Physics of Fluids

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Table of Contents for Physics of Fluids. List of articles from both the latest and ahead of print issues.
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Numerical study on gas–liquid two phase flow characteristic of multistage electrical submersible pump by using a novel multiple-size group (MUSIG) model

Thu, 06/23/2022 - 13:53
Physics of Fluids, Volume 34, Issue 6, June 2022.
Electrical submersible pumps (ESPs) face enormous challenges in the petroleum industry while handling gas–liquid two-phase flow. The major difficulty is caused by the accumulation of gas bubbles inside ESP-impellers, which results in mild to severe degradation in pump performance. Therefore, to analyze the influence of gas entrainment and bubble size, a combination of experimental and numerical analysis is performed on a five-stage mixed-flow ESP in the present study. The experiments are first conducted to analyze the performance of ESP under pure water conditions at different rotating speeds, followed by the gas–liquid two-phase flow experiments that are performed at constant rotating speed (1475 r/min) and for a wide range of inlet gas void fractions (IGVFs). For numerical calculations, a novel multiple-size group (MUSIG) model is applied in ANSYS CFX to analyze the performance and different flow patterns in ESP in different IGVFs and understand the coalescence and breakup phenomena of gas bubbles in the impeller flow passage. The simulation results from the MUSIG model are compared with the Euler–Euler two-fluid model and test results. The MUSIG model can more accurately predict the changes in the performance and internal flow-field of ESP under two-phase flow conditions. Moreover, when the MUSIG model is used to calculate the two-phase flow of the ESP, the first-stage impeller has a higher head than other stages because the flow inside the second and other stages is affected by the disoriented flow coming from the first-stage diffuser and other return channels. Furthermore, this study gives an insight into the comprehensive application of the novel MUSIG model for complex turbo-machine designs such as ESP.

Near-surface gas discharge effect on a steady bow shock wave position in a supersonic flow past a cylindrically blunted body in the air

Thu, 06/23/2022 - 13:53
Physics of Fluids, Volume 34, Issue 6, June 2022.
The problem of the bow shock wave control using a near-surface gas discharge in a supersonic flow past a semi-cylindrical body at Mach number M = 4 in the air is investigated experimentally and numerically. The possibility of controlling the position of a steady bow shock wave and the characteristics of a streamlined body by creating a volumetric plasma region using a surface gas discharge organized on the entire front surface of the body is shown. An increase in the stand-off distance of a steady bow shock is experimentally and numerically obtained, which is the greater, the higher the discharge power and the greater the adiabatic index in the plasma region created by the discharge. A comparison of the numerical and experimental data showed good agreement. It is established that the relative value of the steady bow shock stand-off distance increases linearly in the power range from 1.5 × 105 to 2.4 × 105 W at the discharge current from 430 to 670 A, and the adiabatic index in the plasma region can be estimated as 1.3. It is also found that at higher values of the discharge power, the adiabatic index in the plasma region decreases. The average plasma parameters were expressed as functions of the discharge specific power and the adiabatic index. The mechanism of the gas discharge effect on the bow shock wave is established, and it is shown that the plasma parameters in the region created by the discharge, including the degree of ionization and the degree of nonequilibrium, affect the position of the steady bow shock wave.

Linear instability of a liquid sheet in a transverse standing acoustic field

Thu, 06/23/2022 - 11:26
Physics of Fluids, Volume 34, Issue 6, June 2022.
This work examines the instability of a plane liquid sheet under the action of a transverse acoustic field. The mechanical definition of the acoustic field is introduced first, and the Floquet theory is applied to derive the dispersion equation and dispersion curve. The dominant instability mechanism of each unstable region on the dispersion curve is distinguished by calculating the oscillation frequencies of the disturbance waves. Next, the parameters within the dispersion equation are set as variables to analyze the development of the instability mechanisms of the unstable regions on the dispersion curve and the oscillation modes on the two surfaces of the liquid sheet. The results prove that the distribution of unstable regions can be affected by the amplitude and frequency of the acoustic field, the viscosity and surface tension of the liquid sheet, and the density ratio of the two gas–liquid phases. Variation in the thickness of the liquid sheet causes development and competition within the oscillation modes, which were found to be related to the development of the instability mechanism. Such evolutionary competition between the sinuous and the varicose oscillation modes was also reflected in the experimental study, where it was observed that the disturbance wave has the characteristics of Faraday waves.

