Experiments in Fluids最新文献

In nature as well as in industrial applications, turbulent mixing is ubiquitous. In most cases, these are different fluids with different physical properties (density and/or viscosity). Moreover, all important changes such as mass, momentum and scalar fluxes occur across the turbulent/non-turbulent interface, a thin and sharp layer that separates the turbulent core from the irrotational surrounding fluid. In this paper, we present statistics conditioned on the instantaneous interface position in the very near field of a variable viscosity jet to study the birth and growth of turbulence. The simultaneous scalar concentration and velocity fields are obtained from planar laser-induced fluorescence, where the images undergo an original correction and normalization process, and stereo-particle image velocimetry, respectively. We show that the turbulence is much more advanced in the variable viscosity flow (VVF), which exhibits some features that are visible much later in the constant viscosity flow (CVF). Furthermore, this study reveals a change in the nature of the mixing process between VVF and CVF, which needs to be further investigated.

This review focuses on flow sensors in microfluidics specific to on-chip detection inside a microchannel. These sensors are distinct from external, off-chip flow sensors that are often associated with microfluidics. We explore the various mechanisms and physical principles involved in their working and compare their pros and cons. We consider the working principles that can be used for sensing at the microscale and prepare a typical designer’s perspective with respect to flow sensors that can be integrated on a microfluidic chip. Developing an accurate on-chip flow sensor would enable autonomous flow control leading to advancements in point-of-care applications of microfluidics. We also highlight some of the challenges that have kept researchers at bay from developing an all-weather on-chip flow sensor for microfluidics. Also included is a brief discussion on the relevant applications of on-chip flow sensors including preventive healthcare, drug development, and microreactors. This review should give an impetus to development of better and larger variety of on-chip flow sensors.

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.