Close to 50% of the world's oil and gas reserves are held in naturally fractured carbonate reservoirs. In addition to the presence of fractures, vugs at different scales and distributions are scattered throughout the porous matrix. These cavities cause fluid flow characteristics to significantly differ from those of conventional homogeneous pore structure reservoirs and bring the need to evaluate equivalent petrophysical properties of the heterogeneous medium. In this study, a microfluidic approach is used to determine water and oil relative permeability curves and phase distribution in two-dimensional micromodels of vugular porous media. Steady-state water–oil injection experiments were performed at different fractional flows, while monitoring the dynamics of the pressure drop and visualizing the fluid displacement at the pore scale. Live-image acquisition through fluorescence microscopy made it possible to examine the evolution of the saturation of water and oil phases. A direct comparison between the relative permeability curves of well-characterized vugular porous media and their porous matrix showed that the incorporation of vugs leads to (i) higher equivalent absolute permeability, especially with longer cavities and higher vug density, (ii) increased oil occupancy in the porous matrix, due to less efficient water invasion into the porous matrix, and (iii) higher relative permeability to water, which flows preferentially through the vugular space. These results are consistent with the oil-wet nature of micromodels, since the vugs offer less capillary resistance to the flow of the non-wetting phase.
Thin liquid films are ubiquitous in nature and have many practical applications. From biological films to the curtain coating process, thin films are present in both large and small scales. Despite their importance, understanding the stability of these films remains a significant challenge due to the fluid–fluid interface that is free to deform, affected by interfacial tension and complex rheological behavior. Instabilities in thin films are often caused by van der Waals attractions, which can lead to the rupture of the layer. To investigate the rupture dynamics, numerical methods are commonly used, such as asymptotic derivations of the lubrication theory or interface tracking methods. In this paper, we present a computational study of the breakup dynamics of a stationary thin liquid sheet bounded by a passive gas with a viscous interface, using the arbitrary Lagrangian–Eulerian method and the Boussinesq–Scriven constitutive law to model the rheological behavior. Our results demonstrate that the stability of thin liquid films is influenced by both surface rheology and disjoining effects and that the viscous character of the interface can delay sheet breakup, leading to more stable films.
For investigating the nonlinear acoustic behavior of circular orifices under high amplitude sound excitation, an approach based on the three-dimensional time-domain computational fluid dynamics simulation is proposed to extract the nonlinear acoustic impedance of circular orifices. By solving laminar flow equations, the propagation of the acoustic signal in the vicinity of orifice is simulated, and the effect of the interaction among adjacent orifices is considered by using the lateral periodic boundary condition. The effects of diameter, thickness, and porosity on acoustic impedance under different amplitude sound excitations are studied. By using nonlinear regression analysis of the dimensionless parameters composed of particle velocity amplitude, frequency, and geometric parameters, fitting formulas of nonlinear acoustic impedance of circular orifices are presented. As an application of engineering computation, transmission loss of perforated silencers under low and high amplitude sound excitations is predicted by using the present acoustic impedance fitting formulas and via the finite element computation. By comparing the predicted and measured results, the accuracy and practicability of fitting formulas of nonlinear acoustic impedance of circular orifices are validated.
Numerical investigation of the coupled Kelvin-Helmholtz Rayleigh-Taylor instability (KHRTI) is presented here by solving the compressible Navier-Stokes equations for two air streams differentially heated in two halves of a three-dimensional (3D) box. Here, we explore the role of a stabilizing and destabilizing thermal gradient and that of reversing the direction of the air streams considered for Atwood numbers of {plus minus}0.1567 and dimensionless tangential shear of ΔU = 0.68 and 4.1. The onset of the KHRTI and development of the turbulent mixing layer are explored via time-resolved and instantaneous distributions of temperature and vorticity. Early stages of the KHRTI with reversed air streams follows a Kelvin-Helmholtz (KH) mechanism, with Rayleigh-Taylor (RT) dynamics becoming important at later times. This leads to an earlier development of the turbulent mixing layer. The KHRTI with stabilizing or destabilizing thermal gradients shows a dominance of the buoyancy-driven mechanism, right from the onset. The transition from laminar to turbulent mixing layer involves the creation of coherent structures of spikes, bubbles and KH whirls for the destabilizing, stabilizing thermal gradient and reversed shear cases, respectively. The spectra of the turbulent signals reveal a -5/3 scaling when the shear-driven mechanism is prevalent in the flow and -11/5 scaling when the buoyancy-driven effects become prominent. The compressible enstrophy budget of the KHRTI shows that the onset process is dominated by vortex stretching or compressibility effects, followed by a sharp rise in baroclinic torque contribution once the buoyancy effects become relevant.
