Latest papers in fluid mechanics

Variational approach to droplet transport via bendotaxis: Thin film dynamics and model reduction

Physical Review Fluids - Wed, 04/20/2022 - 11:00

Author(s): Zhen Zhang and Tiezheng Qian

Bendotaxis has recently been proposed as a mechanism for self-transport of droplets at small scales. When an active droplet undergoes self-transport via bendotaxis, interfacial, elastic, and active forces jointly determine the droplet motion in a deformable channel. Through simulations of thin-film dynamics and a model reduction based on Onsager’s variational principle, we show that wettability and activity can jointly operate to enhance or weaken the self-transport effect of bendotaxis, depending on the sign of wettability (hydrophobic or hydrophilic) on the channel wall and the sign of activity (contractile or extensile) in the droplet.

[Phys. Rev. Fluids 7, 044002] Published Wed Apr 20, 2022

Elliptical particle suspensions in Couette flow

Physical Review Fluids - Wed, 04/20/2022 - 11:00

Author(s): Xuechao Liu, Haibo Huang, and Xi-yun Lu

The roles of inertia (Re) and particle aspect ratio (Ar) on the rheology of elliptical particle suspensions are investigated. Scaling trends are found between the viscosity and the particle alignment for any Re considered here. Different mechanisms of stress are calculated. Besides the major contribution of stresslet, Reynolds stress contributes more as Ar and Re increase.

[Phys. Rev. Fluids 7, 044303] Published Wed Apr 20, 2022

Analysis and modeling of bubble-induced agitation from direct numerical simulation of homogeneous bubbly flows

Physical Review Fluids - Wed, 04/20/2022 - 11:00

Author(s): A. du Cluzeau, G. Bois, N. Leoni, and A. Toutant

Using direct numerical simulations of homogeneous bubbly flows, an analysis of velocity fluctuations is performed and a methodology for development of a bubble-induced agitation (pseudoturbulence) model described. This process is based on separating two causes of velocity fluctuations in the liquid: Agitation resulting from wakes and their collective interactions; and nonturbulent fluctuations due to averaged wakes and potential flows around bubbles. An energy conversion signature is observed, revealing the importance of nonlinear interactions. A model is proposed which gives satisfactory results on our database for a wide range of bubble Reynolds numbers and is consistent with experiments.

[Phys. Rev. Fluids 7, 044604] Published Wed Apr 20, 2022

Instability of a heavy gas layer induced by a cylindrical convergent shock

Physics of Fluids - Wed, 04/20/2022 - 10:59
Physics of Fluids, Volume 34, Issue 4, April 2022.
The instability of a heavy gas layer (SF6 sandwiched by air) induced by a cylindrical convergent shock is studied experimentally and numerically. The heavy gas layer is perturbed sinusoidally on its both interfaces, such that the shocked outer interface belongs to the standard Richtmyer–Meshkov instability (RMI) initiated by the interaction of a uniform shock with a perturbed interface, and the inner one belongs to the nonstandard RMI induced by a rippled shock impacting a perturbed interface. Results show that the development of the outer interface is evidently affected by the outgoing rarefaction wave generated at the inner interface, and such an influence relies on the layer thickness and the phase difference of the two interfaces. The development of the inner interface is insensitive (sensitive) to the layer thickness for in-phase (anti-phase) layers. Particularly, the inner interface of the anti-phase layers presents distinctly different morphologies from the in-phase counterparts at late stages. A theoretical model for the convergent nonstandard RMI is constructed by considering all the significant effects, including baroclinic vorticity, geometric convergence, nonuniform impact of a rippled shock, and the startup process, which reasonably predicts the present experimental and numerical results. The new model is demonstrated to be applicable to RMI induced by a uniform or rippled cylindrical shock.

