Physics of Fluids

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ISSN / EISSN : 1070-6631 / 1089-7666
Published by: AIP Publishing (10.1063)
Total articles ≅ 29,569
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Ludovico Fossa', Shahbozbek Abdunabiev, Mina Golshan,
Published: 10 May 2022
Abstract:
Recent results have shown that there is an acceleration in the spread of the size distribution of droplet populations in the region bordering the cloud and undersaturated ambient.We have analyzed the supersaturation balance in this region, which is typically a highly intermittent shearless turbulent mixing layer, under a condition where there is no mean updraft. We have investigated the evolution of the cloud - clear air interface and of the droplets therein via direct numerical simulations.We have compared horizontal averages of the phase relaxation, evaporation, reaction and condensation times within the cloud-clear air interface for the size distributions of the initial monodispersed and polydisperse droplets.For the monodisperse population, a clustering of the values of the reaction, phase and evaporation times, that is around 20-30 seconds, is observed in the central area of the mixing layer, just before the location where the maximum value of the supersaturation turbulent flux occurs.This clustering of values is similar for the polydisperse population but also includes the condensation time.The mismatch between the time derivative of the supersaturation and the condensation term in the interfacial mixing layer is correlated with the planar covariance of the horizontal longitudinal velocity derivatives of the carrier air flow and the supersaturation field, thus suggesting that a quasi-linear relationship may exist between these quantities.
Mona Kanso, Marwa Naime, Vikash Chaurasia, Khemapat Tontiwattanakul, ,
Published: 10 May 2022
Abstract:
The coronavirus is always idealized as a spherical capsid with radially protruding spikes. However, histologically, in the tissues of infected patients, capsids in cross section are elliptical, and only sometimes spherical [B.W. Neuman, J Virol, 80, 7918 (2006)]. This capsid ellipticity implies that coronaviruses are oblate or prolate or both. We call this diversity of shapes, pleomorphism. Recently, the rotational diffusivity of the spherical coronavirus in suspension was calculated, from first principles, using general rigid bead-rod theory [M.A. Kanso, Phys Fluids, 32, 113101 (2020)]. We did so by beading the spherical capsid, and then also by replacing each of its bulbous spikes with a single bead. In this paper, we use energy minimization for the spreading of the spikes, charged identically, over the oblate or prolate capsids. We use general rigid bead-rod theory to explore the role of such coronavirus cross-sectional ellipticity on its rotational diffusivity, the transport property around which its cell attachment revolves. We learn that coronavirus ellipticity drastically decreases its rotational diffusivity, be it oblate or prolate.
Yagya Narayan, Sanghamitro Chatterjee, ,
Published: 10 May 2022
Abstract:
A face mask is essential personal protective equipment to mitigate the spread of COVID-19. While a cloth mask has the least ability to prevent the passage of infectious respiratory droplets through it compared to surgical and N95 masks, the surgical mask does not fit snugly and causes significant air leakage. The synthetic fibers in the latter reduce comfortability and are an allergen for facial eczema. Moreover, the N95 mask causes CO$_2$ inhalation and reduces heat transfer in the nose. Therefore, the objective of the present work is to improve the effectiveness of a two-layer cloth mask by introducing an intermediate, high-efficiency particulate air (HEPA) filter layer. A significant volume of impacted droplets penetrates through a single-layer cloth mask, ejecting secondary droplets from the rear side. However, a two-layer cloth mask prevents this ejection. Despite slowing down the liquid penetration, capillary imbibition through cloth due to its hydrophilicity causes the transport of the liquid into the second layer, resulting in a thin-liquid layer at the mask's rear-side surface and contaminating it. Conversely, the HEPA filter insert in the cloth mask prevents the imbibition, making the second cloth layer free of contamination. We attribute the impedance to the imbibition by the intermediate HEPA filter layer to its hydrophobic characteristics. We experimentally and analytically assess the role of wettability on capillary imbibition. The breathability measurements of masks show that the HEPA insert in the cloth mask does not reduce its breathability compared to that of the surgical mask.
