Showing papers on "Ohnesorge number published in 2015"

Splash behaviour and oily marine aerosol production by raindrops impacting oil slicks

Abstract: The high-speed impact of a droplet on a bulk fluid at high Weber number (We) is not well understood but is relevant to the production of marine aerosol by raindrop impact on the sea surface. These splashes produce a subsurface cavity and a crown which closes into a bubble canopy, but a floating layer of immiscible oil, such as a crude oil slick, alters the splash dynamics. The effects of oil layer fluid properties and thickness, droplet size and impact speed are examined by high-speed visualization. Oil layer rupture and crown behaviour are classified by dimensional scaling. The subsurface cavity volume for impact on thick layers is shown to depend on the Reynolds number (Re), although canopy formation at high Re introduces a competing We effect since rapid canopy closure is found to retard cavity expansion. Time-resolved kinematic measurements show that thin crude oil slicks similarly alter crown closure and cavity growth. The size and spatial distributions of airborne droplets are examined using high-speed holographic microscopy. The droplets have a bimodal distribution with peaks at 50 and and are clustered by size at different elevation angles. Small droplets ( ) are ejected primarily at shallow angles, indicating production by splashing within the first and by breakup of microligaments. Larger droplets ( ) are found at steeper elevation angles, indicating later production by capillary instability acting on large ligaments protruding upward from the crown. Intermittent droplet release while the ligaments grow and sweep upward is thought to contribute to the size-dependent spatial ordering. Greater numbers of small droplets are produced at high elevation angles when a crude oil layer is present, indicating satellite droplet formation from ligament breakup. A crude oil layer also increases the target fluid Ohnesorge number, leading to creation of an intact ejecta sheet, which then ruptures to form aerosolized oil droplets.

Droplet impact on deep liquid pools: Rayleigh jet to formation of secondary droplets.

Abstract: The impact of droplets on a deep pool has applications in cleaning up oil spills, spray cooling, painting, inkjet printing, and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension, and density. Despite the exhaustive research on different aspects of droplet impact, it is not clear how liquid properties can affect the instabilities leading to Rayleigh jet breakup and number of daughter drops formed after its pinch-off. In this article, through systematic experiments we investigate the droplet impact phenomena by varying viscosity and surface tension of liquids as well as impact speeds. Further, using numerical simulations, we show that Rayleigh-Plateau instability is influenced by these parameters, and capillary time scale is the appropriate scale to normalize the breakup time. Based on Ohnesorge number (Oh) and impact Weber number (We), a regime map for no breakup, Rayleigh jet breakup, and crown splash is suggested. Interestingly, crown splash is observed to occur at all Ohnesorge numbers; however, at high Oh, a large portion of kinetic energy is dissipated, and thus the Rayleigh jet is suppressed regardless of high impact velocity. The normalized required time for the Rayleigh jet to reach its peak varies linearly with the critical height of the jet.

Planar microfluidic drop splitting and merging

Abstract: Open droplet microfluidic platforms offer attractive alternatives to closed microchannel devices, including lower fabrication cost and complexity, significantly smaller sample and reagent volumes, reduced surface contact and adsorption, as well as drop scalability, reconfigurability, and individual addressability. For these platforms to be effective, however, they require efficient schemes for planar drop transport and manipulation. While there are many methods that have been reported for drop transport, it is far more difficult to carry out other drop operations such as dispensing, merging and splitting. In this work, we introduce a novel alternative to merge and, more crucially, split drops using laterally-offset modulated surface acoustic waves (SAWs). The energy delivery into the drop is divided into two components: a small modulation amplitude excitation to initiate weak rotational flow within the drop followed by a short burst in energy to induce it to stretch. Upon removal of the SAW energy, capillary forces at the center of the elongated drop cause the liquid in this capillary bridge region to drain towards both ends of the drop, resulting in its collapse and therefore the splitting of the drop. This however occurs only below a critical Ohnesorge number, which is a balance between the viscous forces that retard the drainage and the sufficiently large capillary forces that cause the liquid bridge to pinch. We show the possibility of reliably splitting drops into two equal sized droplets with an average deviation in their volumes of only around 4% and no greater than 10%, which is comparable to the 7% and below splitting deviation obtained with electrowetting drop splitting techniques. In addition, we also show that it is possible to split the drop asymmetrically to controllably and reliably produce droplets of different volumes. Such potential as well as the flexibility in tuning the device to operate on drops of different sizes without requiring electrode reconfiguration, i.e., the use of different devices, as is required in electrowetting—therefore makes the present method an attractive alternative to electrowetting schemes.

