# Latest papers in fluid mechanics

### The role of entrance functionalization in carbon nanotube-based nanofluidic systems: An intrinsic challenge

In this work, experiments, molecular dynamics (MD) simulations, and theoretical analysis are conducted to study ion transport in thin carbon nanotubes (CNTs). Diverse nonlinear relationships between the ionic conductance (G) and the ion concentration (C) are observed. MD simulations show that the distinct G–C dependences are caused by the functionalization of the CNT entrance, which affects the energy barrier for ion transport and changes the ionic conductance. The various G–C relationships are also predicted using the electrokinetic theory by considering the potential generated by the functional groups at the CNT entrance. Practically, the number of functional groups at the CNT entrance is influenced by several factors, including both intrinsic and external effects, which make it difficult to regulate the ionic conductance and pose a challenge to CNT-based nanofluidic systems in practical applications.

### Irreversibility and chaos in active particle suspensions

Author(s): Sergio Chibbaro, Astrid Decoene, Sebastien Martin, and Fabien Vergnet

The collective behavior of active suspensions of microswimmers immersed in a viscous fluid is investigated through numerical studies. It is shown that a bioturbulent state emerges both in suspensions of pushers and of pullers. Which conditions are needed to trigger such phenomenology and what mechanisms underlie such phenomena are discussed. An investigation into whether the difference in puller and pusher dynamics is due to a spontaneous breaking of the hydrodynamics time-reversal symmetry is presented, and the mechanisms underlying such broken symmetry in biological swimmers are examined.

[Phys. Rev. Fluids 6, 013104] Published Tue Jan 26, 2021

### Monotonic instability and overstability in two-dimensional electrothermohydrodynamic flow

Author(s): Yifei Guan, Xuerao He, Qi Wang, Zhiwei Song, Mengqi Zhang, and Jian Wu

Electrothermohydrodynamic convection between parallel electrodes with unipolar injection is investigated using a two-relaxation-time lattice Boltzmann method. The interactions between the stabilizing buoyancy force and the destabilizing electric force lead to either monotonic instability or overstability, depending on the Rayleigh number and the Taylor number. A two-stage bifurcation is observed for overstability near the threshold Rayleigh number with a significant change in phase and amplitude.

[Phys. Rev. Fluids 6, 013702] Published Tue Jan 26, 2021

### Biphase as a diagnostic for scale interactions in wall-bounded turbulence

Author(s): G. Cui and I. Jacobi

Biphase is introduced as a nonlinear alternative to traditional amplitude modulation coefficients for studying the interaction delays between large- and small-scale motions in wall-bounded turbulent flows. The biphase combines energetic and geometric interpretations to provide an integrated diagnostic for the scale interaction problem.

[Phys. Rev. Fluids 6, 014604] Published Tue Jan 26, 2021

### On airborne virus transmission in elevators and confined spaces

The impact of air ventilation systems on airborne virus transmission (AVT), and aerosols in general, in confined spaces is not yet understood. The recent pandemic has made it crucial to understand the limitations of ventilation systems regarding AVT. We consider an elevator as a prototypical example of a confined space and show how ventilation designs alone, regardless of cooling or heating, contribute to AVT. Air circulation effects are investigated through multiphase computational fluid dynamics, and the performance of an air purifier in an elevator for reducing AVT is assessed. We have investigated three different flow scenarios regarding the position and operation of inlets and outlets in the elevator and a fourth scenario that includes the operation of the air purifier. The position of the inlets and outlets significantly influences the flow circulation and droplet dispersion. An air purifier does not eliminate airborne transmission. The droplet dispersion is reduced when a pair of an inlet and an outlet is implemented. The overall practical conclusion is that the placement and design of the air purifier and ventilation systems significantly affect the droplet dispersion and AVT. Thus, engineering designs of such systems must take into account the flow dynamics in the confined space the systems will be installed.