Published: 9 May 2022
Abstract:
Helicity is an important quantity in fluid mechanics that indicates the presence of linked or knotted hydrodynamic vortex filaments. Such flow structures are not only promising elementary structures to study mass and momentum transfer in turbulent flows, but also potent analogs for other topological problems arising in particle physics, liquid crystals, and plasma physics. However, experimental studies of knots and links are highly challenging due to the limited control over helicity generation and difficult observation of the resulting fast-paced multiscale flow evolution. In this paper, we propose using acoustic streaming to link hydrodynamic filaments in fluids. The method is contactless, almost instantaneous, and is relatively insensitive to viscosity. Importantly, it allows starting from quite arbitrary three-dimensional flow structures without relying on external boundary conditions. We demonstrate our approach by using an acoustic screw dislocation to link two hydrodynamic vortex filaments in a sessile droplet. We observe an inversion of the flow chirality (measured by the hydrodynamic helicity) as the topological charge of the screw dislocation is increased. Combined with recent progress in acoustic field synthesis, this work opens a window to study more complex hydrodynamic knots and links topology at a broader range of space- and time- scales.
Saul Piedra, Jaziel Alberto Rojas, Antonio Ivan Rivera, Aldo Figueroa
Published: 9 May 2022
Abstract:
The electromagnetically driven flow in the wide gap of a concentric spheres system is studied experimentally and numerically in the laminar regime ($Re \leq 1540$). The mainly azimuthal driving Lorentz force is promoted by the interaction of a direct current and a dipolar magnetic field. The current is injected through two ring-shaped copper electrodes located at the equatorial zone of each sphere and the magnetic field is produced by a permanent magnet located inside the inner sphere. Velocity profiles for the azimuthal component in the equatorial plane were obtained with Particle Image Velocimetry and the radial velocity component of the flow was recorded using Ultrasonic Doppler Velocimetry. Laser-fluorescein technique was used for flow visualization. It was found that for a critical electric current ($Re=1140$), an instability occurs and the flow becomes time-dependent. We found, theoretically and experimentally a vortex breakdown structure at each of the polar zones of the spherical gap, which to the best knowledge of the authors, this is the first time it is reported with electromagnetic forcing. A full three-dimensional numerical simulation reproduces the experimental observations qualitatively and quantitatively.
Pooja Kumari,
Published: 9 May 2022
Abstract:
Non-Newtonian shear-thinning droplet formation mechanism in a T-junction microchannel is experimentally investigated using aqueous solutions of xanthan gum as the dispersed phase and mineral oil as the continuous phase. Influences of both phase flow rates and polymer concentration on flow regime transition are explored. It is observed that the initial vertical expansion stage is present only for the Newtonian and lower shear-thinning systems. Droplet evolution rate shows the influence of continuous phase flow rate and shear-thinning properties on the dynamics of necking stages, viz. squeezing, transition, pinch-off, and filament thinning. Analysis of Ohnesorge number (Oh) reveals that inertial force dominates in the squeezing stage, whereas viscous and interfacial force control in the filament thinning stage. Longer and stable filament generation is detected as a discerning feature for non-Newtonian systems that appears more prominent with increasing dispersed phase shear-thinning properties. The results also indicate an inverse relation of droplet length with continuous phase flow rate and xanthan gum concentration, while the droplet formation frequency and its polydispersity vary directly with those parameters.