We use large-eddy simulation (LES) and theoretical analyses to study the turbulent flow over fast opposing water waves. A novel nonlinear viscous model for the airflow perturbations induced by Stokes waves is developed, which can account for the wave-perturbation viscous stress and the nonlinear forcing by the multimode interactions of wave-correlated quantities in the wind field. Overall, the nonlinear viscous model can describe the wave-coherent airflow in the LES results for high-order Stokes waves, which demonstrates the negligible effects of wave-perturbation turbulent stress in the wind. According to the model, the dominant components of the fundamental mode of the airflow perturbation primarily result from the linear response of the wind to the wave, and, thus, are not substantially affected by the nonlinear forcing. However, the weak components of the fundamental mode, which produce the form drag on the wave, are created by the combined effects of the nonlinear forcing and the wave-perturbation viscous stress. We found that the main mechanism for generating the nonlinear forcing is the interaction between the second harmonic and the fundamental mode of the wave-correlated quantities in the air. In this mechanism, wave nonlinearity exerts its effects mainly through the second harmonic of the wave surface, instead of the second harmonic of the wave kinematics. Therefore, it is further demonstrated that a second-order Stokes wave is sufficient to capture the wave nonlinearity effects on the form drag.
We study the response of an endothelial cell monolayer (ECM) lining the bottom surface of a cartesian Couette geometry in variations of critical shearing parameters that affect the fluid environment, such as the gap distance between the upper moving and the bottom stationary plates and the velocity of the moving plate. Specifically, we propose an in-silico rheometric emulation based on start-up shear experiments in a representative two-dimensional domain of the monolayer that accounts for the interaction of the blood plasma and the deformable multilayer poroelastic endothelial cells (ECs). We present quantitative predictions for the shear and normal stresses on each cell compartment (membrane, cytoplasm, and nucleus) and their structural changes. We show that the variation of the Wall Shear Stress (WSS) along the cell membrane is considered significant and strongly dependent on the shape of the cell, while membrane thinning is more prominent at the locus of high WSS in the range of physiological velocities. However, under extreme velocities, wall thinning prevails at the locus of flow stagnation.
This work develops a data analysis procedure, namely Proper Orthogonal Decomposition (POD)-Dynamic Mode Decomposition (DMD) augmented analysis, to isolate the energy- and evolution-wise dominant features of flow field in a street canyon. This combination aims to extract modes imposing critical influence on pollutant dispersion from both energetic and dynamic perspectives. The two techniques were first conducted based on large-eddy simulation (LES) results. Subsequently, based on the POD and DMD ranking, the extracted modes were classified into three types: (1) type 1: energetically & dynamically significant mode; (2) type 2: energetically significant & dynamically insignificant mode; (3) type 3: energetically insignificant & dynamically significant mode. Results show that mode type 1 contribute to the mainstream flow and the main vortex structures, which can be observed near the stagnation point, the separating point, and the fluid reattachment area. Mode type 2 throws light on where the turbulent kinetic energy is the largest, leading to periodically sudden pollutants increase on the building roof and the wake region. Mode type 3 contributes to the long-term reversed flow structures occurring near the stagnation point, inside the street canyon, and in the wake region. This technique can provide a systematic analysis of the flow field within a street canyon, and it also provides help for potential applications at a city scale, such as solving pollutant dispersion issues in urban areas.
Coalescence-induced droplet jumping on superhydrophobic surfaces has attracted significant attention in recent years. In this paper, by using a three-dimensional multiphase lattice Boltzmann model, we numerically investigated the droplet jumping on V-shaped groove superhydrophobic surfaces induced by the coalescence between two droplets located in the asymmetric V-shaped groove. Firstly, it is found that the self-jumping process gradually becomes inefficient when the groove angle decreased, which is caused by the increasing viscous dissipation with the decrease of the groove angle. In order to overcome the weakness of the V-shaped superhydrophobic surface and enhance the droplet jumping performance, an improved V-shaped superhydrophobic surface with a triangular prism was conceived. Numerical results showed that the normalized jumping velocity and the energy conversion efficiency of the V-shaped superhydrophobic surface with a triangular prism can be increased by up to 80% and 210%, respectively, in comparison with those of the surface without the triangular prism. The jumping enhancement mainly arises from the combined effect of the redirection of the expanding liquid bridge by the V-shaped sidewalls, as well as the earlier and sufficient impact of the liquid bridge on the triangular prism in the groove. Moreover, using the improved V-shaped superhydrophobic surface, a guided jumping can be achieved due to the reaction forces exerted by the V-shaped sidewalls and the triangular prism, and the jumping angle can be more accurately predicted based on the groove angle.