### On airborne virus transmission in elevators and confined spaces

The impact of air ventilation systems on airborne virus transmission (AVT), and aerosols in general, in confined spaces is not yet understood. The recent pandemic has made it crucial to understand the limitations of ventilation systems regarding AVT. We consider an elevator as a prototypical example of a confined space and show how ventilation designs alone, regardless of cooling or heating, contribute to AVT. Air circulation effects are investigated through multiphase computational fluid dynamics, and the performance of an air purifier in an elevator for reducing AVT is assessed. We have investigated three different flow scenarios regarding the position and operation of inlets and outlets in the elevator and a fourth scenario that includes the operation of the air purifier. The position of the inlets and outlets significantly influences the flow circulation and droplet dispersion. An air purifier does not eliminate airborne transmission. The droplet dispersion is reduced when a pair of an inlet and an outlet is implemented. The overall practical conclusion is that the placement and design of the air purifier and ventilation systems significantly affect the droplet dispersion and AVT. Thus, engineering designs of such systems must take into account the flow dynamics in the confined space the systems will be installed.

### Expansion and combustion of droplets that contain long-chain alcohol alternative fuels

This paper studies the expansion, micro-explosion, and combustion behaviors of base fuels blended with long-chain alcohols. Diesel, biodiesel, and aviation kerosene are chosen as the base fuels, while n-butanol and n-pentanol are representative long-chain alcohols. Upon addition of a long-chain alcohol, deformation of the blended-fuel droplet becomes more violent. Expansion and ejection of internal liquid and gas occur throughout the process; larger proportions of long-chain alcohols lead to larger ejection holes. The degree of expansion first increases and then decreases with the proportion of alcohol. The effect of the alcohol type on d* (normalized droplet diameter) is substantial at low φ (volume fraction of long-chain alcohol) but negligible at high φ. The aviation kerosene-based fuel exhibits the smallest changes in d*. The effects of φ and the alcohol type on the micro-explosion delay time are also analyzed. The ignition delay time of the diesel-based fuel decreases monotonically with the increasing alcohol proportion and that of the biodiesel-based fuel first decreases and then increases, while that of the aviation kerosene-based fuel increases and then decreases. The combustion rate of a pure base fuel accelerates upon addition of alcohol. The ignition delay time is greatly shortened at higher temperatures, and the combustion duration shortens significantly at temperatures lower than 800 °C. The biodiesel-based fuel offers the shortest ignition delay time and the longest combustion duration, while aviation kerosene exhibits the opposite characteristics. Finally, the micro-explosion and comprehensive combustion indices are proposed to estimate the comprehensive micro-explosion and combustion performances, respectively, of blended fuels.

### Experimental analysis of supercritical-assisted atomization

Supercritical CO2 is used in supercritical-assisted atomization (SAA) systems to promote the atomization of nanoparticle suspensions in powder generation in pharmaceutical, electronics, and coating applications. Due to the sensitivity of the mixture properties to the operational conditions, the SAA process is not fully resolved to date. This study experimentally investigates the underlying mechanisms behind SAA utilizing CO2 or N2 as the assisted-atomization fluid (CO2-A or N2-A) using high-speed imaging and laser diffraction techniques. The effects of injection temperature, pressure, and gas-to-liquid ratio (GLR) are explored, and empirical droplet size models are developed. It is found that the primary breakup of CO2-A is governed by the emergence of the near-nozzle gas bubbles originated from the dissolved CO2, which expand radially and squeeze the liquid due to the inertial forces. As a result, the edges of the liquid core become thinner and deform into relatively long ligaments that further break up into droplets. CO2-A exhibited a shorter liquid length, wider spray angle, and smaller droplet size compared to N2-A. The discrepancies observed in the breakup process are mainly attributed to the higher solubility of CO2 in water and lower surface tension of the CO2–water system. The smallest droplet size distribution and the narrowest droplet size distribution are detected for CO2-A injected at the critical pressure of the CO2–water binary system where the solubility of CO2 in water significantly rises. Linear instability analysis indicates that both shear and acceleration that indirectly incorporate the experimentally observed bubble expansion are the main factors driving the instabilities.