Reynolds Addo-Akoto, ,
Published: 8 May 2022
Abstract:
Experimental investigations are made for the combined effects of aspect ratio (AR), slack (βS) and pitch angles on the aerodynamic characteristics of flexible flapping wings in hover. βS is introduced as a way to indirectly alter the flexibility of the wing. An optimum AR range of 3 to 5 based on the lift coefficient is observed depending on the flexibility. For a constant AR, the intensity of the leading-edge vortex (LEV) with corresponding circulatory-based lift mitigates as βS increases beyond 2.5{degree sign}. The variation of βS affects the magnitude of the shed trailing-edge vortices (TEVs) but the vorticity core is maintained. We found the shed TEVs to be the key vortical feature of twistable flexible wings in comparison to the rigid (untwisted) cases. More intriguingly, the negative wing twist played a significant role in sustaining the circulatory lift at the outboard section for even high AR cases. The primary LEV trace is found to be an indicator for the effective spanwise limit of the LEV. Although an increase in AR reduces the effective spanwise limit, it is found that wing flexibility further decreases the radial distance. Again, the study reveals that lift enhancement in the rigid wing requires a wider effective downwash area induced by the outward movement of the LEV traces to merge with the tip vortex. Contrarily, the flexible wing requires an elongated downwash area induced by the wing twist to enhance the aerodynamic performance.
Elad Elmakies, Oleg Shildkrot, Nathan Kleeorin, , , Alexander Eidelman
Published: 8 May 2022
Abstract:
We study experimentally turbulent thermal diffusion of small particles in inhomogeneous and anisotropic stably stratified turbulence produced by one oscillating grid in the air flow. The velocity fields have been measured using a Particle Image Velocimetry (PIV). We have determined various turbulence characteristics: the mean and turbulent velocities, two-point correlation functions of the velocity field and an integral scale of turbulence from the measured velocity fields. The temperature field have been measured with a temperature probe equipped with 12 E thermocouples. Spatial distributions of micron size particles have been determined by a PIV system using the effect of the Mie light scattering by particles in the flow. The experiments have demonstrated that particles are accumulated at the minimum of mean fluid temperature due to phenomenon of turbulent thermal diffusion. Using measured spatial distributions of particles and temperature fields, we have determined the effective turbulent thermal diffusion coefficient of particles in inhomogeneous temperature stratified turbulence.This experimental study has clearly detected phenomenon of turbulent thermal diffusion in inhomogeneous turbulence.
, Daria Rusova, Konstantin Zvonarev
Published: 8 May 2022
Abstract:
Unusual treelike thermal structures can be formed on the free surface of the evaporating water in small containers. These structures are studied in detail for the first time and it has been established that under certain parameters (the container diameter is about 90 mm, the water layer thickness is 6-15 mm, and the container wall temperature is 28-47 {degree sign}C) they are the most probable and pronounced. The presence of both horizontal and vertical temperature gradients is fundamental for the formation of such structures. An attempt has been made to numerically model treelike structures. It is shown that the classical model of heat and mass transfer in a fluid, taking into account the mechanisms of heat transfer on the free surface, usually used for water, and the reference coefficient of the surface tension of water, does not even allow one to qualitatively predict the observed treelike structures on the water surface. A hypothesis about the need to consider the influence of impurities on heat and mass transfer near the water surface has been proposed.
Chunlei Cao, Xiaojing Ma, , Haiwang Li, Guanglin Liu
Published: 7 May 2022
Abstract:
Leidenfrost droplet possesses ultra-low flow resistance, but it is challenging to obtain large thrust force for fast transportation and regulate the direction of droplet motion. Here, for the first time, we demonstrate a novel mechanism for the control of droplet dynamics by explosive boiling. Our system consists of two surfaces that have different functions: a smooth surface running in the Leidenfrsot state for droplet levitation, and a skirt ring edge surface (SRES) as an explosive boiling trigger. For droplet-wall collision with SRES, micro/nano scale roughness not only enhances energy harvesting from the skirt ring to the droplet due to increased radiation heat transfer, but also provides nucleation sites to trigger explosive boiling. The symmetry breaking of explosive boiling creates a thrust force that is sufficient to propel the droplet. The suppression of the thrust force relative to the inertia force regulates the droplet trajectory as it passes through a target location. We show orbit lines passing through a focusing spot that is ~1% of the Leidenfrost surface area around its center, with a maximum traveling speed of ~85 cm/s, which is ~2 times of that reported in the literature. The scale law analysis explains the droplet size effect on the self-propelling droplet dynamics. Our work is attractive for applications under the conditions of required traveling speed and direction of droplet.
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