Latest papers in fluid mechanics

Author(s): Gaurav Singh and Rajaram Lakkaraju

A high level of mixing by passive means is a desirable feature in microchannels for various applications, and use of flexible obstacles (or plates) is one of the prime choices in that regard. To gain further insight, we carry out two-dimensional numerical simulations for flow past one or two flexibl...

[Phys. Rev. E 100, 023109] Published Tue Aug 20, 2019

Author(s): Wen Zhang, Peiqing Liu, Hao Guo, Minping Wan, Jianchun Wang, and Shiyi Chen

The laminar-turbulent transition induced by two-dimensional steplike roughness is investigated focusing on the pattern of breakdown. The statistics of the turbulent burst rate is found to be significantly different from the prediction of the classical theory. A systematic investigation of the patter...

[Phys. Rev. E 100, 023110] Published Tue Aug 20, 2019

Author(s): Itzhak Fouxon and Michael Mond

We study the statistics of fluid (gas) density and concentration of passive tracer particles (dust) in compressible turbulence. As Ma increases from small or moderate values, the density and the concentration in the inertial range go through a phase transition from a finite continuous smooth distrib...

[Phys. Rev. E 100, 023111] Published Tue Aug 20, 2019

Author(s): J. B. Sokoloff

A mechanism for removal of salt from salt water is discussed, which results from friction due to Ohm's law heating, resulting from motion of an electron charge induced in the tube walls by the water molecules’ dipoles and the ions’ charges. The desalination occurs because this friction is larger for...

[Phys. Rev. E 100, 023112] Published Tue Aug 20, 2019

Author(s): Joshua A. Dijksman, Shomeek Mukhopadhyay, Robert P. Behringer, and Thomas P. Witelski

Temperature gradients affect fluid interfaces by changing the local surface tension. Both experiments and numerics show that in rotating thin liquid films, these Marangoni stresses significantly affect fluid film height profile dynamics and equilibria.

[Phys. Rev. Fluids 4, 084103] Published Mon Aug 19, 2019

Author(s): Matteo Foresti and Renzo L. Ricca

In this paper we demonstrate that new phase defects of the Gross-Pitaevskii equation (GPE) can be produced as a Aharonov-Bohm effect resulting from pure phase twist injection on existing defects. This is a phenomenon that has physical justification in the hydrodynamic interpretation of GPE. Here we ...

[Phys. Rev. E 100, 023107] Published Fri Aug 16, 2019

Author(s): Yixin Zhang, James E. Sprittles, and Duncan A. Lockerby

This work combines molecular dynamics simulations with analytical techniques to study the effect of thermal fluctuations on the stability of thin liquid films. Results from a stochastic thin-film equation agree well with those from the simulations, but disagree with a deterministic equation. The authors conclude that thermal fluctuations play an important role in the dynamics of thin films at the nanoscale.

[Phys. Rev. E 100, 023108] Published Fri Aug 16, 2019

Author(s): H. Chen, M. Marengo, and A. Amirfazli

An experimental study finds that when a drop impacts close to the edge of a surface, part of the lamella spreads out of the surface. Depending on the normalized distance to the edge, the free lamella can either recede back onto the surface, or completely break off at the surface edge.

[Phys. Rev. Fluids 4, 083601] Published Fri Aug 16, 2019

Physics of Fluids, Volume 31, Issue 8, August 2019.
In this work, we revisit the temporal stability of slip channel flow. Lauga and Cossu [“A note on the stability of slip channel flows,” Phys. Fluids 17, 088106 (2005)] and Min and Kim [“Effects of hydrophobic surface on stability and transition,” Phys. Fluids 17, 108106 (2005)] have investigated both modal stability and non-normality of slip channel flow and concluded that the velocity slip greatly suppresses linear instability and only modestly affects the non-normality. Here, we study the stability of channel flow with streamwise and spanwise slip separately as two limiting cases of anisotropic slip and explore a broader range of slip length than previous studies did. We find that, with a sufficiently large slip, both streamwise and spanwise slip trigger three-dimensional leading instabilities. Overall, the critical Reynolds number is only slightly increased by streamwise slip, whereas it can be greatly decreased by spanwise slip. Streamwise slip suppresses the nonmodal transient growth, whereas the spanwise slip enlarges the nonmodal growth, although it does not affect the base flow. Interestingly, as the spanwise slip length increases, the optimal perturbations exhibit flow structures different from the well-known streamwise rolls. However, in the presence of equal slip in both directions, the three-dimensional leading instabilities disappear and the flow is greatly stabilized. The results suggest that earlier instability and larger transient growth can be triggered by introducing anisotropy in the velocity slip.

