Ambient Fluid Research Articles

My journey in structural acoustics

Structural acoustics refers to problems of the vibrational response of structures in the presence of an ambient compressible fluid. In this presentation, I present my introduction to the subject by virtue of my interaction with Miguel Junger, one of the pioneering investigators in this field. At his suggestion and with his insight, I addressed two canonical problems that launched my journey into the subject. The first is that of a spherical object spring mounted, moving axially, and radiating into a compressible fluid medium. The other is that of a force excited elastic plate radiating into a fluid half space. Because of their simplicity both problems can be solved exactly, and their solution provides physical insight into the solution of more practical configurations, which were pursued further and included collaboration with Carderock colleagues Gideon Maidanik and Murray Strasberg.

A unified theory for bubble dynamics

In this work, we established a novel theory for the dynamics of oscillating bubbles such as cavitation bubbles, underwater explosion bubbles, and air bubbles. For the first time, we proposed bubble dynamics equations that can simultaneously take into consideration the effects of boundaries, bubble interaction, ambient flow field, gravity, bubble migration, fluid compressibility, viscosity, and surface tension while maintaining a unified and elegant mathematical form. The present theory unifies different classical bubble equations such as the Rayleigh–Plesset equation, the Gilmore equation, and the Keller–Miksis equation. Furthermore, we validated the theory with experimental data of bubbles with a variety in scales, sources, boundaries, and ambient conditions and showed the advantages of our theory over the classical theoretical models, followed by a discussion on the applicability of the present theory based on a comparison to simulation results with different numerical methods. Finally, as a demonstration of the potential of our theory, we modeled the complex multi-cycle bubble interaction with wide ranges of energy and phase differences and gained new physical insight into inter-bubble energy transfer and coupling of bubble-induced pressure waves.

Flow-induced transition of compound droplet to composite microfiber in a channel with sudden contraction

The deformation behavior and hydrodynamic stability of a three-dimensional Newtonian single-core compound droplet during flow in a channel with sudden contraction were studied by numerical modeling. This research was motivated by the quest for conditions of the steady transition of a compound droplet into a composite microfiber, whose core is stretched as much as the shell. With this aim, the dynamics and morphology evolution of the compound droplet were analyzed in detail as functions of capillary number, core-to-shell relative viscosities, interfacial tensions, and the relative initial core radius. It was found that the effective elongation of the core occurs either with a significant increase in the shell viscosity relative to the ambient fluid or with a decrease in the core viscosity with respect to the shell. In this case, as the composite droplet advances into the narrowing zone of the canal, it continues to stretch, becoming a bullet-shaped composite microfiber. A new mechanism of disintegration of the compound droplet was revealed, which is caused by the core destabilizing effect and manifests itself either with an increase in the relative core/shell interfacial tension or the relative core viscosity.

Convective Boundary Layer Flow Adjacent to an Inclined and Linearly Heated Semi-Infinite Plate

Abstract The transient convective flow adjacent to an inclined semi-infinite plate which is heated by a linear temperature gradient is investigated with scaling analysis and direct numerical simulation (DNS) in this study. Both Pr < 1 and Pr > 1 fluids are considered. The initial ambient fluid is quiescent and thermally homogeneous. Important parameters characterizing the thermal boundary layer flow are thickness, characteristic velocity, and time to reach the steady stage. Scaling analysis is carried out to obtain scales for these flow parameters. Compared to previous similar studies, the obtained scale relations are more generalized and they can be utilized for different inclination angles. The derived scales are compared against the DNS results for a variety of flow parameters, e.g., Rayleigh number Ra, Prandtl number Pr, stratification factor s (s = dθw(y)/dy, where θw(y) is the local temperature at a streamwise location of y), inclination angle of the heated plate α, evolutionary time τ, and streamwise location y. The scale relations and the DNS results compare well suggesting the proposed scale laws can provide a sound description for the dynamics of the convective flow subjected to a tilted surface and a linear heating condition.