Unexpected stability of micrometer weakly viscoelastic jets

Thu, 06/23/2022 - 11:26
Physics of Fluids, Volume 34, Issue 6, June 2022.
We study experimentally the stability of micrometer weakly viscoelastic jets produced with transonic flow focusing. Highly stable jets are formed when a low molecular weight polymer is added to water at a given low concentration, and the injected flow rate is reduced to its minimum value. In this case, the capillary instability is delayed, and the jet breakup occurs at distances from the ejector of the order of tens of thousands the jet diameter. The results indicate that the intense converging extensional flow in the ejection point builds up viscoelastic stress that does not relax in the jet even for times much longer than the polymer relaxation time. We hypothesize that the drag (shear) force exerted by the outer gas stream prevents the stress relaxation. It is also possible that partial polymer entanglement at the jet emission point contributes to this effect. We measure the jet length and the diameter at the ejector orifice and breakup point. The diameter takes values just above 2 μm at the breakup point regardless of the liquid flow rate and gas pressure.

Inverted conical methane/air flame shape transformation under acoustic excitation: Gravity impact

Wed, 06/22/2022 - 12:03
Physics of Fluids, Volume 34, Issue 6, June 2022.
In this paper it is shown that under certain conditions under reverse gravity, acoustics increase the stability of the flame in comparison with normal gravity. It is shown that there is a hysteresis in the V–M and M–V transitions under reverse gravity as well as under normal one. In contrast to normal gravity, the conditions for hysteresis degeneracy under reverse gravity are proven to be independent of the excitation frequency.

Diffusiophoresis of a highly charged conducting fluid droplet

Wed, 06/22/2022 - 12:03
Physics of Fluids, Volume 34, Issue 6, June 2022.
Diffusiophoresis of a perfectly conducting droplet-like liquid metal in electrolyte solutions is investigated theoretically, focusing on the chemiphoresis component, the very heart of diffusiophoresis, where the droplet motion is induced solely by the chemical gradient. The resulting electrokinetic equations are solved with a pseudo-spectral method based on Chebyshev polynomials. For the isothermal electrokinetic system of a perfectly conducting droplet considered here, there is no Marangoni effect, which is a motion-inducing effect due to the variation of interfacial tension along the droplet surface. No Maxwell traction is present as well. The droplet motion is full of hydrodynamic nature. It is found, among other things, that contrary to a dielectric droplet, a conducting droplet always moves up the chemical gradient toward the region with a higher concentration of ions in chemiphoresis. This implies that a perfectly conducting droplet like a gallium or its alloy droplet is superior to the commonly utilized dielectric droplet like a liposome in drug delivery in terms of self-guarding itself toward the desired destination of injured or infected area in the human body, as specific ionic chemicals are often released there. Optimum droplet size yielding the fastest migration rate is predicted.

Wetting failure in the early stage of water drop impact on a smooth solid surface

Wed, 06/22/2022 - 12:03
Physics of Fluids, Volume 34, Issue 6, June 2022.
A water drop impacting a dry solid surface can eject a thin liquid sheet, which is forced to expand on the surface to wet the solid surface. Wetting failure, which produces defects in applications based on the impact of drops, including coating, cooling, cleaning, and printing, may occur with a sufficiently large liquid-sheet velocity. However, the exact onset of wetting failure when a drop impacts the surface has yet to be determined. Therefore, we examine the dependence of rim instability immediately after liquid-sheet ejection on the static contact angle of the solid surface at the instant of water drop impact. This study is the first attempt to solve this problem and is made possible only by using an ultra-high-speed camera. We revealed that wetting failure can occur by investigating the rim instability of the liquid sheet.

A complete classification of sub-shocks in the shock structure of a binary mixture of Eulerian gases with different degrees of freedom

Wed, 06/22/2022 - 12:03
Physics of Fluids, Volume 34, Issue 6, June 2022.
The shock structure in a binary mixture of polyatomic Eulerian gases with different degrees of freedom of a molecule is studied based on the multi-temperature model of rational extended thermodynamics. Since the system of field equations is hyperbolic, the shock-structure solution is not always regular, and discontinuous parts (sub-shocks) can be formed. For given values of the mass ratio and the specific heat of the constituents, we identify the possible sub-shocks as the Mach number M0 of the shock wave and the equilibrium concentration c0 of the constituents change. In the plane (c0, M0), we identify the possible regions for the sub-shock formation. The analysis is obtained to verify when the velocity of the shock wave meets a characteristic velocity in the unperturbed or perturbed equilibrium states, which gives a necessary condition for the sub-shock formation. The condition becomes necessary and sufficient when the velocity of the shock becomes greater than the maximum characteristic velocity in the unperturbed state, namely, the regions with no sub-shocks, a sub-shock for only one constituent, or sub-shocks for both constituents are comprehensively classified. The most interesting case is that the lighter molecule has more degrees of freedom than that of the heavy one. In this situation, the topology of the various regions becomes different. We also solve the system of the field equations numerically using the parameters in various regions and confirm whether the sub-shocks emerge or not. Finally, the relationship between an acceleration wave in one constituent and the sub-shock in the other constituent is explicitly derived.