A simple method for structuring natural oleosome emulsions by polymer-bridging mechanism is proposed. Polymer bridging of oleosome droplets was induced by the addition of two different adsorbing polymers. Over a range of polymer/oleosome ratios, the mixture results in the formation of a particle gel network of aggregated oleosome droplets. It is found that polymer bridging ability is heavily influenced by the strength of binding between polymer and oleosome surface where sodium alginate interacted stronger to oleosome surface than ι-carrageenan. These effects are associated with the different molecular architecture and physical differences between the two hydrocolloids. Alginate has a co-block arrangement of charged and uncharged units. The polymer promotes stronger adsorption to the oleosome surface, in contrast to ι-carrageenan, where the negative charges are distributed uniformly along its chain. The polymer bridging ability will influence the resulting microstructure and therefore rheological properties. Confocal scanning laser microscopy showed that the difference in microstructure is mainly in the extent of heterogeneity over different length scales where sodium alginate produced the most heterogeneous microstructures. Bridging-flocculated emulsions showed power-law scaling behavior of the storage modulus with the oleosome concentration which was explained using percolation theory.
Interfacial Rayleigh-Taylor mixing is crucial to describing important natural and engineering processes such as exploding supernovae, laser micro-machining, hot spots in inertial confinement fusion, and optical telecommunications. These require the characterization of the time dependence of the driving acceleration. We compare our theoretical formulation based on group theory foundations with interface-capturing numerical simulations for linear and nonlinear two-dimensional Rayleigh-Taylor instabilities in a finite-sized domain with time-varying acceleration over broad ranges of Atwood numbers and acceleration exponents. Detailed corroboration between theory and simulations is provided for this foundational case. Both demonstrate the strong interfacial nature of Rayleigh-Taylor instabilities, which suggests that practical flow fields can be reconstructed from the derived fluid potential using the proposed theory. Robust agreement is also obtained for the early- and late-time evolution of the amplitudes of the bubble and spike, which demonstrate that the Rayleigh-Taylor flow can transition to the mixing regime even for a single-mode initial perturbation. Corroboration with experiments in high energy density plasmas motivated by studies of supernovae is also achieved. In addition, a long-standing puzzle in Rayleigh-Taylor dynamics on the interplay between shear and curvature in theory and simulations is resolved by accounting for finite viscosity in the latter. The characterization of Rayleigh-Taylor instabilities as a highly interfacial phenomenon provides valuable insights into its multi-scale nature, which enhances the design and understanding of numerous processes of practical interest.
The study of heat transfer and peristaltic pumping of magnetohydrodynamic (MHD) biofluids have many physiological applications such as heart-lung machines during surgeries, dialysis, vitamin injections, cancer treatment. Also, it has many industrial applications such as pharmaceutical fluid production, filtration, dispensing cosmetic/glue emulsions with no contamination. Furthermore, the bi-viscous Bingham nanofluid model is the best model for several bio/industrial fluids. Therefore, the impact of thermal radiation and cross diffusion on the mixed convection peristaltic pumping of a bi-viscous Bingham nanofluid in a porous flexible conduit is considered. Also, we focus on the flexibility of the walls along with the convective boundary conditions. We adopted the lubrication strategy to reduce the system's complexity. The system of non-dimensional partial differential equations (PDEs) along with the pertinent boundary conditions is solved for several sets of values of the dimensionless parameters. The expressions for the temperature, concentration, velocity, and heat transfer coefficient are obtained analytically. The impact of the relevant parameters on the velocity, temperature, coefficient of heat transfer, concentration, and trapping are discussed in depth with the help of graphical illustrations. The results indicate that the velocity distribution is reduced with growing Darcy parameter and concentration Grashof number. Intensifying the magnetic parameter results in shrinking the trapped bolus. Decay in the heat transfer coefficient is observed for rising values of the radiation parameter. The current findings are compared with the existing studies in the literature and are found to agree very well for special cases.