Inspiratory leakage flow fraction for surgical masks with varying gaps and filter materials

Physics of Fluids - Wed, 04/20/2022 - 10:59
Physics of Fluids, Volume 34, Issue 4, April 2022.
Leakage flows due to a poor fit can greatly reduce the mask protection efficiency. However, accurate quantification of leakages is lacking due to the absence of standardized tests and difficulties in quantifying mask gaps. The objective of this study is to quantify the leakage flows around surgical masks with gaps of varying areas and locations. An integrated ambient–mask–face–airway model was developed with a pleated surgical mask covering an adult's face, nose, and chin. To study the gap effects, the mask edge along the facile interface was divided into different domains, which could be prescribed either as the mask media or air. A low Reynolds number k-ω turbulence model with porous media was used to simulate inspiratory flows. Experimentally measured resistances of two surgical masks were implemented in porous media zones. Results show that even a small gap of 1-cm2 area could cause a 17% leakage. A gap area of 4.3 cm2 at the nose bridge, the most frequent misfit when wearing a surgical mask, led to a leakage of 60%. For a given mask, the increase rate of leakage slowed down with the increasing gap area. For a given gap, the leakage fraction is 30–40% lower for a mask with a resistance of 48.5 Pa than a mask of 146.0 Pa. Even though the flow dynamics were very different among gaps at different locations, the leakage intensity appeared relatively insensitive to the gap location. Therefore, correlations for the leakage as a function of the gap area were developed for the two masks.

Numerical analysis of interaction between turbulent structures and transient sheet/cloud cavitation

Physics of Fluids - Wed, 04/20/2022 - 10:59
Physics of Fluids, Volume 34, Issue 4, April 2022.
This paper through the in-house code numerically examines the cavitation–vortex–turbulence interaction mechanism. The high grid resolution can obtain a more detailed flow field structure, which is helpful to reveal the relationship between cavitation occurrence and development and local turbulent flow field. Results are presented for a three-dimensional NACA66 hydrofoil fixed at an 8° angle of attack under a moderate Reynolds number of 1 × 106 and sheet/cloud cavitating conditions. Numerical simulations are performed via the boundary data immersion method coupled with the artificial compressibility method through a Fortran-based code. The results show that the numerical predictions are capable of capturing the unsteady cavitation characteristics, in accordance with the quantitative features observed in high-speed cavitation tunnel experiments. The evolution of the transient cavitating flow can be divided into three stages: growth of the attached sheet cavity, development of a re-entrant jet, and cloud shedding downstream. The Liutex method is applied to capture the vortex structure. Further analysis of the process of enstrophy transport reveals that cavitation promotes vortex production and increases the enstrophy as the cavity becomes more unstable. Moreover, the structure of the vortex gradually evolves from a vortex tube to a U-type vortex, Ω-type vortex, and streamwise vortex. Finally, the interaction between cavitation and turbulence is expounded using the turbulent energy transport equation, which demonstrates that cavitation promotes the production, diffusion, and dissipation of turbulent kinetic energy, while the viscous transport term only acts during the process of cloud cavity shedding.

Profiles of high-order moments of longitudinal velocity explained by the random sweeping decorrelation hypothesis

Physical Review Fluids - Tue, 04/19/2022 - 11:00

Author(s): Kelly Y. Huang and Gabriel G. Katul

The generalized log law for the high-order moments of longitudinal velocity with distance from a boundary in the inertial region is derived from the following assumptions: that the random sweeping decorrelation hypothesis applies; that the velocity statistics are near-Gaussian; and that the longitudinal velocity spectrum scales as k−1 in the intermediate region. Measurements of longitudinal velocity collected within the first meter from the surface in the western deserts of Utah show good agreement with the proposed theory even under mild thermal stratification.

[Phys. Rev. Fluids 7, 044603] Published Tue Apr 19, 2022

Pattern formation on the surface of the granular medium in a horizontal rotating cylinder filled with fluid

Physical Review Fluids - Mon, 04/18/2022 - 11:00

Author(s): Veronika Dyakova and Denis Polezhaev

We observe a new type of pattern formation at the interface between fluid and heavy granular medium in a horizontal rotating cylinder. Gravitational force perturbs the surface of the granular medium and induces azimuthal motion of suspended granules relative to the rotating fluid. The analysis of the experimental data shows that the length of the observed ripples is consistent with the predictions of the theory of the Kelvin-Helmholtz instability.

[Phys. Rev. Fluids 7, 044302] Published Mon Apr 18, 2022

Experimental investigation on the impingement of synthetic jet vortex rings on a spherical wall

Physical Review Fluids - Mon, 04/18/2022 - 11:00

Author(s): Changlong Chen, Donglai Gao, and Wen-Li Chen

The interaction between vortex rings and walls widely exists in natural phenomena and engineering practices. In this paper, we experimentally explore synthetic jet vortex rings impinging on a spherical wall. The impingement behavior is revealed in detail by vortex ring evolution and trajectories. An important observation is that driven by the curve, the strength of the induced vortex ring increases with the decrease in the sphere diameter.

