can be well described by molecular kinetic theory using local contact line velocities, even with local surface effects and abrupt geometry changes. Moreover, a smaller crevice width, a sharper edge and a larger flow velocity tend to enhance the interface pinning effect and prevent fluid penetration into the crevice. The rounding radius has a more significant effect on the interface pinning compared with crevice width. The experimental data and image processing algorithm are made publicly available.

When producing metal-oxide nanoparticles via flame spray pyrolysis, precursor-laden droplets are ignited and undergo thermally induced disintegration, called ‘puffing’ and ‘micro-explosion’. In a manner that is not fully understood, these processes are associated with the formation of dispersed phases inside the droplets. This work aims at visualizing the interior of precursor-laden burning single droplets via diffuse back illumination and microscopic high-speed imaging. Solutions containing iron(III) nitrate nonahydrate (INN) and tin(II) 2-ethylhexanoate (Sn-EH) were dispersed into single droplets of sub-100 μm diameter that were ignited by passing through a heated coil. At low precursor concentration, 50% of the INN-laden droplets indicate a gas bubble of about 5 μm diameter in the center of the droplet. The bubble persists for several hundred microseconds at a similar size. In almost all of these cases, the bubble expands at some point and the droplet ends up in a micro-explosion. In some of these instances, the droplet’s surface shows spatial brightness modulations, i.e., surface undulations, indicating the formation of a viscous shell. With increasing INN concentration, the fraction of droplets showing surface undulations, gas bubbles, and micro-explosions drastically decreases. This may be associated with a more rigid viscous shell and reduced mobility of bubbles. Bright incandescent streaks originating from the disrupting INN-laden droplets, may indicate sub-micrometer droplets or particles from within the droplets or formed in the gas phase. In contrast, Sn-EH-laden droplets show very fast disruptions, typically less than 10 μs from first visible deformation to ejection of secondary droplets. Bubbles and surface undulations were not observed.

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The wake dynamics of the flow past a confined circular cylinder and its active control by sweeping jets (SWJs) and steady jets (SJs) positioned at the front stagnation points were experimentally investigated using particle image velocimetry and pressure measurements. Experiments were conducted across a range of Reynolds numbers (Re, based on the incoming flow velocity and the cylinder diameter) from 10,000 to 45,000 and blockage ratios ((beta)) of (1/2), (1/3), (1/4), and (1/5). A comprehensive comparison between the current results and existing literature on natural flow dynamics fills the knowledge gap and reveals that confinement gradually reduces the time-average pressure coefficient ((C_{{text{p}}})) and increases the drag coefficient ((C_{{text{D}}})) and Strouhal number (St). The interaction between the wake and lateral wall shear layer gradually increased as (beta) increased. Both SWJs and SJs effectively suppressed wake fluctuations, and the statistical characteristics of the flow field and proper orthogonal decomposition analysis indicated a consistent flow control mechanism between the two methods. However, the SJs introduced external fluctuations and unbalanced forces in the forward flow field, resulting in a wake flow asymmetry. By contrast, SWJs provide more uniform control and superior flow control effectiveness and efficiency.

Measuring dynamic liquid surfaces is a significant challenge in fluid mechanics and sloshing dynamics, with a notable lack of high-precision, effective full-field measurement methods. To resolve this challenge, this research proposes a speckle projection-based 3D digital image correlation (3D-DIC) method for the measurement of dynamic liquid surfaces. The approach employs liquid staining and speckle projecting to create textured patterns on the liquid surface, which are then captured by binocular cameras. The binocular cameras are calibrated using a ratio-invariant method to accurately obtain the internal and external parameter matrices. Subsequently, algorithm based on zero-mean normalized cross-correlation (ZNCC) is utilized to reconstruct the dynamic liquid surface wave height field. To validate the accuracy of the method, a geometric optical numerical model is established to simulate binocular images of regular wave liquid surfaces with projected speckle patterns. The results show that full-field root mean square (RMS) error in simulated liquid surface measurement is less than 0.019 mm. Physical experiments were further conducted to confirm the method's applicability, achieving a maximal measurement error of 0.133 mm for real dynamic liquid surfaces. Results demonstrate that the proposed method achieves high-precision, non-contact, and full-field measurements of dynamic liquid surfaces, making it ideal for laboratory measurements of flowing liquids.

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In the recent years, increased use of hydraulic turbines in off-design operating conditions such as no-load and deep part-load has resulted in increased damage to the turbines. A detailed understanding of the fluctuating flow phenomena can help to identify and mitigate the potentially damaging flow structures. This paper presents a comprehensive experimental and numerical study of the flow phenomena at the inlet of a Francis turbine at four no-load operating conditions, including speed-no-load and a deep part-load operating condition. Measurements are taken using a high-frequency stereoscopic endoscopic particle image velocimetry method on radial–azimuthal planes, covering the vaneless space and a large part of the interblade channels at different spans. For the speed-no-load condition, experimental data are enriched with unsteady RANS simulation data to understand the three-dimensional behavior of the flow. The average flow phenomena, transient structures and velocity fluctuations are discussed and compared among different operating points. At all operating points, the strongest average flow circulation zone (strong enough to form a vortex only at one operating condition) consistently exhibits the highest velocity fluctuation energy. The results show that the highest velocity fluctuations, and thus the most energetic dynamic structures, are in a no-load operating point with a guide vane opening smaller than speed-no-load. Position and intensity of the interblade vortices varies not only with the guide vane opening but also with the amount of torque extracted by the runner.