Physics of Fluids, Volume 31, Issue 8, August 2019.
Despite significant progress, cell viability continues to be a central issue in droplet-based bioprinting applications. Common bioinks exhibit viscoelastic behavior owing to the presence of long-chain molecules in their mixture. We computationally study effects of viscoelasticity of bioinks on cell viability during deposition of cell-loaded droplets on a substrate using a compound droplet model. The inner droplet, which represents the cell, and the encapsulating droplet are modeled as viscoelastic liquids with different material properties, while the ambient fluid is Newtonian. The model proposed by Takamatsu and Rubinsky [“Viability of deformed cells,” Cryobiology 39(3), 243–251 (1999)] is used to relate cell deformation to cell viability. We demonstrate that adding viscoelasticity to the encapsulating droplet fluid can significantly enhance the cell viability, suggesting that viscoelastic properties of bioinks can be tailored to achieve high cell viability in droplet-based bioprinting systems. The effects of the cell viscoelasticity are also examined, and it is shown that the Newtonian cell models may significantly overpredict the cell viability.

Physics of Fluids, Volume 31, Issue 8, August 2019.
The purpose of this work is twofold: to present a computational strategy to simulate the dynamics of a rigid sphere during water sloshing and to validate the model with original experimental data. The numerical solution is obtained through the coupling between a two-fluid Navier-Stokes solver and a rigid solid dynamics solver, based on a Newton scheme. A settling sphere case reported in the literature is first analyzed to validate the numerical strategy by ascertaining the settling velocity. In addition, an experiment is carried out based on a sphere submerged into a communicating vessel subjected to sloshing. Experimental data are captured using image processing and statistically treated to provide sphere dynamics quantitative information. The effects of different classical models used to describe drag coefficients, added mass, and wall effects are considered in the study to evaluate their influence on the results. The numerical model provides results that are consistent with the physical data, and the trajectory analysis shows good agreement between the simulations and the experiments.

Physics of Fluids, Volume 31, Issue 8, August 2019.
We examine the turbulent boundary layers developing over convergent-divergent riblets (C-D riblets) with three different heights (h+ = 8, 14, and 20) at Reθ = 723 using particle image velocimetry. It is observed that although a logarithmic region presents in the velocity profiles over the converging and diverging line, Townsend’s outer-layer similarity hypothesis is invalid. Compared to the smooth-wall case, C-D riblets with a height of 2.4% of the smooth-wall boundary layer thickness can cause a significant increase in the turbulence production activities over the converging region, as evidenced by a more than 50% increase in the turbulent shear stress and in the population of prograde and retrograde spanwise vortices. In contrast, the impact of riblets on the diverging region is much smaller. The slope of vortex packets becomes steeper, and they are more streamwise stretched in the outer layer over the diverging region, whereas their shape and orientation is less affected over the converging region. Furthermore, the number of uniform momentum zones across the boundary layer increases over the converging region, causing a reduction in the thickness of uniform momentum zones in the outer part of the boundary layer. Overall, while an increased riblet height affects a large portion of the boundary layer away from the wall over the converging region, the impact on the diverging region is largely confined within the near-wall region. Such distinct differences in the response of the boundary layer over the diverging and converging region are attributed to the opposite local secondary flow motion induced by C-D riblets.

Physics of Fluids, Volume 31, Issue 8, August 2019.
While the mechanisms that drive breakup and aerodynamic dispersion of traditional liquids such as water have been extensively studied, it is not yet clear if models for traditional liquids can be used to accurately describe the behavior of molten metals. In this paper, multiphase simulations with the interface-capturing combined level-set volume-of-fluid approach are used to provide time-resolved morphology and breakup data for a liquid column subject to a shock-induced cross-flow. For the first time, numerical simulation of the behavior of a liquid metal (Galinstan alloy composed of gallium, indium, and tin) is compared to the well-documented behavior of water. Simulations consider a gas cross-flow Weber number between 10 and 12, which produces a multimode breakup morphology consisting of multiple baglike structures. Up to bag breakup, we confirm that the deformation rate of Galinstan follows the same dependence on the gas cross-flow Weber number as ordinary liquids when time is nondimensionalized by including the liquid-gas density ratio. Moreover, we determine that the appearance of a central stem along the column upstream surface in multimode bag breakup is consistent with the occurrence of Rayleigh-Taylor instability. We also resolve bag stretching and fragmentation, to the full extent allowed by our computational resources, and carry out a direct comparison with the measurements of size and velocity of secondary droplets from high-speed digital inline holography. For Galinstan, we illustrate the differences between simulation and experiment that emerge because of the modification of the surface properties of the metal exposed to air.

Physics of Fluids, Volume 31, Issue 8, August 2019.
A previously proposed classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo (DSMC), the Macheret-Fridman for direct simulation Monte Carlo (MF-DSMC) model [Luo et al., “Classical impulsive model for dissociation of diatomic molecules in direct simulation Monte Carlo,” Phys. Rev. Fluids 3, 113401 (2018)], is extended in this work. To improve the prediction of state-specific rates at high vibrational energy, the anharmonic vibrational phase angle distribution function is first incorporated into the model. Then, to improve the prediction of thermal equilibrium dissociation rates, the general concept of calculating total collision cross sections with the MF-DSMC model is discussed and the framework of implementing a collision model based on exponential potential is constructed. The improved model is validated by comparisons with quasiclassical trajectory calculations, empirical estimations, and experimental measurements. In general, better agreement compared with the original version of the model is obtained. The improved model is also evaluated by simulating O2 reacting shock experiment.