Numerical simulation of density-driven non-Newtonian fluid flow

Abstract Density-driven flow is numerically studied. The sinking fluid is set as a power-law non-Newtonian fluid with a higher density than the environmental fluid. During the simulation process, saturation concentration is fixed on the upper boundary; thus downward plumes are formed because of gravitational instability. The dissolution flux undergoes a series of changes, from the initially diffusion-dominated regime to the convection-dominated regime due to the appearance of finger structures, and then to the transition of finger structures merging into larger plumes. Finally, it enters the shutdown regime as the plumes start to reach the impermeable bottom boundary. In the process of plume sinking, different fluid properties have an important impact on the downward velocity, shape of plumes and the dissolution flux of the flow field. The tip velocity of the plumes is slowed until the high-concentration fluid is supplied to further push the plumes downward. For the shear-thinning fluid ambient fluid, this phenomenon is even more drastic. However, for shear-thickening fluid, this phenomenon is almost not observed. In addition, unlike the condition of a Newtonian fluid, protoplumes on the original interface appear at the early stage. Prominent protoplumes have developed between the primary plumes in non-Newtonian fluids throughout the entire process.

Axisymmetric plumes due to fluid injection through a small source in a wet porous medium

A small spherical source discharges a fluid into a porous medium that is already fully saturated with another fluid. The injected fluid has higher density than the ambient fluid, and so it forms a plume that moves downward under the effects of gravity. We present a simple asymptotic analysis assuming the two fluids do not mix that gives the width of the plume far from the source as a function of the injected volume flux. A spectral method is then developed for solving the full nonlinear problem in Boussinesq theory. Accurate numerical solutions are presented, which show in detail the evolution of the plume of heavier injected fluid over time. Close agreement with the asymptotic plume shape far from the source is demonstrated at later times.

The near-field shape and stability of a porous plume

When a fluid is injected into a porous medium saturated with an ambient fluid of a greater density, the injected fluid forms a plume that rises upwards due to buoyancy. In the near field of the injection point, the plume adjusts its speed to match the buoyancy velocity of the porous medium, either thinning or thickening to conserve mass. These adjustments are the dominant controls on the near-field plume shape, rather than mixing with the ambient fluid, which occurs over larger vertical distances. In this study, we focus on the plume behaviour in the near field, demonstrating that for moderate injection rates, the plume will reach a steady state, whereby it matches the buoyancy velocity over a few plume width scales from the injection point. However, for very small injection rates, an instability occurs in which the steady plume breaks apart due to the insurmountable density contrast with the surrounding fluid. The steady shape of the plume in the near field depends only on a single dimensionless parameter, which is the ratio between the inlet velocity and the buoyancy velocity. A linear stability analysis is performed, indicating that for small velocity ratios, an infinitesimal perturbation can be constructed that becomes unstable, whilst for moderate velocity ratios, the shape is shown to be stable. Finally, we comment on the application of such flows to the context of CO $_2$ sequestration in porous geological reservoirs.

On the definition, evolution, and properties of the outer edge of gravity currents: A direct-numerical and large-eddy simulation study

Gravity currents are flows driven by the action of gravity over fluids with different densities. Here, we focus on gravity currents where heavier fluid travels along the bottom of a sloping bed, underneath a large body of stagnant lighter ambient fluid. The thickness of the current increases due to entrainment of ambient fluid into the current. Direct numerical and large eddy simulations of gravity currents and a wall-jet transporting a passive scalar field are performed. We focus on the rate of penetration of mean momentum and mean concentration of the agent responsible for the density difference (temperature, salinity, or sediment volume fraction) into the ambient fluid. The rates of penetration of turbulence-related quantities (i.e., turbulent kinetic energy, Reynolds flux, and stress) into the ambient are analyzed. A robust methodology for defining the upper edge of these quantities and thereby defining the current thickness using these different mean and turbulent quantities is presented. A comparison between downstream evolution of the gravity current with the corresponding behaviors of canonical wall-bounded turbulent flows is presented. The present understanding of turbulent/non-turbulent interface (TNTI) is extended to include subcritical flows where, due to the strong effect of stratification, the TNTI is buried well within the upper edge of the current and confined right above the inner near-bed layer. The present work sheds light on the striking difference between the different definitions of thickness (momentum, concentration, turbulence, etc.) in subcritical gravity currents, where stratification suppresses turbulence in the upper region of the current.

