Latest papers in fluid mechanics

Analysis of depinning behavior of drop on chemically heterogeneous surface

Physical Review Fluids - Fri, 11/13/2020 - 10:00

Author(s): Bing He, Chunyan Qin, Sihao Zhou, and Binghai Wen

Three types of depinning behaviors of a drop on a heterogeneous surface are numerically investigated. Depinning is found to occur due to two processes: slow- and fast-moving. A dynamic equilibrium results from competition between gravity and the capillary force in the whole drop. At the microscopic scale, based on measurement of the real-time contact angle, the local force balance in the contact line region is maintained by the unbalanced Young’s force and the substrate resistance in the slow-moving stage, while the unbalanced Young’s force provides the driving force for contact line motion in the fast-moving stage.


[Phys. Rev. Fluids 5, 114003] Published Fri Nov 13, 2020

Volumetric measurement of turbulence and flow topology in an asymmetric diffuser

Physical Review Fluids - Fri, 11/13/2020 - 10:00

Author(s): Prashant Das and Sina Ghaemi

Three-dimensional particle tracking velocimetry is used to characterize the time-resolved turbulent flow in an asymmetric diffuser. The measurements cover the full diffuser and show that the separated flow consists of large-scale structures with a Strouhal number smaller than 0.03. Three large vortices are identified; two vortices originate from the wall with the largest diverging angle and the third vortex from the neighboring diverging wall.


[Phys. Rev. Fluids 5, 114605] Published Fri Nov 13, 2020

Temporal dynamics of the alignment of the turbulent stress and strain rate

Physical Review Fluids - Fri, 11/13/2020 - 10:00

Author(s): Joseph G. Ballouz, Perry L. Johnson, and Nicholas T. Ouellette

Energy transfer between scales in the turbulent cascade requires geometric alignment between scale-dependent turbulent stresses and strain rates. However, each of these tensors evolves dynamically and differently, leading to nontrivial temporal behavior of this alignment. A numerical simulation of turbulence is used to study this process and, by formulating the energy flux in a fully Lagrangian way, the important role played by deformation in the energy cascade is highlighted.


[Phys. Rev. Fluids 5, 114606] Published Fri Nov 13, 2020

Self-similar velocity profiles and mass transport of grains carried by fluid through a confined channel

Physics of Fluids - Thu, 11/12/2020 - 12:01
Physics of Fluids, Volume 32, Issue 11, November 2020.
Confined fluid-driven granular flows are present in a plethora of natural and industrial settings, yet even the most fundamental of these is not completely understood. While widely studied grain flows such as bed load and density-matched Poiseuille flows have been observed to exhibit exponential and Bingham style velocity profiles, respectively, this work finds that a fluid-driven bed of non-buoyant grains filling a narrow horizontal channel—confined both from the sides and above—exhibits self-similar Gaussian velocity profiles. As the imposed flow rate is increased and the grain velocity increases, the Gaussian flow profiles penetrate deeper into the packing of the channel. Filling fractions were observed to be also self-similar and qualitatively consistent with granular theory relating to the viscous number I, which at a given position on the self-similar Gaussian curve is found to be generally constant regardless of the imposed flow rate or velocity magnitude. An empirical description of the flow is proposed, and local velocity and filling fraction measurements were used to obtain the local grain flux and accurately recover a total grain flow rate.

Energy stable and mass conservative numerical method for a generalized hydrodynamic phase-field model with different densities

Physics of Fluids - Thu, 11/12/2020 - 11:18
Physics of Fluids, Volume 32, Issue 11, November 2020.
A general hydrodynamic phase-field model for two-phase flows with general average velocity and variable densities is rigorously derived using thermodynamic laws and Onsager’s variational principle. The pressure is naturally involved in both the Cahn–Hilliard equation and Navier–Stokes equation. The proposed model includes two famous phase-field models as its special cases. The model admits a natural energy dissipation law. A semi-implicit, totally linear, and energy stable numerical scheme is proposed for the model, which uses an intermediate velocity involving all driving forces, including surface tension, pressure, and gravity. The tight coupling relationship between pressure and velocity is decoupled. Another advantage of the proposed scheme is that the intermediate velocity allows us to preserve the mass conservation, and consequently, there is no need to impose any mass balance equation in the Navier–Stokes equation as usual. The discrete energy dissipation law is proved rigorously. Several numerical examples are simulated to demonstrate that the proposed method can preserve the energy stability and total mass conservation for complex hydrodynamical flow problems with large density contrasts and gravity.

