Experiments in Fluids最新文献

In the paper, a novel experimental setup is introduced that is designed to investigate the dynamics of two-layer liquid–liquid systems under vertical piston oscillations. It can operate within a wide range of fluid properties, including miscible and immiscible, reactive, and non-reactive pairs of liquids. The oscillations are driven by the motion of a high-precision linear motor, whose forcing is transmitted to fluids via membranes and a hydraulic circuit. The system’s flexibility allows for precise adjustment of vibration parameters (frequency and amplitude), facilitating a detailed examination of their effects on fluid dynamics. The setup developed is used in the present work for experiments on a two-layer system composed of aqueous sugar and salt solutions, focusing on the study of double-diffusive convection dynamics. Under oscillatory conditions, experimental results demonstrate a significant reduction in the mixing time between the salt and sugar solutions in the zone of contact between the layers, with nearly one order of magnitude enhancement in the mixing rate compared to non-vibrational experiments. Meanwhile, the oscillations have little impact on the development of finger patterns within the considered duration of experiments. This acceleration in mass transfer processes is attributed to the disruptive effect of oscillations on stable layer stratification, promoting efficient interlayer mixing. The findings underscore the potential applications of controlled vibrations in enhancing fluid mixing dynamics across various scientific and engineering disciplines, especially in real-world marine environments, such as improving oceanic productivity through enhanced mixing strategies.

This study presents a novel experimental setup designed to release a buoyant scalar plume and measure concentration and velocity simultaneously. Tracer gas is released from a point source in a low-Reynolds-number boundary layer ((Re_tau approx 1600)). The buoyant plume released consists of a mixture of varying proportions of tracer gas and a chemically stable base gas, resulting in density ratios relative to ambient air ((rho _s/rho _infty)) of 1.48, 1, and 0.15. The concentration of the tracer gas (iso-butylene) is detected using a photo-ionisation detector (PID) that ionises a small volume of the tracer gas within its chamber. Additionally, an (times)-wire is employed to measure the streamwise and vertical components of velocity. Results of the mean and root-mean-square (RMS) concentration profiles for the positively and negatively buoyant plumes exhibit a Gaussian or reflected-Gaussian behaviour similar to a neutral plume, albeit with altered parameters such as the plume centreline that now vary with (rho _s/rho _infty). The data indicate that the half-width of the positively buoyant plume is wider than that of the neutral plume, and the spread of negatively buoyant plumes is thinner. Consequently, the maximum concentration of the negatively buoyant plume is the largest among the three (rho _s/rho _infty). Although power laws are fitted to describe the downstream evolution of plume spread and maximum mean and RMS values of concentration, the accuracy of the fit appears to be limited.

This study introduces a fast 1D nuclear magnetic resonance (NMR) imaging method based on multi-slice imaging with a stepper motor to study sedimentation dynamics of clayey soils. Traditional NMR is limited by long acquisition times due to water’s T₁ relaxation time. Our approach combines multi-slice imaging with a stepper motor and frequency-based selection, reducing measurement time while maintaining sub-millimeter resolution, at the same time overcoming the limitations by the slow relaxation of water. This nondestructive method provides detailed insights into the sedimentation and consolidation of suspensions, including pore size distribution and density profiles within a single measurement. The technique is demonstrated with kaolinite clay suspensions, highlighting the technique’s ability to capture the dynamics of gravity-driven systems rapidly and accurately, even for fast-sedimenting soils such as kaolinite in the first hours of sedimentation. This advancement is valuable for geotechnical and environmental applications where understanding sedimentation is crucial.

Graphical abstract

The impingement of liquid sprays on hot walls is used extensively in both spray-cooling systems and in combustor fuel injection applications. At low and moderate wall temperatures, the secondary size distributions have been reported in the literature. For high wall superheat conditions, particularly for real multicomponent fuels, this secondary size distribution has received less attention. Understanding the resultant size distribution for a spray-wall impact is key to capturing vaporization and local mixture for fuel-spray impingement. In this study, single drop impacts for a range of single-component (n-decane) and multicomponent jet fuel (F-24) are characterized through dual-view imaging. Secondary droplets are captured for impact Weber numbers of 100–600 and wall temperatures spanning the nucleate and film boiling (Leidenfrost) regimes. Imaging through a transparent sapphire substrate is used to capture the impact phenomena and impact-induced breakup of impacting drops. We report empirical correlations for the secondary droplet size for single-component (n-decane) and multicomponent (F-24) liquid fuels with varying wall temperature to provide validation datasets for spray-wall simulations.