Modal analysis of propeller wakes under different loading conditions

Wed, 06/22/2022 - 12:03
Physics of Fluids, Volume 34, Issue 6, June 2022.
Propeller wakes under different loading conditions obtained by the improved delayed detached eddy simulation method were studied based on the flow decomposition technique. The sparsity-promoting dynamic mode decomposition was used to study the flow physics in the wake of a propeller, with particular emphasis placed on identifying the underlying temporal and spatial scales that play important roles in the onset of propeller wake instabilities. The morphology of flow structures of different modes selected by the sparsity-promoting algorithm at different frequencies characterizes the instability process of the wake system. It shows that the circumferential diffusion of tip vortex structures promotes the approaching of adjacent tip vortices, enhancing the interaction of the vortex pairs, which plays an important role in the instability triggering mechanism of the propeller wake, especially the mutual inductance between neighboring tip vortices. The present study further extends knowledge of propeller wake instability inception mechanisms under different loading conditions.

Numerical investigation on movement of triple points on oblique detonation surfaces

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
A normal detonation wave in a gaseous mixture is a transient, multidimensional structure containing triple points (TPs) that collide in pairs and then propagate oppositely. However, the TPs on an oblique detonation wave (ODW) almost propagate along the same direction in most studies. In this study, the reactive Euler equations coupled with a two-step induction–reaction kinetic model are used to solve a two-dimensional wedge-induced ODW. Two novel movement patterns are observed in most cases. Results show that the TPs of the ODW can propagate upstream and even stand on the wave surface. The movement patterns of TPs include downstream, upstream, and steady according to their propagation direction relative to the wedge. We find that the ratio of the post-ODW flow speed Uτ to the transverse wave speed UT dominates the TP movement types. When the speed ratio Uτ/UT is approximately equal to 1, the TPs can stand on the wave surface. Above unity, downstream TPs form, and upstream TPs correspond to a value smaller than 1. Furthermore, the inflow Mach number has little influence on UT, while Uτ changes significantly. This is largely due to the high sensitivity of the ODW angle to the inflow. The high heat release rate benefits upstream TPs, and steady TPs form under a large wedge angle. The results are confirmed by varying the inflow Mach number, wedge angle, and chemical parameters.

Interface evolution characteristics of dual droplet successive oblique impact on liquid film

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
The dynamic characteristics of dual droplet successive oblique impact on a thin liquid film are numerically studied by using the coupled level set and volume-of-fluid model. This three-dimensional model effectively predicts the evolution of crown and crater, which is validated qualitatively and quantitatively by comparing with experimental observations. Some interesting interface features during the collision and coalescence of crowns are revealed in the present simulations, such as the gas cavity, liquid crest, and air gap. In addition, the crater contour takes on different phases with time in the case of dual droplets impact. The evolution characteristics of crater contours in front view and side view have been summarized within a certain time period. Furthermore, the variations of the maximum crater radius in upstream, downstream, and lateral directions as time are quantitatively analyzed. It is found that in the circumferential direction of the crater, the radial kinetic energy of the liquid decreases gradually from the upstream to the downstream direction. This research establishes a foundation for industrial and agricultural applications involving droplet impact.

Ram to scram mode transition in a simulated flight acceleration

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
Thrust abruption due to mode transition could be catastrophic for hypersonic vehicles. To understand the underlying physics, a direct-connected transient Flight Trajectory Simulator 1 (FTS-1) has been developed at the Institute of Mechanics, Chinese Academy of Sciences. This facility uses advanced high-speed measuring techniques, including thrust and static wall pressure measurement, and Schlieren and hydrocarbon radical chemiluminescence (CH* chemiluminescence) imaging. Kerosene-fueled dual-mode combustor experiments are designed in an acceleration trajectory. The basic operation parameters and the flame and flow dynamics of the acceleration induced mode transition are evaluated. Discussions are given on the triggering mechanisms responsible for the ram to scram mode transition in a simulated flight acceleration.