Recirculation zone downstream lateral expansions of open channel flow

Physics of Fluids - Thu, 11/12/2020 - 11:18
Physics of Fluids, Volume 32, Issue 11, November 2020.
The present work combines experiments, computational fluid dynamics, and hydraulic models to explain the recirculation length variations downstream of sudden lateral expansions of an open-channel flow. This situation can be compared to the classical turbulent backward facing step situation, but the flow shallowness brings substantial differences. Experiments show that the recirculating zone length relative to the expansion width, L/d, varies in a quite wide range from 2 to 14. The expansion ratio Rb and mostly the bed friction number S influence these variations. Three dimensional numerical simulations confirm these results but also detail the role of a third parameter, the relative water depth h/d. A one dimensional hydraulic model reveals the mechanisms leading to so huge L/d variations. Finally, two dimensional depth-averaged flow equations reveal the prevailing role of bed friction for high S values. Thanks to all these results, the present paper explains the evolution of the recirculation zone length with the different parameters. The shortening of this length is caused by two different mechanisms, corresponding to two asymptotical regimes. For low bed friction numbers S and high relative water depth h/d, large-scale vortices shed from the expansion corner drive the momentum exchanges from the freestream to the recirculation. For high bed friction numbers, their role is negligible: the recirculation zone is accelerated by a velocity leveling due to bed friction, leading to L/d ∝ S−0.7.

Dissipative instability of converging cylindrical shock wave

Physics of Fluids - Thu, 11/12/2020 - 11:18
Physics of Fluids, Volume 32, Issue 11, November 2020.
The condition of linear instability for a converging cylindrical strong shock wave (SW) in an arbitrary viscous medium is obtained in the limit of a large stationary SW radius when it is possible to consider the same Rankine–Hugoniot jump relations as for the plane SW. This condition of instability is substantially different from the condition of instability for the plane SW because a cylindrical SW does not have a chiral symmetry in the direction of the SW velocity (from left to right or vice versa) as in the case of a plane SW. The exponential growth rate of perturbations for the converging cylindrical SW is positive only for nonzero viscosity in the limit of high, but finite, Reynolds numbers as well as for the instability of a plane SW.

Far-field particle manipulation scheme based on X wave

Physics of Fluids - Thu, 11/12/2020 - 11:18
Physics of Fluids, Volume 32, Issue 11, November 2020.
The construction of particle manipulation in the near-field sound field has been extensively studied. In this article, a scheme for far-field particle control through a non-diffracted wave based on the X wave is proposed, which has theoretical completeness and algorithmic simplicity for the construction of far-field acoustic tweezers. The analytical expression of the acoustic radiation force (ARF) acting upon spherical particles of any order X wave is deduced. The spectral pattern of ARF exerted by the X wave in the fluid medium is given. The change law of ARF exerted by the X wave with specific parameters is obtained through calculation. Through the drawn image, the possibility of obtaining a wide range of negative ARFs through X wave is verified. In addition, the far-field invariance of the ARF exerted by the X wave as non-diffraction wave is verified, which provides the possibility of the subsequent construction of far-field acoustic tweezers as well as the basis algorithms for designing acoustic schemes for specific particle manipulation.

Effect of the conditional scalar dissipation rate in the conditional moment closure

Physics of Fluids - Thu, 11/12/2020 - 11:18
Physics of Fluids, Volume 32, Issue 11, November 2020.
In the context of modeling turbulent scalar mixing using probability density function (PDF) methods, the treatment of molecular mixing is of paramount importance. The conditional moment closure (CMC) offers a high-fidelity description for molecular mixing in nonpremixed flows. Recent work has demonstrated that first-order CMC can be implemented numerically using the moments of the conditioning variable and first-order joint moments of the scalar of interest. When solving the CMC using, for example, quadrature-based moment methods (QBMM), a functional form must be chosen for the conditional scalar dissipation rate (CSDR) of the conditioning variable. In prior work, the CSDR was chosen to produce a β-PDF for the conditioning variable (mixture fraction) at steady state. This choice has the advantage that the system of moment equations used in QBMM-CMC can be written in closed form. In this work, an alternative choice for the CSDR is investigated, namely, the amplitude mapping closure (AMC). With the AMC, the moment equations can be closed using the quadrature method of moments incorporated into a realizable ordinary differential equation solver. Results are compared with the β-CSDR closure for binary, passive scalar mixing in homogeneous single- and disperse-phase turbulent flows. It is also demonstrated that the moment formulation of CMC provides a straightforward method for modeling the effect of differential diffusion in the context of CMC.