Planar Microfluidic Drop Splitting and Merging

Abstract: Open droplet microfluidic platforms offer attractive alternatives to closed microchannel devices, including lower fabrication cost and complexity, significantly smaller sample and reagent volumes, reduced surface contact and adsorption, as well as drop scalability, reconfigurability, and individual addressability. For these platforms to be effective, however, they require efficient schemes for planar drop transport and manipulation. While there are many methods that have been reported for drop transport, it is far more difficult to carry out other drop operations such as dispensing, merging and splitting. In this work, we introduce a novel alternative to merge and, more crucially, split drops using laterally-offset modulated surface acoustic waves (SAWs). The energy delivery into the drop is divided into two components: a small modulation amplitude excitation to initiate weak rotational flow within the drop followed by a short burst in energy to induce it to stretch. Upon removal of the SAW energy, capillary forces at the center of the elongated drop cause the liquid in this capillary bridge region to drain towards both ends of the drop, resulting in its collapse and therefore the splitting of the drop. This however occurs only below a critical Ohnesorge number, which is a balance between the viscous forces that retard the drainage and the sufficiently large capillary forces that cause the liquid bridge to pinch. We show the possibility of reliably splitting drops into two equal sized droplets with an average deviation in their volumes of only around 4% and no greater than 10%, which is comparable to the 7% and below splitting deviation obtained with electrowetting drop splitting techniques. In addition, we also show that it is possible to split the drop asymmetrically to controllably and reliably produce droplets of different volumes. Such potential as well as the flexibility in tuning the device to operate on drops of different sizes without requiring electrode reconfiguration, i.e., the use of different devices, as is required in electrowetting—therefore makes the present method an attractive alternative to electrowetting schemes.

Simulations of surfactant effects on the dynamics of coalescing drops and bubbles

Abstract: We present simulations of coalescence in the presence of surfactant We consider a fluid-fluid interface where we track surfactant concentration Our model is applicable to a soap bubble merging with a suspended soap film and to a surfactant covered liquid drop merging with a reservoir In both cases, we determine the regime in which coalescence is only partial Along with viscous effects, represented by the Ohnesorge number, the elasticity of the surface tension relative to the surfactant concentration is seen to play a key role and exhibits a surprising nonmonotonic influence, for which we present a physical mechanism The effects of gravity are also simulated, along with effects of differing initial conditions, as well as those of uneven initial surfactant concentration, as are likely to arise in physical applications We describe how the presence of surfactants can influence a coalescence cascade

Thin-sheet flow between coalescing bubbles

Abstract: When two spherical bubbles touch, a hole is formed in the fluid sheet between them, and capillary pressure acting on its tightly curved edge drives an outward radial flow which widens the hole joining the bubbles. Recent images of the early stages of this process (Paulsen et al., Nat. Commun., vol. 5, 2014) show that the radius of the hole at time grows proportional to , and that the rate is dependent on the fluid viscosity. Here, we explain this behaviour in terms of similarity solutions to a third-order system of radial extensional-flow equations for the thickness and velocity of the sheet of fluid between the bubbles, and determine the growth rate as a function of the Ohnesorge number . The initially quadratic sheet profile allows the ratio of viscous and inertial effects to be independent of time. We show that the sheet is slender for if , where is the bubble radius, but only slender for if due to a compressional boundary layer of length , after which there is a change in the structure but not the speed of the retracting sheet. For , the detailed analysis justifies a simple momentum-balance argument, which gives the analytic prediction , where is the surface tension and is the density.

A Combined Experimental and Numerical Modeling Study of the Deformation and Rupture of Axisymmetric Liquid Bridges under Coaxial Stretching.