The hydrodynamic mechanism of drag reduction by flapping a pair of flexible filaments behind a cylinder was explored using the penalty immersed boundary method. The effects of the phase difference between two filaments, the attachment height, and the flapping amplitude on drag reduction were examined. The flapping filaments weaken the vortex shedding via the destructive interaction between the vortices with the opposite signal. The clapping (out-of-phase) flexible filaments experience a lower friction drag and reduce a form drag of the cylinder, showing a better drag reduction than the snaking (in-phase) flexible filaments and the clapping rigid filaments. A minimum drag is obtained at an appropriate attachment height and flapping amplitude that avoid collision of the filaments and weaken the shear-layer-filaments interaction. The effectiveness ratio of the clapping filaments is higher than that of the snaking filaments. Energy saving can be achieved by avoiding the shear layer-filament interaction at a low flapping amplitude, whereas the filaments can further reduce drag with greater energy consumption at an appropriate flapping amplitude. In addition, the total drag decreases with increasing Reynolds number, accompanied by a transition of the wake pattern from the 2S mode to the P + S mode.
This study reports the development of a semi-empirical Force Balance-based Moving Front kinetic Monte Carlo (FB-MFkMC) model to describe droplet spreading on a smooth surface. The proposed model depicts the state-by-state evolution of a sessile droplet in a stochastic manner that captures the molecular-level events taking place in an accurate yet efficient manner. In the developed model, the movement of the droplet triple contact line is depicted using rate expressions that detail the probability that the contact line will locally advance over a set distance at each time point. These rate expressions are derived based on the force balance acting upon the droplet interface, which is captured using analytical inertial and capillary expressions from the literature. This work furthermore derives a new semi-empirical expression to depict the viscous damping force acting on the droplet. The derived viscous force term depends on a fitted parameter , whose value was observed to vary solely depending on the droplet liquid as captured predominantly by the droplet Ohnesorge number. The proposed FB-MFkMC approach is subsequently validated using data obtained both from conducted experiments and from the literature to support the robustness of the framework. The predictive capabilities of the developed model are further inspected to provide insights on the sessile droplet system behaviour.
This investigation considers the effect of axial pressure gradient on the dynamics of flame-vortex interaction for a lean premixed bluff-body stabilized flame. Large eddy simulations (LES) of four different combustor geometries generated through combustor wall adjustments that resulted in mild to strong pressure gradients are studied. A bluff-body stabilized combustor for a propane/air flame is analyzed first. Results are compared with all available experimental data with the purpose of validating the LES methodology used in OpenFOAM and obtaining a base solution for the study of the pressure gradient effect on flame-vortex interaction. The role of the pressure gradient on flame structure, emission characteristics, vortex dynamics, and flame stability are presented.The mild favorable pressure gradient due to the decelerated flow in diffuser configurations influences flame-vortex dynamics by suppressing flame-induced vorticity sources; baroclinic torque and dilatation, and hence resulting in augmented hydrodynamic instabilities. The sustained hydrodynamic instabilities maintain the large flame wrinkles and sinusoidal flame mode in the wake region. The nourished near lean blowoff dynamics also affect the emission characteristics and the emission of species increases. However, the accelerated flow in the nozzle configuration amplifies the flame-induced vorticity sources that preserve the flame core hence resulting in a more organized, symmetric, and stable flame. Ultimately, the combustion performance and operation envelope in the lean premixed flames can be increased by maintaining the flame stability and suppressing the limiting lean blowoff dynamics and emissions with the help of a strong favorable pressure gradient generated through adjusting the combustor geometry.
High-enthalpy gas streams in a real engine or wind tunnel will contain some trace gases; however, this is often ignored in many studies. To investigate transverse fuel-jet mixing and combustion characteristics in a high-enthalpy inflow with trace-gas species, large-eddy simulations based on the HyShot II configuration have been adopted and verified. It was found that a trace amount of atomic oxygen (O) and nitrous oxide has a direct significance on ignition delay times and can, therefore, influence the overall flame distribution. In addition, the results show a greater sensitivity to the levels of O. The complex shock-wave system generated by the transverse jet is found to be a key factor in enhancing mixing and inducing combustion. It not only facilitates vortex generation by increasing the baroclinic term but also provides regions with high pressure and temperature, which accelerate the chemical-reaction rates for radical generation. The initial ignition locations, characterized by HO2 production rates, are mainly located in the low-speed region close to the injector, e.g., in the recirculation region, while OH is mainly formed downstream and accompanied by strong heat release. Based on the analysis of instantaneous contours and statistical results, the overall combustion was found to be in scramjet mode. The partially premixed flame dominates the combustor, where the combustion mainly lies in the flamelet regime, while the diffusion flame dominates the nozzle, where the combustion is spread over the flamelet regime, the broken flamelet regime, and the perfectly stirred reactor regime.