[Phys. Rev. Fluids 7, 044703] Published Mon Apr 18, 2022

Flow and mixing induced by single, colinear, and colliding contractile waves in the intestine

Physical Review Fluids - Fri, 04/15/2022 - 11:00

Author(s): Richard J. Amedzrovi Agbesi and Nicolas R. Chevalier

The flow of liquid food bolus in different intestinal contraction regimes is studied experimentally, analytically, and numerically. We show that a particle subjected to a peristaltic wave has a nonintuitive propulsion-reflux motion. When multiple waves are generated sequentially, as happens in the gut, reflux is found to be maximized for an inter-wave length corresponding to that observed physiologically in animals, indicating a possible evolutionary bolus absorption optimization. We find that counter-propagating waves generate a high-pressure region from which high-velocity bolus jets emerge. As a result, these waves generate 80 times more mixing than waves going in the same direction.

[Phys. Rev. Fluids 7, 043101] Published Fri Apr 15, 2022

Controlling instabilities of electrified liquid jets via orthogonal perturbations

Physical Review Fluids - Fri, 04/15/2022 - 11:00

Author(s): Chenghao Xu, Wuqing He, Weiwei Yang, Weiwei Deng, and Huihui Xia

We present an experimental study on controlling electrified jet instabilities via imposing orthogonal perturbations. A steady helicoidal whipping structure is achieved for the first time in air ambience, which is of significant importance in producing uniform fibers using electrospinning. The flexibility of orthogonal perturbations in output also allows more types of perturbation patterns following Lissajous curves, which demonstrate great potential in applications such as film deposition.

[Phys. Rev. Fluids 7, 043702] Published Fri Apr 15, 2022

Linear stability analysis of nonisothermal glass fiber drawing

Physical Review Fluids - Fri, 04/15/2022 - 11:00

Author(s): Julien Philippi, Mathias Bechert, Quentin Chouffart, Christophe Waucquez, and Benoit Scheid

We present a model of the draw resonance instability for glass fiber drawing including inertia, gravity, surface tension, and temperature. Using linear stability analysis, we have evidenced, through an alternative scaling, the crucial role of the fiber aspect ratio as a control parameter. It appears that a strong destabilization of the system occurs as this parameter increases. We also show the significant influence of nonhomogeneous ambient temperature on system stability. Contrary to the film casting problem, the high critical draw ratio in industrial applications could be rationalized only for a heat transfer coefficient dependent on both the velocity and cross-sectional area of the fiber.

[Phys. Rev. Fluids 7, 043901] Published Fri Apr 15, 2022

Effect of wing sweep on a perching maneuver

Physical Review Fluids - Fri, 04/15/2022 - 11:00

Author(s): Dibya Raj Adhikari, George Loubimov, Michael P. Kinzel, and Samik Bhattacharya

During landing flights, birds often perform a perching maneuver, which allows them to land smoothly. In this work, we investigated the effect of wing sweep on the evolution of the instantaneous forces and the flow field during the perching maneuver. Our results indicate that swept wing generates higher aerodynamic forces, which is contributed by a stable leading-edge vortex (LEV).

[Phys. Rev. Fluids 7, 044702] Published Fri Apr 15, 2022

Experimental studies on the frequency selection in flat plate wakes: Mean-flow stability analyses and low-dimensional modeling

Physical Review Fluids - Thu, 04/14/2022 - 11:00

Author(s): Dipankar Dutta, Indra Kanshana, Shyam Sunder Gopalakrishnan, and A. C. Mandal

The global frequency selection of two-dimensional vortex shedding in the flat plate wake is investigated experimentally. By performing a local stability analysis, and low-dimensional modeling based on the time-averaged mean flow velocity profiles, we have shown that the time-averaged velocity profiles give an accurate estimate of the global shedding frequency. Furthermore, we have analyzed the interaction strengths between the mean flow and the higher harmonics thereby experimentally supporting the theoretical criterion outlined previously by Sipp and Lebedev.