Experimental investigation of fountains in homogeneous fluid with coexistent temperature and concentration effects using PIV/PLIF

Fountains formed due to the injection into homogeneous fluids with the combined effect of temperature and concentration occur widely in nature and in environmental improvement and industrial applications such as brine water discharge, living sewage disposal, release of waste hot gases into the atmosphere from chimneys, solar desalination discharge, and sewage wastewater discharge for agriculture irrigation. In this study, the behavior of round fountains with salt water as a homogeneous ambient fluid in a transparent glass tank over 0.25 ≤ FrT ≤ 2.0, 0 ≤ N ≤ 5.0, and 47 ≤ Re ≤ 824 was studied using particle image velocimetry/planar laser-induced fluorescence (PIV/PLIF). The recorded PIV/PLIF images provide a better understanding of the transient behavior and intrinsic laws of buoyant jets in an ambient fluid and optimally use their transport mechanisms and entrainment characteristics in environmental improvement and industrial applications. Scaling for the initial maximum fountain height at the development stage, the obtained scaling correlation from the experimental results is consistent with previous results from direct numerical simulation results.

Global existence of free-energy solutions to the 2D Patlak-Keller-Segel-Navier-Stokes system with critical and subcritical mass

We consider a coupled Patlak-Keller-Segel-Navier-Stokes system in $\mathbb{R}^2$ that describes the collective motion of cells and fluid flow, where the cells are attracted by a chemical substance and transported by ambient fluid velocity, and the fluid flow is forced by the friction induced by the cells. The main result of the paper is to show the global existence of free-energy solutions to the 2D Patlak-Keller-Segel-Navier-Stokes system with critical and subcritical mass.

Correction method for dynamic measurement with an optical lever AFM (Effects of ambient fluid and surface force)

Correction method for dynamic measurement with an optical lever AFM (Effects of ambient fluid and surface force)

A Multi‐Layer Stacked Triboelectric Nanogenerator Based on a Rotation‐to‐Translation Mechanism for Fluid Energy Harvesting and Environmental Protection

AbstractEnergy shortage and environmental degradation are two important challenges facing humanity. Here, a multi‐layer stacked triboelectric nanogenerator (MLS‐TENG) based on a rotation‐to‐translation mechanism is reported for fluid energy harvesting and environmental protection. The mechanism transforms fluid‐induced rotation into a reciprocal translation of the MLS‐TENG, enabling the conversion of fluid energy into electrical energy. In addition, benefiting from a multi‐layer stacked structural design, the open‐circuit voltage is increased from 860 to 2410 V and an efficient energy harvesting rate of 2 mJ min−1 is obtained in an actual river. Furthermore, with the assistance of the MLS‐TENG, a self‐powered wireless temperature and humidity monitoring system and a metal anticorrosion system are successfully established. Ambient monitoring data can be transmitted continuously at an interval of 49.7 s, and the corrosion rate of steel is significantly slowed down. This study provides guidance for efficient harvesting of ambient fluid energy, with promising applications in environmental monitoring and protection.

On simple models for gravity currents from moving sources

The propagation of the gravity current generated from a moving source of buoyancy is of interest in deep-sea mining and related technologies. The study by Ouillon et al. (J. Fluid Mech., vol. 924, 2021, A43) elucidated some salient patterns of the flow concerning a source close to the bottom on the basis of direct numerical simulation on a supercomputer. Here, we present a simple box model that provides further insights and useful analytical approximations for this gravity-current flow system. We show that this flow is very different from that produced by a moving source at the top, studied by Hogg et al. (J. Fluid Mech., vol. 539, 2005, pp. 349–385). The model confirms that the main governing parameter is the ratio $a$ of speed of source to that of buoyancy propagation. The model points out dependency also on the front-jump Froude number (which implies dependency on the height of the ambient fluid). For a sufficiently large $a >a_{crit}$ , a supercritical regime appears in which the gravity current forms a wedge behind the moving source; in the subcritical regime, the upstream propagation attains a maximum $x_m$ at time $t_m$ . The model predicts the value $a_{crit}$ , the distance and time $x_m$ and $t_m$ in the subcritical case, and the shape of the wedge in the supercritical case, without any adjustable constant. Comparisons with the numerical data show fair agreement.