Recent advantages in the depth from defocus technique for the size and location determination of particles in dispersed two-phase flows have enabled the technique to detect and analyze spherical particle images in flow systems with high number concentrations. In the present study, the use of convolutional neural networks for this task will be explored, with comparisons to the conventional analyses in terms of accuracy, tolerable concentration limits and computational speed. This approach requires a large teaching dataset of images, which is only practical and feasible if the dataset can be synthetically generated. Thus, the first development to be presented is an image generation procedure for out-of-focus neighboring spherical particles resulting in a known blurred image overlap. This image generation procedure is validated using laboratory images of known particle size distribution, position and image overlap, before creating a teaching dataset. The trained processing scheme is then applied to both synthetic datasets and to experimental data. The synthetic datasets allow limits of image overlap and tolerable volume concentration limits of the technique to be evaluated as a function of particle size distribution.(https://github.com/xu200911/Generate-overlapping-out-of-focus-particles)

Recent research highlights the need for comprehensive three-dimensional (3D) analysis of laryngeal flow to better understand voice production, as traditional 2D methods fail to capture the full complexity of supraglottal jet dynamics. This study employed tomographic particle image velocimetry to capture the volume velocity flow fields in a synthetic multilayered vocal fold model. The impact of increased airway resistance from different vocal tract configurations was examined. Results indicated that adding a vocal tract reduced the maximum axial velocity and jet displacement, particularly at low subglottal pressure (Psg). Higher Psg increased both the maximum axial velocity and jet displacement. For all configurations, with and without a vocal tract, the vocal folds were observed to open at the posterior and anterior edges first, indicated by a double jet formation at the beginning of the opening phase, followed by an elongated jet during peak flow and a double jet at the posterior and anterior edges during the closing phase. Contrary to previous studies, the glottal flow waveforms became more symmetric between the opening and closing phases with higher Psg and the presence of a vocal tract. Additionally, vocal efficiency (VE) decreased while cepstral peak prominence increased with higher Psg. Overall, this study provides further insights into the influence of vocal tract configurations on the supraglottal jet and supports the correlation between glottal flow skewing and VE.

This research revisits the free-surface synthetic Schlieren technique (FS-SS) (Moisy et al. in Exp Fluids 46(6):1021–1036, 2009) for the topographical measurement of the free surface of a liquid. In contrast to the aforementioned method, which utilizes the image refraction of a random dot pattern through a free surface to determine the surface gradients, this present study mathematically derives a method where the pattern may take an arbitrary three-dimensional shape. This is shown to be theoretically valid under a small pattern slope approximation. Our method is then verified against the previously mentioned, flat-pattern, free-surface synthetic Schlieren technique by resolving the free-surface elevation of plane waves across a channel, showing similar results in constant strain conditions, with improved results in variable strain conditions, particularly in cases where there is a very large difference in strain across the channel. The validation test cases investigated include a rectangular channel containing a transparent liquid with a random dot pattern placed below at a constant angle and a pattern placed on top of a cosine-shaped profile. Both of these setups are validated against the classical FS-SS technique involving a flat pattern. The new method involving an arbitrarily shaped pattern proposed here may increase the resolution in low-amplitude regions by increasing the surface–pattern distance below these regions and correspondingly reducing the sensitivity in high-strain regions by decreasing the surface–pattern distance. Geometries shown to produce advantageous results in waves that include both regions of very high strains and regions of very low amplitudes are explored, resolving the wave in both regions simultaneously. This shows promise in resolving multi-scale surface waves in highly viscous liquids, which may include very high-amplitude regions quickly followed by very low-amplitude regions due to damping effects.

This study presents an experimental investigation of the aerodynamics relevant to the landing of a small Uncrewed Aerial System on a fast-moving ground vehicle. A Representative Ground Vehicle and small Uncrewed Aerial System have been constructed for experimental measurements of the wake interactions in a low-speed, recirculating wind tunnel. Quantitative flow image techniques are employed to probe how the sUAS interacts with the wake structures shed by the ground vehicle when the two are in close proximity. Tools are developed for quantitative comparisons of flow fields and turbulence spectra. Using this tool, it is possible to identify regions of the flow and relative positions of the two vehicles where wake interactions are mostly linear in nature. This near-linear wake interaction was observed to extend to the wake spectra in regions where the time-averaged flow fields were also near-linear. Finally, it is shown that these observations of a near-linear wake interaction do not hold when the sUAS interacts with highly decelerated regions of the ground vehicle wake.

Droplet impact on substrates is the cornerstone of several processes relevant to many industrial applications. Imposing substrate oscillation modifies the impact dynamics and can, therefore, be used to control the ensuing heat, mass, and energy transfer between the substrate and the impacting droplet. Previous research has shown that substrate oscillation strongly influences the spreading behavior of the droplet. In this study, we extend this understanding to examine how substrate oscillations can further modulate the retraction dynamics of the droplet, consequently affecting its long-term behavior, with a particular focus on induced jetting and subsequent breakup. We systematically examine the breakup of jets formed by the recoiling droplet through experimental investigations across a range of oscillation frequencies and amplitudes. Our findings reveal two distinct jet breakup modes: early and late, each governed by different time scales. Subsequently, we present a mechanistic description of the jetting process. Furthermore, we derive a simple scaling analysis based on energy balance to identify the critical condition required for jet breakup. Finally, we compare the experimental data with the scaling analyses to show its efficacy.