Experiments in Fluids最新文献

The plasma around a reentry spacecraft causes the charged particles to interact with the electromagnetic waves emitted by the on-board antennas, and the vehicle experiences radio communication difficulties. A proposed way to mitigate the radio blackout is the magnetic field alleviation technique that consists of superimposing a magnetic field onto the flow, converting the plasma into an anisotropic medium, and changing its refractive index. The applied magnetic field leads to the creation of an extraordinary wave that can propagate for plasma frequencies higher than the radio signal frequency. In this work, a probe containing a cryogenically-cooled high-temperature superconducting magnet is used to study the effect of an applied magnetic field on the plasma flow and on the radio signal propagation, in the VKI Plasmatron facility. The magnetized plasma is characterized by optical emission spectroscopy, stagnation heat flux, and dynamic pressure measurements. The experimental radio signal measurements are conducted using conical horn antennas, operating at frequencies in the K(_text{a})-band. An antenna is placed inside of the magnetic probe, transmitting toward a stagnant air plasma flow. The applied magnetic field causes an increase of the flow temperature, leading to an augmentation of the plasma frequency and stagnation heat flux, due to the Hall effect. No significant effects are observed in the signal transmission and attenuation, while the signal reflection trend is consistent with the variation of magnetic field strength, and plasma and collision frequencies. The dependency of the Faraday rotation with the magnetic field and its direction is observed. While a clear demonstration of the magnetic window is not conclusively observed in the transmission parameters, the behavior of the reflection coefficient shows that the radio blackout mitigation is feasible at optimal combinations of flow ionization.

Bubble departure from walls in cross-flow conditions has been studied. This process plays a fundamental role in numerous applications, from gas-liquid contacting, heterogeneous nucleation in cavitation, to flow boiling. Predicting the size of the detaching bubble is essential for controlling the liquid–gas interfacial area in gas/liquid systems encountered in these applications. However, existing data on bubble departure in cross-flow are often restricted to low wall shear rates where bubble deformation is minor and thus negligible. Moreover, the effect of surface wettability on bubble departure is usually not reported in detail. Therefore, in this study, we analyse bubble deformation and departure from artificial nucleation sites for elevated wall shear rates using high-speed imaging. In addition, the surface wettability was modified and quantities related to dynamic wetting phenomena were examined. Our findings clearly show that wettability of the surface and the dynamic pressure imposed on the bubble by the flow jointly dictate how a bubble deforms prior to departure and how large its size is on departure. In addition, different mechanisms of departure are identified and reported in a regime map based on the critical deformation and the wall shear rate.

An experimental study of the stability of a centrifuged liquid–liquid interface at the non-uniform rotation is hereby presented. Two immiscible liquids, different in density and having high viscosity contrast, fill a horizontal, circular Hele-Shaw (H-S) cell rotating about the vertical axis. At the continuous rotation, the liquids form an axisymmetric core-annular configuration. Then, the rotation rate is modulated as a sum of constant and periodically varying values, and the oscillatory Kelvin–Helmholtz (K–H) instability is sought for. The interface dynamics is assessed by taking photographs and numerically processing the images recorded. With the increase in the amplitude of rotation rate modulation, the interface loses stability giving rise to an azimuthally periodic, quasi-stationary wavy pattern—the “frozen wave” driven by the oscillatory K–H instability. The evolution of its properties in the supercritical regime of oscillations is studied depending on the values of the rotation rate and the frequency and amplitude of librations. The experimental results are analyzed in comparison to the case of a vertical H-S cell at non-uniform rotation. The role of orientation of the gravity field is discussed, and its impact on the threshold of instability onset is found. A conclusion is made that the gravity-induced asymmetry in combination with the choice of the dimensionless frequency may reduce the interface stability by engaging an additional vibrational mechanism.

The effect of free rotation of the cylindrical container wall on the vortex flow of a liquid in a model of a centrifugal reactor is studied. A method for intensifying the vortex flow in a cylindrical reactor by adaptive free rotation of the side wall under the action of viscous fluid drag is proposed. The medium in the cylindrical container is agitated by the upper disk rotation. Other parts of the container can freely rotate relative to their axes with a rotational speed determined by the level of friction in the bearing. Thus, the thermal friction energy of the working fluid with a fixed wall is redistributed to the kinetic energy of rotation of the entire reactor system. The kinematic characteristics and flow structure are examined using experimental Particle Image Velocimetry (PIV) measurements and numerical simulations in Star-CCM + . Results show that even minimal free rotation significantly suppresses vortex breakdown, which is typically observed when the cylindrical container is fixed. Furthermore, the maximum values of the tangential velocity with a rotating wall are 2–3 times higher than in a stationary cylinder, and the flow swirl is an order of magnitude higher, which indicates a significant decrease in hydraulic resistance. These findings are relevant for the development of energy-efficient vortex flow control technologies and the improvement of reactor systems, in particular, in the development of new models of compact centrifugal reactors for biological, chemical, and energy industries.

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.