Boundary layer transition triggered by a discrete roughness element generates a turbulent wedge that spreads laterally as it proceeds downstream. The historical literature reports the spreading half angle is approximately 6(^{circ }) in zero-pressure gradient flows regardless of Reynolds number and roughness shape. Recent simulations and experiments have sought to explain the lateral spreading mechanism and have observed high- and low-speed streaks along the flanks of the wedge that appear central to the spreading process. To better elucidate the roles of Reynolds number and of streaks, a naphthalene flow visualization survey and hotwire measurements are conducted over a wider range of Reynolds numbers and longer streamwise domain than previous experiments. The naphthalene results show that while the mean spreading angle is consistent with the historical literature, there may be a weak dependency on x-based Reynolds number, which emerges as a result of the large sample size of the survey. The distance between the roughness element and the wedge origin exhibits a clear trend with the roughness–height-based Reynolds number. The hotwire measurements explain that this difference originates from whether breakdown occurs first in the central lobe or flanking streaks of the turbulent wedge. This observation highlights different transition dynamics at play within the supercritical regime. In agreement with past experiments, the hotwire measurements reveal that breakdown occurs in the wall normal shear layer above low-speed streaks. Due to the elongated streamwise extent of this experiment, secondary streak dynamics are also uncovered. A high-speed streak is produced directly downstream of the initiating low-speed streak. Subsequently, a new low-speed streak is observed outboard of the previous high-speed streak. This self-sustaining process is the driving mechanism of turbulent wedge spreading.

The generation process and behavior of non-Newtonian viscoelastic vortex rings generated with a piston cylinder apparatus are studied through fluorescent dye visualizations. The generalized Reynolds numbers targeted by this study are ({varvec{Re}}in mathbf{[10,600]}) and allow the identification of different regimes leading gradually from the generation of a blob that remains attached to the cylinder to that of a starting jet and then to a self-propagating vortex ring. Experiments are performed in eight different viscoelastic solutions and allow to evaluate the influence of the rheological properties of the fluid on the dynamics of the coherent structure. Regardless of the viscoelastic fluid used, the kinematics of the structure exhibits two steps: an initial propagation until reaching a maximum penetration position which depends on the parameters of the experiment (Reynolds number and elasticity number) and then a backward movement over a distance which also depends on these same parameters. The visualizations highlight important deformations of the structure envelope, in particular a spanwise flattening just before reaching the maximum position and a refocusing around the propagation axis during the backward movement. Results are interpreted in terms of Reynolds number and elasticity number.

Extensive experimental research on high-pressure spray has been conducted for decades to deepen our understanding and optimize its use in transportation, aviation, and propulsion applications; however, the near-field and in-nozzle flow characteristics are not fully understood. Dense near-field spray is among the most challenging diagnostic tasks since light is severely scattered and diffused by the liquid droplets and columns. In this work, the near-field spray and in-nozzle flow characteristics of an aeration nozzle at elevated pressures were characterized by neutron radiography imaging at the Oak Ridge National Laboratory High Flux Isotope Reactor. Neutron imaging benefits via strong penetration depths for some metals (i.e., aluminum, lead, and steel) and is sufficiently sensitive to detection of light elements, especially for hydrogen-based molecules, due to the large incoherent scattering cross section of neutrons. Both two-dimensional snapshots of the near-field spray and a three-dimensional tomographic scan of the nozzle geometry and in-nozzle water were obtained. This work provides new quantitative characterization of practical metal nozzle geometry for accurate boundary conditions, internal flow patterns inside the nozzle, and high-pressure spray flows. The findings may be used to improve performance and operating conditions of transportation vehicles and propulsion systems.

We present a novel bifocal imaging method enabling three-dimensional particle tracking and size determination employing a single camera only. The method is based on double refraction causing a particle to be imaged twice, each particle image of different blur. From these double images, a linear calibration function can be derived allowing to determine the three-dimensional particle position unambiguously over the entire depth of measurement volume. As this calibration function is independent of the particle size used, the particle size can be determined simultaneously by relating size of the double images and depth position of the particle. To prove the applicability, a co-laminar flow of two particle suspensions with particles of 1.14 (upmu)m and 2.47 (upmu)m in diameter was measured in a Y-shaped microchannel. While the laminar flow field was measured with very low uncertainty and independent of the particle size, the particle size distributions determined reproduced reliably the size distributions expected for the co-laminar flow applied, with a precision of about 98.6 (%) regarding the particle size discrimination. The progress for research is a new method readily to implement in common optical setups, promising, for example, valuable insights in polydisperse suspension flows—the vast majority of flows in fundamental research and applications.

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