### Probing the high mixing efficiency events in a lock-exchange flow through simultaneous velocity and temperature measurements

Gravity currents produced by a lock-exchange flow are studied using high-resolution molecular tagging techniques. Instead of employing salt to produce density stratification, an initial temperature difference is introduced in the system to generate the ensuing gravity currents. The experiments focus on the interface between the hot and cold fluids to characterize the resultant mixing across the interface. The present measurements spatially resolve the flow to smaller than the Kolmogorov scale and close to the Batchelor scale. This enables reasonably accurate estimates of velocity and density gradients. The measured density (temperature) distribution allowed estimation of the background potential energy of the flow that is used to quantify mixing. These measurements yield a mixing efficiency of about 0.13 with a standard deviation of 0.05 for the present Reynolds number range [[math]]. An analysis combining flow visualization and quantitative measurements reveals that spatially local values of high mixing efficiency occur after the occurrence of certain dissipative stirring events. These events, largely associated with vortical overturns, are commonly observed near the interface between the two fluids and are a precursor to locally efficient mixing.

### A high-throughput method to characterize membrane viscosity of flowing microcapsules

Microcapsules have many industrial applications and also serve as a widely used mechanical model of living biological cells. Characterizing the viscosity and elasticity of capsules at a high-throughput rate has been a classical challenge, since this is a time-consuming process in which one needs to fit the time-dependent capsule deformation to theoretical predictions. In the present study, we develop a novel efficient method, by integrating a deep convolutional neural network with a high-fidelity mechanistic capsule model, to predict the membrane viscosity and elasticity of a microcapsule from its dynamic deformation when flowing in a branched microchannel. Compared with a conventional inverse method, the present approach can increase the prediction throughput rate by five orders of magnitude while maintaining the same level of prediction accuracy. We also demonstrate that the present approach can deal with capsules with large deformation in inertial flows.

### Force distribution within a barchan dune

Barchan dunes, or simply barchans, are crescent-shaped dunes found in diverse environments such as the bottom of rivers, Earth’s deserts, and the surface of Mars. In our recent paper [“Shape evolution of numerically obtained subaqueous barchan dunes,” Phys. Rev. E 101, 012905 (2020)], we investigated the evolution of subaqueous barchans by using the computational fluid dynamics-discrete element method, and our simulations captured well the evolution of an initial pile toward a barchan dune in both the bedform and grain scales. The numerical method having shown to be adequate, we obtain now the forces acting on each grain, isolate the contact interactions, and investigate how forces are distributed and transmitted in a barchan dune. We present force maps and probability density functions for values in the streamwise and spanwise directions and show that stronger forces are experienced by grains at neither the crest nor the leading edge of the barchan but in positions just upstream the dune centroid on the periphery of the dune. We also show that a large part of grains undergo longitudinal forces of the order of 10−7 N, with negative values around the crest, resulting in decelerations and grain deposition in that region. These data show that the force distribution tends to route a large part of grains toward the crest and horns of subaqueous barchans, being fundamental to comprehend their morphodynamics. However, to the best of the authors’ knowledge, they are not accessible from the current experiments, making our results an important step toward understanding the behavior of barchan dunes.