Plasma formation in ambient fluid from hypervelocity impacts

The generation of plasma from hypervelocity impacts is an active research topic due to its important science and engineering ramifications in various applications. Previous studies have mainly focused on the ionization of the solid materials that constitute the projectile and the target. In this letter, we consider impact events that occur in a fluid (e.g., gas) medium, and present a multiphysics computational modeling approach and associated analysis to predict the behavior of the dynamic fluid–solid interaction that causes the surrounding fluid to ionize. The proposed computational framework is applied to a specific case involving a system of three interacting domains: a copper rod projectile impacting onto a soda lime glass target in a neon gas environment. The impact velocity is varied between 3km/s and 6km/s in different simulations. The computational model couples the compressible inviscid Navier–Stokes equations with the Saha ionization equations. The three material interfaces formed among the projectile, the target, and the ambient gas are tracked implicitly by solving two level set equations that share the same velocity field. The mass, momentum, and energy fluxes across the interfaces are computed using the FInite Volume method with Exact two-material Riemann problems (FIVER). The simulation result reveals a region of neon gas with high velocity, temperature, pressure, and mass density, formed in the early stage of the impact mainly due to the hypersonic compression of the fluid between the projectile and the target. For impact velocities higher than 4km/s, ionization is predicted in this region.

Numerical simulation for thermally stratified Powell–Eyring fluid due to the Riga plate: application of the Cattaneo–Christov theory

The present study demonstrates the behavior of Powell–Eyring fluid flow deformed by a linearly stretchable Riga plate. The analysis of heat characteristics is scrutinized with the thermal stratification phenomenon conducted through generalized Fourier’s law (i.e. the Cattaneo–Christov theory). Heat generation/absorption is assumed to disclose the features of thermal transport phenomena. The temperature of the plate is higher than the ambient fluid temperature. It is noted that the Cattaneo–Christov model is responsible to generate some extra terms of heat absorption or generation in the energy conservation law. Appropriate transformations are implemented to achieve nonlinear governing equations in the form of an ordinary differential equation. Numerical solutions are computed for the governing equations through the Range–Kutta Fehlberg technique. In summary, the modified Hartman number is responsible to grow up velocity distribution while the opposite trend is noted for the variable thickness parameter. Heat transport phenomenon can be minimized by incorporating higher thermal relaxation parameters (i.e. it can be used as a cooling phenomenon).

Study of the trapping mechanism of merging drops moving under thermocapillary migration on a surface with wettability contrast

Trapping of drops due to wettability contrast in porous materials emerges in a variety of subsurface and manufacturing applications. In carbon capture and storage, carbon dioxide must be trapped in order to avoid its release into the atmosphere, while trapped oil must be displaced. Both carbon dioxide and oil can exist as drops in porous media. The migration of a single drop attached to a wall can be hindered if the wall surface has a different wettability. However, the trapping condition becomes more complicated in the presence of two or more drops. In this work, we aim to study the trapping mechanism during thermocapillary migration of a merging drop on a heterogeneous surface. To do so, numerical simulations have been performed using the Front-Tracking/Finite Volume Method, where the Navier-Stokes equations are coupled with the conservation equations. The generalized Navier boundary condition (GNBC) has been used as the slip model to remove the viscous singularity. The combined finite-volume and tracking method is able to deal with different types of discontinuities in compressible or incompressible fluid flows, as e.g., interfaces. The material properties of the drop and the ambient fluid are different, and surface tension depends on the temperature. The results have shown that there exist three regimes characterizing the motion of merging drops, including the passage of the leading drop, the trapping of the merging drop, and the partial trapping, which correspond to the trapping of the leading drop but the passage of the merging drop. We show that there is a critical wettability contrast at which the merging drop gets trapped. The effects of Marangoni number (Ma) and viscosity ratio are investigated. The critical wettability contrast decreases with the increase of Ma number and the regime shift as a function of the viscosity and Marangoni number. These findings have implications for the design of geological carbon dioxide storage, improvement of oil recovery and microfluidic device development.