Unified modeling and kinetic analysis of the near-cathode region and hot cathode in atmospheric-pressure arc discharges

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
The near-cathode region plays a crucial role in exploring the transport characteristics of the transition from arc column to the hot cathode in atmospheric-pressure arc discharges because of the existing non-equilibrium phenomena. A one-dimensional unified model, including the near-cathode region and the cathode body, is developed for an argon arc discharge with the tungsten cathode at atmospheric pressure in this paper. The electrostatic model coupled with an external circuit in the near-cathode region is solved based on the implicit particle-in-cell coupled Monte Carlo collision method without any assumptions of thermal or ionization equilibrium or quasi-neutrality. A detailed description of the arc plasma–cathode and cathode–gas interactions is obtained by calculating the nonlinear heat conduction equation in the cathode. It is shown that the space-charge sheath strongly affects particle transport in the near-cathode region and energy transport from arc plasma to the thermionic cathode. The total current density has significant effects on the kinetic characteristics of arc plasma by feedback-like mechanisms. The Joule heating by the external circuit and charged particles deposited into the cathode are dominating mechanisms of energy transfer from the near-cathode region to the cathode, while energy loss by radiation is more significant compared with natural convection.

Shock-wave structure in non-polar diatomic and polyatomic dense gases under rotation and vibration

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
This study investigates the effect of rotation and vibration on the structure of shock waves in moderately dense diatomic and polyatomic non-polar gases using the one-temperature Navier–Stokes–Fourier approach. The modified Enskog equation of state of the gas is taken to include the denseness and shielding effects. The specific heat at constant volume has been taken to be temperature-dependent. The shear viscosity, the bulk viscosity, and the thermal conductivity have been assumed to follow the temperature-dependent power-law model. Nitrogen and oxygen gas have been taken as the test cases for diatomic gases while carbon dioxide was taken for the polyatomic gases. The implicit system of equations is derived and solved numerically for density and temperature. The inclusion of denseness, rotational, and vibrational modes of molecular motion have a significant effect on the density and temperature profiles, the inverse shock thickness, the bulk to shear viscosity ratio, and the molar specific heat at constant pressure. The gas having a low characteristic vibrational temperature has been found to have a high value of inverse shock thickness. The inverse shock thickness, the bulk to shear viscosity ratio, and the molar specific heat at constant pressure for nitrogen and carbon dioxide are found to be in good agreement with the experimental values.

Double-diffusive convection in a magnetic nanofluid-filled porous medium: Development and application of a nonorthogonal lattice Boltzmann model

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
In this work, to fill the rare reports on double-diffusive convection (DDC) considering the effects of porous medium, nanofluid, and magnetic field at the same time, we first developed a full nonorthogonal multiple-relaxation-time lattice Boltzmann (LB) model for DDC in a nanofluid-filled porous medium subjected to a magnetic field. The capability of the newly proposed model is then verified. By solving specific problems via the full model with specific control parameters, we show that the nonorthogonal LB model is accurate for handling the effects of the porous medium, nanofluid, and magnetic field. Finally, we apply the model to DDC in an Fe3O4–water nanofluid-filled porous cavity with a hot left boundary and examine the effects of magnetic field intensity and inclination angle on the flow, heat, and mass transfer inside the porous medium. Results show that heat and mass transfer can both be adjusted by varying the intensity and inclination angle of the magnetic field. When the external magnetic field is applied, the heat and mass transfer along the hot wall declines monotonously with increasing the strength of the magnetic field. In contrast, the average Nu and Sh increase at first and then decrease with the inclination angle of the magnetic field, reaching the maximum at around γ = 45°. Results in this work pave a tunable way for heat and mass transfer regulation inside a magnetic nanofluid-fill porous medium. In addition, this work provides essential reference solutions for further study on DDC in a nanofluid-filled porous medium subjected to a magnetic field.

Convective dissolution of carbon dioxide in two- and three-dimensional porous media: The impact of hydrodynamic dispersion

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
Convective dissolution is the process by which CO2 injected in geological formations dissolves into the aqueous phase and thus remains stored perennially by gravity. It can be modeled by buoyancy-coupled Darcy flow and solute transport. The transport equation should include a diffusive term accounting for hydrodynamic dispersion, wherein the effective diffusion coefficient is proportional to the local interstitial velocity. We investigate the impact of the hydrodynamic dispersion tensor on convective dissolution in two-dimensional (2D) and three-dimensional (3D) homogeneous porous media. Using a novel numerical model, we systematically analyze, among other observables, the time evolution of the fingers' structure, dissolution flux in the quasi-constant flux regime, and mean concentration of the dissolved CO2; we also determine the onset time of convection, [math]. For a given Rayleigh number Ra, the efficiency of convective dissolution over long times is controlled by [math]. For porous media with a dispersion anisotropy commonly found in the subsurface, [math] increases as a function of the longitudinal dispersion's strength (S), in agreement with previous experimental findings and in contrast to previous numerical findings, a discrepancy that we explain. More generally, for a given strength of transverse dispersion, longitudinal dispersion always slows down convective dissolution, while for a given strength of longitudinal dispersion, transverse dispersion always accelerates it. Furthermore, a systematic comparison between 2D and 3D results shows that they are consistent on all accounts, except for a slight difference in [math] and a significant impact of Ra on the dependence of the finger number density on S in 3D.