Abstract: The deformation and rupture of axisymmetric liquid bridges being stretched between two fully wetted coaxial disks are studied experimentally and theoretically. We numerically solve the time-dependent Navier-Stokes equations while tracking the deformation of the liquid-air interface using the arbitrary Lagrangian-Eulerian (ALE) moving mesh method to fully account for the effects of inertia and viscous forces on bridge dynamics. The effects of the stretching velocity, liquid properties, and liquid volume on the dynamics of liquid bridges are systematically investigated to provide direct experimental validation of our numerical model for stretching velocities as high as 3 m/s. The Ohnesorge number (Oh) of liquid bridges is a primary factor governing the dynamics of liquid bridge rupture, especially the dependence of the rupture distance on the stretching velocity. The rupture distance generally increases with the stretching velocity, far in excess of the static stability limit. For bridges with low Ohnesorge numbers, however, the rupture distance stay nearly constant or decreases with the stretching velocity within certain velocity windows due to the relative rupture position switching and the thread shape change. Our work provides an experimentally validated modeling approach and experimental data to help establish foundation for systematic further studies and applications of liquid bridges.

Lagrangian Approach for Simulating Supercooled Large Droplets’ Impingement Effect

Abstract: This document is the Accepted Manuscript version of the following article: C Wang, S Chang, and H Wu, "Lagrangian Approach for Simulating Supercooled Large Droplets’ Impingement Effect", Journal of Aircraft, Vol 52, No 2 (2015), pp 524-537 The Version of Record is available online at doi: https://doiorg/102514/1C032765

Accounting for Surface Concentrations Using a VOF Front Tracking Method in Multiphase Flow

Abstract: In this dissertation, we present a numerical method for trackingsurfactants on an interface in multiphase flow, along withapplications of the method to two physical problems. We alsopresent an extension of our method to track charged droplets. Ourmethod combines a traditional volume of fluid (VOF) method withmarker tracking. After describing this method in detail, wepresent a series of tests we used to validate our method. Theapplications we consider are the coalescence of surfactant-ladendrops, and the rising of surfactant-laden drops instratifications.In our study of the coalescence of surfactant-laden drops, wedescribe conditions under which coalescence is partial, ratherthan total. In particular, we examine the dependence of thecritical Ohnesorge number, above which coalescence is total, onsurfactant effects. We find that the surfactant potency has asurprising non-monotonic effect on the critical Ohnesorge number.This effect is explained by a balancing interface area loss andtangential stresses, which we describe using a scaling argument.Our argument is confirmed by forming a predicted criticalOhnesorge number profile, which qualitatively matches the data. Wealso discuss gravity effects, varying initial conditions, anddaughter drops resulting from partial coalescence.In our study of rising drops, we examine three distinct physicalsetups. In the first setup, we examine a drop coated in insolublesurfactant rising in a uniform ambient. Our results for anunstratified ambient show good agreement with earlier work, andfill a gap between results for zero Reynolds number andintermediate Reynolds number. In our second setup, we study dropsrising in a linear density stratification, with and withoutsurfactant. Entrainment effects on the rising drop are isolatedand used to compute an effective buoyancy of entrained fluid. Inour third setup, we present velocity profiles of a clean dropentering a layer of soluble surfactant. The surfactant layer``sucks'' the drop in, before it transitions to a terminal Risingspeed.Lastly, we extend our method to track electric fields and chargesin the bulk fluid and on the surface. Such a numerical method hasapplications to electrically induced drop deformation, thecoalescence of charged droplets, and electro-wetting. Thisextension of our method is validated by examining a simple testcase.

Effects of Fuel Physical Properties and Breakup Model Constants on Large-Eddy Simulation of Diesel Sprays