New exact solutions describing Rossby waves and vortices in ocean propagating along the zonal direction at a constant velocity are found for the (3 + 1)-dimensional nonlinear Charney–Obukhov equation. These solutions are a partial superposition of previously discovered exact solutions of the Charney–Obukhov equation. Partial superposition is found for that part of the solution of the Charney–Obukhov equation, which complements the zonal stream. The presence of such a superposition in the solutions of a nonlinear equation with two nonlinear boundary conditions is a remarkable property of the Charney–Obukhov equation for the ocean and allows one to simulate a wide class of fluid flows based on exact solutions. As an example, we discuss solutions that include superposition of trigonometric functions and the functions of Bessel in a horizontal plane, and superposition of spherically symmetrical solutions in the vertical coordinate. Visualization of the solutions found shows that, depending on the values of the parameters included in the solutions, they can describe both a flow with a large number of vortices and a periodic structure with alternating high and low pressure fronts.
This work is dedicated to the systematic investigation of wind turbine wakes under the effect of pressure gradients. Wind tunnel experiments are carried out with a wind turbine positioned on straight ramps of increasing angle such that it experiences an approximately linear flow speed-up/slow-down from the induction region into the far wake. Fifteen ramp angles are studied: 7 favorable (FPG), 7 adverse (APG), and 1 zero pressure gradient. The wake center is shown to follow the base flow streamline originating from a virtual turbine hub height. A quasi-linear relationship between the pressure gradient and near wake length is demonstrated. Far wake characteristics, such as the recovery of the wake center velocity deficit and wake growth rate, are observed to systematically vary with the pressure gradient. The wake recovery rate increases (decreases) with the increase in the FPG (APG), and the wake growth rate shows a linear increase from most favorable to most adverse pressure gradient. The turbine power coefficient decreases significantly with increasing APG to a greater degree than the increase in power coefficient under FPG. The engineering approach of superposing the wake deficit predicted by the standard Gaussian model on the modified base flow is shown to work for very moderate pressure gradients. In light of this, a threshold in terms of flow speed-up/slow-down along the wake trajectory is established, below which the engineering approach can be reasonably employed. Finally, a physics-based model for wakes under the pressure gradient is tested. A new theoretical relation for near wake length under the pressure gradient is proposed. Using the theoretical near wake length, the pressure gradient model predicts the turbine wakes for all cases with good accuracy and shows a significant improvement from the engineering approach.
Vortex-induced vibration (VIV) of two transversely vibrating cylinders in a side-by-side (SBS) arrangement is numerically investigated using a combination of direct-forcing immersed boundary and large eddy simulation techniques. The VIV responses of vibrating SBS cylinders at two reduced velocities (UR* = 4.0 and 6.0) are studied for a range of gap ratio 1.0 ≤g*≤ 3.0. Moreover, the influence of mass ratio, damping ratio, and Reynolds number in the amplitude response and efficiency of VIVACE (Vortex-Induced Vibration for Aquatic Clean Energy) from vibrating SBS cylinders are investigated at moderate Reynolds numbers (Re = 1000 and 10 000). The optimal gap ratio for UR* = 4.0 is in the range of 1.0 ≤g*≤ 1.2. Larger than this range, the VIV responses are close to single-cylinder responses. At UR* = 6.0, all gap ratios show lower responses than a single-cylinder case. The vibrating SBS cylinder with a larger damping ratio results in higher maximum VIVACE efficiency with a narrower UR* range for significant efficiency. With almost the same amplitude response, the SBS cylinders with a lower mass ratio result in lower VIVACE efficiency. Using the same mass-damping parameters, it appears that a low mass ratio could be desirable to increase the UR* range of significant VIVACE efficiency and pick the proper damping ratio to reach a high value of maximum VIVACE efficiency. The effect of flow conditions on the amplitude response and VIVACE efficiency of vibrating SBS cylinders with the same VIV parameters is not significant.