[Phys. Rev. Fluids 7, 044102] Published Thu Apr 14, 2022

Mode competition in a plunging foil with an active flap: A multiscale modal analysis approach

Physical Review Fluids - Tue, 04/12/2022 - 11:00

Author(s): Tso-Kang Wang and Kourosh Shoele

Controlling flow-induced fluttering with morphing surfaces has long been a practical solution. However, the background knowledge of how the structure interacts with the flow has been lacking. Using a tightly-coupled fluid-structure interaction algorithm that utilizes conformal mapping techniques to provide geometrical weighting for multiscale decomposition, we isolate the dynamic effects caused by the surface motion and the flow. The energy ratio between the two effects is proven to be a good indicator of the control efficacy of the morphing flap.

[Phys. Rev. Fluids 7, 044701] Published Tue Apr 12, 2022

Formation of dissipative structures in a three-dimensional electro-thermo-convective flow

Physical Review Fluids - Mon, 04/11/2022 - 11:00

Author(s): Kang Luo, Xue-Lin Gao, Xue-Rao He, Hong-Liang Yi, and Jian Wu

Direct numerical simulations and linear stability analysis are performed to study the three-dimensional electro-thermo-convective (ETC) flow between two parallel plates under a simultaneously applied temperature difference and voltage. Entropy generation analysis and hexagonal pattern analysis are u…

[Phys. Rev. Fluids 7, 043701] Published Mon Apr 11, 2022

Analysis of second moments and their budgets for Richtmyer-Meshkov instability and variable-density turbulence induced by reshock

Physical Review Fluids - Mon, 04/11/2022 - 11:00

Author(s): Man Long Wong, Jon R. Baltzer, Daniel Livescu, and Sanjiva K. Lele

A Mach 1.45 shock and subsequent reshock interacting with a high Atwood number interface between sulfur hexafluoride and air is studied with an adaptive mesh simulation with more than 4.5 billion cells. Mechanisms governing the variable-density flow after the shocks’ interactions with the interface are analyzed with transport equations. The figure shows the mole fraction fields in the numerical shock tube around the interface just before (left) and after (right) the reshock. Red and blue colors represent heavier and lighter fluids, respectively. The reshock deposits baroclinic vorticity at both large and small scales and thus rapid breakdown to fully developed turbulence ensues.

[Phys. Rev. Fluids 7, 044602] Published Mon Apr 11, 2022

Onset of turbulence in particle-laden pipe flows

Physical Review Fluids - Mon, 04/11/2022 - 11:00

Author(s): Willian Hogendoorn, Bidhan Chandra, and Christian Poelma

Knowledge of the onset of turbulence in particle-laden pipe flows is important for a range of practical applications. Therefore, in the current study we consolidate both existing and new experimental data to investigate the exact role of size and volume fraction of the suspended particles on the stability of suspension flows. By introducing a new parameter, based on the particle-to-pipe diameter ratio and volume fraction, a wide range of particle-laden flows are united on one single curve, being a function of the suspension Reynolds number. Moreover, this parameter allows us to distinguish between the different transition mechanisms: classical, intermediate, or particle-induced.

[Phys. Rev. Fluids 7, L042301] Published Mon Apr 11, 2022

Influence of added dye on Marangoni-driven droplet instability

Physical Review Fluids - Thu, 04/07/2022 - 11:00

Author(s): Carola Seyfert and Alvaro Marin

When studying multiphase flows, it is customary to add a dye to one of the phases to enhance the contrast between different phases. However, many dyes have surface-active effects which, while minute in stationary measurements, can have a significant influence on interfacial phenomena out of equilibrium. In this study, we quantify the consequences of dye addition for the Marangoni Bursting phenomenon, and we offer some insights on the dramatic effects caused by the dye. Furthermore, we show two different, straightforward approaches to quantify the contrast enhancement gained through dye addition.

[Phys. Rev. Fluids 7, 043602] Published Thu Apr 07, 2022

Multiphase simulations and experiments of subaqueous granular collapse on an inclined plane in densely packed conditions: Effects of particle size and initial concentration

Physical Review Fluids - Thu, 04/07/2022 - 11:00

Author(s): Cheng-Hsien Lee and Jia-You Chen

Subaqueous granular collapse on an inclined plane was investigated numerically in densely packed conditions by using a multiphase model. A new set of laboratory experiments were performed to validate the multiphase model with four different particle sizes (from fine sand to very coarse sand). The simulated results reveal that both the volume of the sliding mass in the early stages (initial sliding volume) and the front speed increase with increasing particle size. Additionally, increasing the initial concentration reduces the initial sliding volume and front speed.

[Phys. Rev. Fluids 7, 044301] Published Thu Apr 07, 2022


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