Abstract: The Kelvin-Helmholtz/Rayleigh-Taylor (KH-RT) wave breakup model is a commonly used model in predicting primary and secondary atomization and breakup processes in Lagrangian-Eulerian Diesel spray simulations. Droplet sizes predicted by this model are dependent on several parameters. The parameters include fuel physical properties, such as density, viscosity, and surface tension, and a number of adjustable model constants, such as KH and RT time constants, KH and RT size constants, and the breakup length constant. The purpose of this study is to investigate the effects of these parameters on predicting spray motions using large-eddy simulation with the dynamic structure sub-grid stress model. The code used in this study is OpenFOAM. This study has three major parts. Firstly, effects of the model constants on the prediction of momentum exchange process were examined by comparing liquid and gas momentum fluxes. Drag Forces exerted on liquid spray by gas phase can be determined from the slopes of gas and liquid momentum fluxes plotted against axial distance. We found that the prediction of momentum exchange between gas and liquid is most sensitive to the KH time constant, B1, among the other model constants. Secondly, effects of fuel physical properties were investigated by using four different fuels in the simulations of non-vaporizing and vaporizing sprays. The four fuels used were n-dodecane, F76 fuel, n-hexadecane, and methyl tetradecanoate. The F76 fuel is a multi-component fuel containing twenty-one hydrocarbons. Global spray quantities such as liquid and vapor penetrations, Sauter mean diameter, total liquid mass, number of parcels, and breakup model quantities such as Ohnesorge number and KH wave speed were compared. The key finding is that not all of these quantities monotonically increase or decrease with fuel molecular weight. Lastly, effects of fuel physical properties on sensitivities of the breakup model constants were studied. We compared liquid penetration and vapor penetration for each fuel using different values of the model constants. We found that the prediction of vapor penetrations is more sensitive to the KH time constant B1 when a fuel with lighter molecular weight was used, and the prediction of liquid penetrations is sensitive to the breakup length constant, Cb, in all of the four fuels. The computational investigations in this study reveal some limitations of the current spray breakup model, and motivate us to develop more advanced models to overcome these limitations.Copyright © 2015 by ASME

Numerical Investigation of Head-On Binary Collision of Alumina Droplets

Abstract: A numerical investigation of the head-on collision of two equal-sized alumina droplets has been conducted. Direct numerical simulations were performed by employing a volume of fluid method for tracking the interface and an adaptive mesh for improving the calculation efficiency. First, simulations of the head-on collision of tetradecane droplets in nitrogen were carried out, and three different outcomes of collision were captured: bouncing, coalescence after substantial deformation, and reflexive separation. These outcomes are in good agreement with the experiments. The numerical critical Weber numbers between the different regimes are also in good agreement with their experimental values. Next, the head-on collision of alumina droplets was investigated at various Weber numbers (1–800) and at one Ohnesorge number (0.1151). In addition to the three outcomes captured earlier, coalescence after a long extension between reflexive separation without satellite and reflexive separation with one satellite is obser...

Deformation and Breakup of an Axisymmetric Falling Drop under Constant Body Force

Abstract: The secondary breakup of liquid drops, accelerated by a constant body force, is examined for small density differences between the drops and the surrounding fluid. a density ratio of ten has been studied. We used Volume of Fluid (VOF) method to simulate the breakup. The breakup is controlled by the Eotvos number (Eo), the Ohnesorge number (Oh), and the viscosity and density ratios. If viscous effects are small (small Oh), the Eotvos number is the main controlling parameter. At a density ratio of ten, as Eo increases the drops break up in a backward facing bag, transient breakup, and a shear breakup mode. Similar breakup modes have been seen experimentally for much larger density ratios. Although a backward facing bag is seen at low Oh, where viscous effects are small, comparisons with simulations of inviscid flows show that the bag breakup is a viscous phenomenon, due to boundary layer separation and the formation of a wake. At higher Oh, where viscous effects modify the evolution, the simulations show that the main effect of increasing Oh is to move the boundary between the different breakup modes to higher Eo. The results are summarized by “breakup maps” where the different breakup modes are shown in the Eo–Oh plane for different values of the viscosity and the density ratios.

Instability of two-phase co-axial jets at small Reynolds number

Abstract: A precise knowledge of co-axial flow dynamics and a better understanding of the mechanisms that act to destabilize the interface between the two fluids is of fundamental interest in many industrial applications like lubricated transport, injection devices, atomization and controlled microdroplet production. The flow is in general unstable, since at least two mechanisms act to destabilize the cylindrical interface: shear and capillary instabilities. While these two mechanisms are active in a jet issuing from a tap, their respective influence strongly depends on the Reynolds number, the Ohnesorge number, but also on the viscosity, density, and aspect ratios. In this thesis, the global stability characteristics of two-phase co-axial flow are determined. The stability analysis follows two successive steps. First the steady base flow is determined, via the resolution of the nonlinear Navier-Stokes equations together with the location of the free interface. Second, these equations are linearized around the base flow and the dominant eigenmodes determined. The novelty lies in the formulation of models that can describe both the qualitative and quantitative characteristics of the two-phase flow configuration and the adaptation of the tools of global stability analysis for this configuration. We find that the dripping to jetting regime transition depends on the Capillary number, the degree of the confinement and the viscosity ratio, and we show that, surprisingly, the nozzle geometry does not affect the stability properties of the flow. Finally, the influence of surface viscosity on these coaxial flows has been considered. The governing and constitutive equations describing the continuum mechanics of the surface in the axisymmetric case are derived. With the addition of surface viscosity at the interface, the base flow evolves over a lengthscale which is much larger than the entry length in the Stokes regimes and than the typical unstable wavelength. We show that while the flow becomes eventually more convectively unstable once it reaches the fully developed profile, the surface viscosity creates an absolute region at the inlet, that is expected to promote droplet formation.