Nonlinear oscillations of a collapsible tube subjected to unsteady external pressure

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
The non-linear dynamics of an extremely thin-walled collapsible tube with internal flow subjected to a time-varying external pressure are studied experimentally and theoretically. For the constant chamber pressure case, we observe the existence of a fixed-point attractor, period-1 attractor, and quasiperiodic attractor. The period-1 limit cycle oscillations are essentially relaxation oscillations with up-down asymmetry in the time domain, and as the Reynolds number increases, the asymmetry becomes greater. With the forcing (varying chamber pressure), the system has no fixed points; its response can be period-n, quasiperiodic, or chaotic, depending upon the Reynolds number, driving amplitude, and frequency. For the forced system, at a low Reynolds number, the external forcing dominates the self-excited oscillations and symmetric oscillations are observed; at a higher Reynolds number, the reverse is true. In experiments and theory, aperiodic oscillations for the forced system are always observed in regimes beyond the Hopf bifurcation point of the unforced system. Distended and collapsed cases, under forcing, exhibit only 1:1 synchronous oscillation. These suggest that a natural oscillation timescale of the system must be present for the external forcing to induce aperiodicity. In the experiments, the forced system exhibits signs of quasiperiodic route to chaos at lower driving amplitude, while period-doubling route to chaos at higher driving amplitude. When the system is forced near its natural frequency, an aperiodic response is totally suppressed.

Field-controlling patterns of sheared ferrofluid droplets

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
We investigate how ferrofluid droplets suspended in a wall-bounded shear flow can organize when subjected to an external magnetic field. By tuning the magnitude of the external magnetic field, we find that the ferrofluid droplets form chain-like structures in the flow direction when the magnetic field is weak, while forming a crystal-like pattern in a strong magnetic field. We provide the phase diagram and the critical conditions for this chain-to-crystal transition, by applying both numerical simulations and analytic calculations. We also examine how the organized patterns of the ferrofluid droplets can be controlled by simply changing the direction of the magnetic field. This work demonstrates new aspects of field-controllable ferrofluid droplets as a configurable and reprocessable metamaterial.

Dynamic evolution of liquid phase disturbance and its critical influence on pre-breakdown process

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
Liquid phase disturbances are often observed in pre-breakdown processes; however, their dynamic behaviors are rarely studied. In this paper, time evolution characteristics of liquid phase disturbance under ultra-long pulses (>100 ms) were investigated. The results showed that the steady expansion of liquid phase disturbance follows the pattern of constant heating power, volume growth rate, and liquid temperature (about 52 °C unvaried with applied voltage). The shrinkage of liquid phase disturbance with the applied voltage leads to the breakdown transition from a full disturbed phase mode to a partial disturbed phase mode. Further research indicated that the liquid phase disturbance has a significant influence on the development of subsonic streamers (especially for positive polarity). In the disturbed phase of liquid, the streamers propagate faster with a plump morphology than in the stationary phase. The local turbulences at the boundary of the disturbed phase can retard the streamer propagation remarkably and lead to the streamer branching. Finally, the abnormal downtrend of positive streamers' average velocity varied with the applied voltage due to the shrinkage of liquid phase disturbance was predicted and observed for the first time.

Lattice Boltzmann model for the low-Mach number variable-density flow

Tue, 06/21/2022 - 12:02
Physics of Fluids, Volume 34, Issue 6, June 2022.
In this work, we present a pressure-based double-population lattice Boltzmann model for the low-Mach number variable-density flow. The model is simple, stable, and purely local. The asymptotic analysis of the model indicates that it recovers the continuity, momentum, and energy equations describing the low-Mach number variable-density flow. The comparisons between the simulation results using the proposed model and the numerical data reported by previous studies demonstrate that the model can accurately predict the drag coefficient and the Nusselt number for a sphere and a prolate ellipsoid in low-Mach number variable-density flow over a wide range of Reynolds numbers.

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