A study of fluid dynamics and human physiology factors driving droplet dispersion from a human sneeze

Physics of Fluids - Thu, 11/12/2020 - 11:17
Physics of Fluids, Volume 32, Issue 11, November 2020.
Recent studies have indicated that COVID-19 is an airborne disease, which has driven conservative social distancing and widescale usage of face coverings. Airborne virus transmission occurs through droplets formed during respiratory events (breathing, speaking, coughing, and sneezing) associated with the airflow through a network of nasal and buccal passages. The airflow interacts with saliva/mucus films where droplets are formed and dispersed, creating a route to transmit SARS-CoV-2. Here, we present a series of numerical simulations to investigate droplet dispersion from a sneeze while varying a series of human physiological factors that can be associated with illness, anatomy, stress condition, and sex of an individual. The model measures the transmission risk utilizing an approximated upper respiratory tract geometry for the following variations: (1) the effect of saliva properties and (2) the effect of geometric features within the buccal/nasal passages. These effects relate to natural human physiological responses to illness, stress, and sex of the host as well as features relating to poor dental health. The results find that the resulting exposure levels are highly dependent on the fluid dynamics that can vary depending on several human factors. For example, a sneeze without flow in the nasal passage (consistent with congestion) yields a 300% rise in the droplet content at 1.83 m (≈6 ft) and an increase over 60% on the spray distance 5 s after the sneeze. Alternatively, when the viscosity of the saliva is increased (consistent with the human response to illness), the number of droplets is both fewer and larger, which leads to an estimated 47% reduction in the transmission risk. These findings yield novel insight into variability in the exposure distance and indicate how physiological factors affect transmissibility rates. Such factors may partly relate to how the immune system of a human has evolved to prevent transmission or be an underlying factor driving superspreading events in the COVID-19 pandemic.

Editorial: Introduction to the 37th Annual Gallery of Fluid Motion (Seattle, Washington, USA, 2019)

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Gwynn J. Elfring and Antonino Ferrante

[Phys. Rev. Fluids 5, 110001] Published Thu Nov 12, 2020

Rico and the jets: Direct numerical simulations of turbulent liquid jets

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): C. R. Constante-Amores, L. Kahouadji, A. Batchvarov, S. Shin, J. Chergui, D. Juric, and O. K. Matar

This paper is associated with a poster winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.20...



[Phys. Rev. Fluids 5, 110501] Published Thu Nov 12, 2020

Gas escape behavior from bursting bubbles

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Ali A. Dasouqi and David W. Murphy

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS....



[Phys. Rev. Fluids 5, 110502] Published Thu Nov 12, 2020

Breaking waves: To foam or not to foam?

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Petr Karnakov, Sergey Litvinov, Jean M. Favre, and Petros Koumoutsakos

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS....



[Phys. Rev. Fluids 5, 110503] Published Thu Nov 12, 2020

Vortex bursting

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Wim M. van Rees

This paper is associated with a poster winner of a 2019 APS/DFD Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.P0035.



[Phys. Rev. Fluids 5, 110504] Published Thu Nov 12, 2020

Swinging jets

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): A. Bertsch, A. Bongarzone, E. Yim, P. Renaud, and F. Gallaire

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS....



[Phys. Rev. Fluids 5, 110505] Published Thu Nov 12, 2020

Gas giant–like zonal jets in the laboratory

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Daphné Lemasquerier, Benjamin Favier, and Michael Le Bars

This paper is associated with a poster winner of a 2019 American Physical Society Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019...



[Phys. Rev. Fluids 5, 110506] Published Thu Nov 12, 2020

Droplets impaling on a cone

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Guillaume Durey, Quentin Magdelaine, Mathias Casiulis, Hoon Kwon, Julien Mazet, Pierre Chantelot, Anaïs Gauthier, Christophe Clanet, and David Quéré

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019...



[Phys. Rev. Fluids 5, 110507] Published Thu Nov 12, 2020

Crystal critters

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Samantha A. McBride, Henri-Louis Girard, and Kripa K. Varanasi

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019...



[Phys. Rev. Fluids 5, 110508] Published Thu Nov 12, 2020

Unraveling the interplay of two counter-rotating helical vortices

Physical Review Fluids - Thu, 11/12/2020 - 10:00

Author(s): Alessandro Capone and Francisco Alves Pereira

This paper is associated with a video winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS....



[Phys. Rev. Fluids 5, 110509] Published Thu Nov 12, 2020

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