Experiments in Fluids最新文献

As the energy industry transitions toward net-zero emissions, underground carbon dioxide ((textrm{CO}_2)) storage plays a crucial role in mitigating global warming caused by anthropogenic (textrm{CO}_{2}) emissions. Capillary trapping is considered one of the most promising mechanisms for (textrm{CO}_2) storage in saline aquifer systems. It is essential for securely immobilizing (textrm{CO}_2) in porous rock formations, improving the long-term stability of geological storage. The lack of understanding of factors affecting the contribution of capillary trapping in sequestration may result in only partial achievement of technical and financial objectives. In this study, the mutual relationship between injection rates, initial saturation of (textrm{CO}_2), and capillary trapping capacity was studied by performing displacement experiments on a micromodel representing a heterogeneous sandstone aquifer. The brine-alternating-(textrm{CO}_2) (BAC) technique was applied under varying injection conditions of brine and proxy supercritical (textrm{CO}_2) (({textrm{ScCO}}_2)) into the micromodel. In this study, it was observed that large clusters of trapped (textrm{CO}_2) were developed due to the dominance of capillary forces at lower injection rates of brine, resulting in a trapped (textrm{ScCO}_2) saturation of 0.62. However, the saturation of trapped ({textrm{proxyScCO}}_2) was decreased to 0.37 due to the increasing influence of viscous forces at higher injection rates. This study highlights the crucial role of capillary and viscous forces during BAC injection in heterogeneous sandstone reservoirs and provides a guideline for laboratory-scale (textrm{ScCO}_2) sequestration studies.

This study explores the melting dynamics of frozen water droplets exposed to a non-thermal Surface Dielectric Barrier Discharge (SDBD) plasma actuator, in the context of developing an active ice protection system. Unlike mechanical actuators, which are relatively energy-efficient but lack robustness, or resistive heaters, which are compact but highly energy-consuming, DBD plasma actuators offer a promising alternative by combining heat generation with reduced size and weight. A key focus is placed on measuring the temperature within the ice using a laser-induced fluorescence (LIF) technique. For this purpose, pyranine dye mixed with sucrose was used, enabling a highly temperature-sensitive fluorescence signal in the solid phase. However, the method becomes ineffective once melting progresses due to the limited emission of the deprotonated form of pyranine in liquid water. Melting of a 2-mm droplet was achieved 22–139 s after discharge activation, depending on operating parameters and dielectric material, with ice temperature rises between 3 and 20 °C. To detect the onset of melting, the discharge current was analyzed, with a notable decrease in the number of micro-discharge peaks observed as a liquid layer forms. Finally, by comparing the electrical power dissipated by the actuator with the thermal energy required to raise the droplet’s temperature to the onset of melting, the efficiency of the SDBD system could be evaluated at 0.1%. Results highlight the dominant role of dielectric heating and provide insight into the limitations and potential of plasma-based de-icing strategies.

Event-based cameras are dynamic vision sensors that acquire asynchronous, pixel-wise data when local changes in light intensity exceed a user-defined threshold. Their high temporal resolution and data efficiency are particularly advantageous for applications with rapid movement of sparse/discrete signals such as in flow velocimetry. While previous studies have demonstrated the application of event-based particle image velocimetry (PIV) and particle tracking velocimetry (PTV), the impact of key hardware limitations, specifically latency and bandwidth, on the velocity dynamic range remains underexplored. This work first characterizes event cameras using single-pulse planar illumination of a water/alumina suspension in a cuvette, where effects of laser pulse energy, particle image density/size, and contrast threshold are systematically investigated. A camera performance map showed that non-retrievable information loss occurred row-wise for event rates above (sim) 400 ev/(upmu)s as the readout interface saturated due to camera bandwidth. The observed correlation between event rate and camera latency constrained the minimum time interval between successive laser pulses, therefore limiting the velocity that can be effectively measured with event-based PIV. A subsequent set of experiments performed in an air jet with double-pulse PIV produced exploitable vector fields for a velocity dynamic range of 0.2–1.8 m/s (0.003–0.03 px/(upmu)s) with particle image densities above 0.018 ppp and uncertainties <0.4 px. Results indicate that camera latency and bandwidth introduce a complex trade-off between the time interval between laser pulses (velocity dynamic range) and particle image density (spatial resolution) to ensure robust and exploitable velocity measurements. The proposed methodology can be readily applied to evaluate other event-based cameras and serve as a practical guideline to set up and optimize event-based PIV.