### Thermodynamics of shear-induced phase transition of polydisperse soft particle glasses

The thermodynamics of the shear-induced phase transition of soft particle glasses is presented. Jammed suspensions of soft particles transform into a layered phase in a strong shear flow from a stable glassy phase at lower shear rates. The thermodynamics of the two phases can be computed based on the elastic energy and excess entropy of the system. At a critical shear rate, the elastic energy, the excess entropy, the free energy, the temperature, and the shear stress undergo discontinuous jumps at the phase transitions from the glassy to the layered phase. An effective temperature is defined from the derivative of the elastic energy and the excess entropy. The Helmholtz free energy is constructed using the elastic energy, excess entropy, and derived temperature. At a fixed shear rate, there is no equilibrium between the states. However, at a fixed temperature, the glassy and layered states may coexist, as indicated by the equality of their Helmholtz free energies. While this first-order phase transition is possible, it cannot be observed in simple shear because the stress is the same in both phases at the same temperature. Thus, shear banding cannot be observed in this system. Finally, an equation of state, which relates the shear stress to the excess entropy, is presented. This equation of state shows that all dynamical properties (e.g., shear-induced diffusivity and first and second normal stresses) of these jammed non-Brownian suspensions can be determined solely by measuring the shear stress.

### On the droplet entrainment from gas-sheared liquid film

We formulate the droplet entrainment detached from a thin liquid film sheared by a turbulent gas in a circular pipe. In a time-averaged sense, the film has a Couette flow with a mean velocity of um. Then, a roll wave of wavelength λ and phase velocity uc is formed destabilized through Kelvin–Helmholtz instability, followed by a ripple wave of wavelength λp due to Rayleigh–Taylor instability, wherein the vorticity thickness of the gas stream is consistently a characteristic length scale. Superposing the two types of waves in axial and transverse directions produces conical cusps as the root of ligaments, from which droplets are torn off. The droplet entrainment rate is derived as [math], validated by recent experimental results.

### Thermodynamics of shear-induced phase transition of polydisperse soft particle glasses

The thermodynamics of the shear-induced phase transition of soft particle glasses is presented. Jammed suspensions of soft particles transform into a layered phase in a strong shear flow from a stable glassy phase at lower shear rates. The thermodynamics of the two phases can be computed based on the elastic energy and excess entropy of the system. At a critical shear rate, the elastic energy, the excess entropy, the free energy, the temperature, and the shear stress undergo discontinuous jumps at the phase transitions from the glassy to the layered phase. An effective temperature is defined from the derivative of the elastic energy and the excess entropy. The Helmholtz free energy is constructed using the elastic energy, excess entropy, and derived temperature. At a fixed shear rate, there is no equilibrium between the states. However, at a fixed temperature, the glassy and layered states may coexist, as indicated by the equality of their Helmholtz free energies. While this first-order phase transition is possible, it cannot be observed in simple shear because the stress is the same in both phases at the same temperature. Thus, shear banding cannot be observed in this system. Finally, an equation of state, which relates the shear stress to the excess entropy, is presented. This equation of state shows that all dynamical properties (e.g., shear-induced diffusivity and first and second normal stresses) of these jammed non-Brownian suspensions can be determined solely by measuring the shear stress.

### Spatiotemporal evolutions of forces and vortices of flow past ellipsoidal bubbles: Direct numerical simulation based on a Cartesian grid scheme

An in-depth investigation of two fixed non-spherical bubbles is an indispensable step toward revealing fundamental mechanisms in complex bubbly flows, where direct numerical simulation (DNS) is one of the most promising approaches to conduct such a task. However, accurately modeling force distribution and efficiently generating satisfactory mesh around a non-spherical bubble pair are challenging to current DNS methods. In this study, an effective non-body-fitted gas–liquid interface tracking scheme based on the Cartesian grid was developed to conduct three-dimensional DNS of two fixed ellipsoidal bubbles with frozen shape in an incompressible Newtonian fluid. The grid-independent analysis and analytical validation prove that our developed non-body-fitted gas–liquid interface tracking scheme is able to accurately retrieve all force components exerted on a bubble with less mesh generation and computational efforts than body-fitted counterparts. Using this non-body-fitted gas–liquid interface tracking scheme, spatiotemporal evolutions of forces and vortices around the two fixed ellipsoidal bubbles were directly simulated under various values of Reynolds numbers, separation distances, and the bubble’s ellipsoidicity. The analysis of drag force shows that the overall drag behaviors of ellipsoidal bubbles are quite similar to those of spherical bubbles though larger ellipsoidicity produces a higher drag coefficient. However, the sign of lift forces, i.e., either the two bubbles attract or repel each other, is highly dependent on ellipsoidicity. For the bubble pair with moderate ellipsoidicity, attractive force dominates at moderate-to-high Reynold numbers, while the two bubbles tend to repel at low Reynolds numbers. For the bubble pair with high ellipsoidicity, the two bubbles repel each other at all values of Reynolds numbers and separation distances. Characteristics of vortex developments, which are the reason behind these ellipsoidicity-dependent force behaviors, are presented and discussed. This study highlights the importance of the bubble’s shape in the interactions and associated vortex between two adjacent fixed ellipsoidal bubbles.