Physics of Fluids, Volume 31, Issue 8, August 2019.
The problem of fully developed laminar fluid flow in pipes, driven by an oscillatory pressure gradient, can be solved exactly for the time-dependent velocity field and related quantities such as flow rate and tidal displacement. When dissipation is neglected and the momentary axial variation of temperature is assumed to be linear, the corresponding thermal energy equation describing heat transfer along a pipe connecting two reservoirs at different temperatures can also be solved to yield exact solutions for the time-dependent temperature field, axial heat flux, and effective axial conductivity. In this paper, it is shown that these exact solutions for the unsteady temperature field are invalid at low Womersley numbers because the momentary axial variation of temperature is not linear. When the thermal energy equation is written in quasisteady form, approximate quasisteady analytical solutions can be found for the temperature field, which yield effective axial conductivities several orders of magnitude greater than those given by the low-Womersley-number, unsteady-flow solution. It is also shown that the conditions under which effects of dissipation on axial heat transfer become significant, at high Womersley numbers, can be determined by a simple criterion. When dissipation is significant, exact solutions for the unsteady temperature field are invalid at high Womersley numbers because the momentary axial variation of temperature is also nonlinear.

Physics of Fluids, Volume 31, Issue 8, August 2019.

Physics of Fluids, Volume 31, Issue 8, August 2019.
General rigid bead-rod theory [O. Hassager, “Kinetic theory and rheology of bead-rod models for macromolecular solutions. II. Linear unsteady flow properties,” J. Chem. Phys. 60(10), 4001–4008 (1974)] explains polymer viscoelasticity from macromolecular orientation. By means of general rigid bead-rod theory, we relate the complex viscosity of polymeric liquids to the architecture of axisymmetric macromolecules. In this work, we explore the zero-shear and complex viscosities of 24 different axisymmetric polymer configurations. When nondimensionalized with the zero-shear viscosity, the complex viscosity depends on the dimensionless frequency and the sole dimensionless architectural parameter, the macromolecular lopsidedness. In this work, in this way, we compare and contrast the elastic and viscous components of the complex viscosities of macromolecular chains that are straight, branched, ringed, or star-branched. We explore the effects of branch position along a straight chain, branched-chain backbone length, branched-chain branch-functionality, branch spacing along a straight chain (including pom-poms), the number of branches along a straight chain, ringed polymer perimeter, branch-functionality in planar stars, and branch dimensionality.

Physics of Fluids, Volume 31, Issue 8, August 2019.
Film boiling over a circular cylinder in a horizontal cross-flow of saturated liquid is studied in the mixed regime that is characterized by a combined influence of buoyancy and flow inertia at low magnitudes of the Froude number (Fr). Liquid-vapor interface evolution and the ensuing vapor wake dynamics together with heat transfer have been determined through a computational framework developed for phase change problems on two-dimensional unstructured grids using a coupled level set and volume of fluid interface capturing method. While the quasisteady nature of ebullition cycle is gradually lost as Fr increases, the effect of cross-flow orientation with respect to gravity is shown to be nontrivial in the mixed regime. A direct consequence of the orthogonal gravity and flow fields is an anomalous impairment of heat transfer with an increase in cross-flow velocity under certain conditions, which is discussed in detail. Simultaneously, the film boiling behavior as influenced by several other hydrodynamic and thermal parameters is also ascertained. The interplay between buoyancy and inertia is further highlighted while discussing the interdependent liquid and vapor wake characteristics in the mixed regime with horizontal cross-flow. The liquid wake behavior is shown to result not only from the bluff body geometry but also the instantaneous vapor wake profiles, with the wall superheat affecting the time scale of wake interaction. Finally, a reduction factor (ξ) is proposed and determined as a function of the Froude number, which is used in conjunction with a correlation for upward cross-flow film boiling to predict the heat transfer.

Author(s): San To Chan, Jesse T. Ault, Simon J. Haward, E. Meiburg, and Amy Q. Shen

Experiments and simulations are combined to study flows in a T-mixer with offset inlets, whose stability is coupled to the vortex breakdown structure in the system. This leads to an unexpected flow regime in which increasing the flow rate can re-stabilize steady-state solutions of the flow.

[Phys. Rev. Fluids 4, 084701] Published Thu Aug 15, 2019

Author(s): R. Abu-Rjal, N. Leibowitz, S. Park, B. Zaltzman, I. Rubinstein, and G. Yossifon

For a sufficiently deep microchannel, both experiments and simulations show a distinct transient nonmonotonic behavior of the system chronopotentiometric and chronoamperometric responses, resulting from the emergence of electroconvective instability in the overlimiting conductance regime.

[Phys. Rev. Fluids 4, 084203] Published Wed Aug 14, 2019