Understanding the gust response of free-falling bodies such as plant seeds and debris is critical in predicting their dispersal. Furthermore, gusts can significantly affect the performance and survivability of low-inertia aerial vehicles. However, current methodologies for studying common gusts, particularly transverse gusts, which are characterised by the sudden appearance of a flow velocity component orthogonal to the flyer’s velocity, are not applicable to untethered or free-falling bodies. This article introduces a novel approach that addresses this limitation through an accelerating reference frame generating a fictitious force that temporarily scales and redirects the gravitational force. This approach is demonstrated through a first-of-its-kind vertical wind tunnel that accelerates horizontally in the direction normal to the flow with the same acceleration as the gust. A preliminary characterisation of the facility is presented. The tunnel acceleration generates the same pressure gradient as irrotational, uniform transverse gusts, without introducing the shear layer typical of Küssner’s gusts. The gust response of a free-falling dandelion diaspore to a discrete transverse gust (Wagner type) is demonstrated, but the proposed approach is suitable for arbitrary time series of transverse gusts, including Theodorsen-type periodic gusts. For the first time, this novel approach will allow investigating the dynamic response of untethered bodies to transverse gusts, including micro- and nanodrones, unpowered microrobots, plant seeds, debris and more.

Effective quantitative visualization of compressible flows across the full field of view is essential for understanding the flow topology and dynamics of supersonic jet oscillations, shock-boundary-layer interactions, and shock reflection and diffraction. The present study provides a methodology for obtaining the time-dependent density field of transient shock-dominated flows within a confined duct. A shock strong enough to induce flow separation and exhibiting unsteady behavior is introduced downstream of the throat within a divergent half duct. The incoming flow Mach number just upstream of the shock is approximately 1.47, and the Reynolds number calculated based on the height and flow properties at the throat is 1.35 (times) (10^5). We employ Mach-Zehnder interferometry with a finite-fringe setup to capture the time-resolved density field including the shock motion, utilizing a He-Ne laser as the light source and a high-speed camera as the recording device. A two-dimensional Fourier fringe analysis is employed to extract the phase information over the entire density field. Fascinating visual representations, such as the pseudo-infinite interferogram and the density field with phase information known as domain coloring, are introduced to illustrate flow topology. The oscillatory characteristics of shock motions are analyzed using both Lagrangian and Eulerian approaches, and the results are compared quantitatively. Furthermore, an uncertainty analysis is conducted to assess the accuracy of the density measurements and to reveal how shock oscillations affect density uncertainty.

This study introduces a single-camera synthetic Schlieren technique for measuring free-surface topography in fluid layers over a smoothly varying solid bed. By modeling light refraction through weakly deformed air–liquid and weakly varying liquid–solid interfaces, we establish a linear relationship between free-surface gradients and pattern displacements that yields an explicit bed-independent formulation for upward-looking configurations. The proposed framework incorporates coordinate mapping to correct refraction-induced parallax distortion influenced by the bed shape. Validation experiments featuring sinusoidal bed topography and wedge-shaped sloped bed topography achieve accurate spatiotemporal reconstruction of both static capillary meniscus profiles and dynamic water drop ripple evolution. The present method advances experimental capabilities for quantifying interfacial hydrodynamics in multi-layer fluid systems.

Surface roughness modifies the flow dynamics over static surfaces and can significantly affect the instantaneous generation of lift and drag. This study presents force and flow measurements on NACA0012 foils covered with simple, commercially available spherical-cap roughness elements. We varied the roughness area coverage relative to the propulsive area from 0% (smooth) to 35% (mid-rough) and 70% (full-rough). Our experiments survey an angle of attack and a Reynolds number range of (-2^circ le alpha le 20^circ) and 10,000 (lessapprox Re lessapprox) 55,000, respectively. Within this parameter space, surface roughness leads to small alterations in time-averaged statistics of lift and drag. In contrast, it leads substantial changes in unsteady force and flow behavior. Specifically, surface roughness reduces lift fluctuations, up to (sim 60%), due to decreased pressure fluctuations on the foil surface. This reduction is accompanied by a modest decrease in time-averaged lift coefficient and an increase in time-averaged drag coefficient. Drag fluctuations increase by up to (sim 30%), except near stall, where both lift and drag fluctuations decrease. Roughness also mitigates flow separation, as indicated by reduced velocity fluctuations and a delayed stall onset in the (C_L(alpha )) curves. These results show that surface roughness influences not only time-averaged statistics but also the instantaneous response of lift, drag, and flow fields. Our findings offer insights into the hydrodynamic function of shark-skin-inspired surfaces and demonstrate how simple, distributed roughness can provide passive control of boundary layer behavior and flow separation.