Cold water ingestion ameliorates increase in core temperature and discomfort during simulated motor racing in a hot environment: a randomised trial

Introduction: Formula One drivers race for prolonged periods (>90min) in hot conditions, wearing protective apparel that impairs heat loss, thus are at risk of prolonged elevated core temperature. Ingestion of cold fluids has been shown to be beneficial in endurance activities such as running and cycling. This study aimed to determine whether the voluntary ingestion of cold fluid aided thermoregulation more effectively than the voluntary ingestion of ambient temperature fluid in a simulated motor racing environment.

Density-driven exchange flow propagating over an array of densified obstacles

The evolution of bottom-propagating gravity currents with the presence of an array of densified obstacles submerged in a channel is investigated using large-eddy simulations. Our attention is particularly focused on the flow transition of gravity currents over rough surfaces with extra resistance that provokes significant dissipative processes. Two geometric parameters of the roughness elements, namely, the submergence ratio of the obstacle D/H and the gap-spacing ratio λ/D between obstacles, govern their kinematic and dynamic effects on the propagation of gravity currents. Physically, D/H plays a significant role in the control of the current diversion, and λ/D regulates the flow pathway of gravity current propagation. The integrated measures show that two distinct flow morphologies are identified. For a low submergence ratio (D/H<0.15), an overtopping flow is formed in which the gravity current travels on the top of the array and undergoes an inconspicuous loss of buoyancy, subject to minimal vertical convective instability interacting with the underlying ambient fluid within the gap regions. For a sufficiently high submergence ratio (D/H≥0.15) and a certain gap spacing (2≤λ/D<4), an overrunning flow is formed in which the current rapidly decelerates to a buoyancy–inertia state and then transitions to a drag-dominated state with a gain in excessive drag, in which the front velocity is proportional to t−0.5. However, the simulation results show a turning point toward an increase in the gap spacing as λ/D≥4, that the maximum drag acting on the gravity current is measured when it impinges on the second obstacle of an array, and that the drag coefficient goes up by 10%–40%, depending on D/H. The propagation of the gravity current does not show a higher sensitivity to the retarding effect instead. Meanwhile, the promotion of energy conversion occurs because of the gravity current encountering the continuous climbing and plunging flow behavior between two adjacent obstacles in regular motions.

Investigation of asbestos concentration in ambient and lavage fluids of patients referred for bronchoscopy, Ahvaz.

The amount of fibers in the lungs is considered to reflect the cumulative intensity of past asbestos exposure, and bronchoalveolar lavage (BAL) has been proposed to be a good indicator of the presence and quantity of asbestos particles in the lungs. This study evaluated the asbestos concentration in BAL fluids of asbestos-exposed and unexposed pulmonary patients and the environment of Ahvaz city. This prospective study was conducted on 80 patients underwent diagnostic fiberoptic bronchoscopy referred to Imam Khomeini Hospital in Ahvaz, Iran, in 2019. Patients with Lung diseases were divided into three groups based on CT scan results: normal (n = 32), lung cancer (n = 40) and Interstitial lung disease (n = 8). The analysis of asbestos fiber concentration in BAL fluid was carried out by Scanning Electron Microscope (SEM). The positive asbestos test was detected in 69% of all subjects, including 64% of whom had asbestos-related jobs and 74.5% of those with non-related jobs (p = 0.240). The concentrations of asbestos fiber in the BAL in normal patients, lung cancer and interstitial fibrosis (ILD) were 8.13 ± 5.38, 9.66 ± 7.30 and 6.31 ± 1.98 f/ml, respectively (P = 0.492). There was no significant difference between the asbestos levels and exposure history (P = 0.877). The mean concentration of asbestos in the ambient air during the current year was 2.69 ± 0.57 f/ml (2.26-3.70), and the correlation between asbestos levels in BAL and the air was not significant (r = 0.147; P = 0.243). The exposure of different occupational and non-occupational groups to this carcinogenic substance indicates the need for environmental and individual control measures to reduce and prevent asbestos exposure.

Sediment characterization of bottom propagating reversing buoyancy particle-bearing jets

Lofting characteristics and sedimentation patterns in reversing buoyancy particle-bearing jets are investigated experimentally and theoretically. The presence of particles and buoyancy show notable differences in the jet dynamics and shape of the sediment deposit. The suction force significantly influences the lofting location of the jet. The sedimentation dynamics are affected by the bottom drag and entrainment of the ambient fluid.