### Cavitation model of the inflationary stage of Big Bang

In this paper, we propose a model for the initial stage of the development of the universe analogous to cavitation in a liquid in a negative pressure field. It is assumed that at the stage of inflation, multiple breaks of the metric occur with the formation of areas of physical vacuum in which the generation of matter occurs. The proposed model explains the large-scale isotropy of the universe without ultrafast inflationary expansion and the emergence of a large-scale cellular (cluster) structure, as a result of the development of cavitation ruptures of a false vacuum. It is shown that the cavitation model can be considered on par with (or as an alternative to) the generally accepted inflationary multiverse model of the Big Bang.

### Dynamic fluid configurations in steady-state two-phase flow in Bentheimer sandstone

Author(s): Ying Gao, Ali Q. Raeini, Martin J. Blunt, and Branko Bijeljic

Fast synchrotron tomography is used to study the impact of capillary number, Ca, on fluid configurations in steady-state two-phase flow in porous media. Brine and n-decane were co-injected at fixed fractional flow, fw=0.5, in a cylindrical Bentheimer sandstone sample for a range of capillary numbers...

[Phys. Rev. E 103, 013110] Published Mon Jan 25, 2021

### Transition from steady to chaotic flow of natural convection on a section-triangular roof

Author(s): Haoyu Zhai, Juan F. Torres, Yongling Zhao, and Feng Xu

Flow phenomena on a roof are investigated by employing direct numerical simulation. A sequence of pitchfork bifurcations of steady plumes on the roof occurs for small Rayleigh numbers, which is analyzed using a topological method. Furthermore, a Hopf bifurcation followed by periodic doubling, quasiperiod bifurcations, and a transition to chaos appear as the Rayleigh number is increased.

[Phys. Rev. Fluids 6, 013502] Published Mon Jan 25, 2021

### Fully coupled model for simulating highly nonlinear dynamic behaviors of a bubble near an elastic-plastic thin-walled plate

Author(s): Wenbin Wu, Moubin Liu, A-Man Zhang, and Yun-Long Liu

On the basis of the boundary element method and explicit finite element method, a three-dimensional fully coupled model is developed to investigate the interaction between a bubble and an elastic-plastic thin-walled plate. The model can accurately calculate the bubble loading acting on the plate surface and describe the structural motion coupling with the flow field on two sides of the plate. As a result of the elastic-plastic effects of the thin-walled plate, the bubble displays attractive motion, repulsive motion, or splitting.

[Phys. Rev. Fluids 6, 013605] Published Mon Jan 25, 2021

### Stability analysis of a resonant triad in a stratified uniform shear flow

Author(s): Lima Biswas and Priyanka Shukla

The existence of a resonant triad interaction among two primary internal waves and a superharmonic wave in a stably stratified uniform shear flow is proved. Under the pump-wave approximation, the resonant triad becomes unstable when the first mode acts as the pump wave. The exact solutions of the amplitude equations reveal that the stability of a triad depends on the mode numbers and initial conditions.

[Phys. Rev. Fluids 6, 014802] Published Mon Jan 25, 2021