Study of Liquid Breakup Process in Solid Rocket Motor Nozzle

Abstract: : In a solid rocket motor (SRM), when the aluminum based propellant combusts, the fuel is oxidized into alumina (Al2O3). It tends to agglomerate into molten droplets, impinge on the chamber walls, and then flow along the nozzle wall. Such agglomerates cause erosivedamage. The focus of the current research is to characterize the agglomerate flow within the nozzle section by studying the breakup process of the liquid film that flows along the wall of a straight test channel while a relatively higher-speed gas moves over it. We have used anunsteady-flow Reynolds-Averaged Navier-Stokes code (URANS) to investigate the interaction of the liquid film flow with the gas flow. The rate of the wave breakup was characterized by introducing Breakup-length, Ohnesorge Number (Oh) and Weber Number (We) for various flow conditions. Based on the Volume Fraction (VF) of the liquid, these three numbers are indicators of a two-phase flow liquid breakup level. We could summarize that larger Breakup-length relates to a high break-up state of the liquid since the appearance of droplets contributes to a larger total boundary length during calculation. The Ohnesorge Number is the ratio of the viscous forces to the inertia and surface tension forces, where a large Oh number reduces breakup activity. The Weber Number indicates the ratio of the inertial force to the surface tension force, and a higher Weber Number relates to higher breakup of the two-phase flow.

Formation and Breakup Patterns of Falling Droplets

Abstract: Some interface front patterns of falling droplets are studied via direct numerical solution of the full Navier–Stokes equations governing the system of droplets and the ambient surrounding media as a single-fluid model. We focus on the mutual interactions of the effects of characterizing nondimensional parameters on the formation and break-up of large cylindrical droplets. The investigation of droplet cross sections and deformation angles shows that for moderate values of the Atwood number, increasing the Eotvos number explicitly increases the deformation rate in formation and breakup phenomena. Otherwise, increasing the Ohnesorge number basically amplifies the viscous effects.

Log-normal diameter distribution of Pd-based metallic glass droplet and wire

Abstract: We have studied the formation of Pd42.5Cu30Ni7.5P20 metallic glass droplets and wires in the gas atomization process. We demonstrate that the sizes of droplets and wires can be distinguished by the Ohnesorge number (Oh), which is the proportion of the spinnability to the capillary instability and the diameter distributions follow a log-normal distribution function, implying cascade fragmentation. For droplets, the number significantly increases at Oh 1 while the diameter steadies below 400 nm. Further, the wire diameter is quadrupled at Oh = 16 due to the high viscosity which suppresses both capillary breakup and ligament elongation.

2D Lattice Boltzmann Simulation of Droplet Jumping in a Viscous Fluid

Abstract: In order to investigate the coalescence and out-of-plane jumping of two incompressible droplets on a non-wetting surface surrounded by an incompressible fluid with matched viscosity in the low Ohnesorge number regime, a two-dimensional lattice Boltzmann phase-field model is implemented. An interfacial force of potential form is used to model the internal surface tension force and capture the fluid-surface interaction, viz. the contact-line dynamics. We evaluate the simulated velocity fields and interface shape evolution during coalescence and the subsequent jumping event. We confirm that the coalescence dynamics of the binary droplet system is similar to the case where the outer fluid viscosity is small compared to that of the droplet fluid, as is the case of condensed water droplet jumping on superhydrophobic surfaces in a gaseous ambient. An argument is also developed to demonstrate that the dynamics in 2D, when appropriately scaled, should be approximately equivalent to the corresponding 3D case. A simple drag model is used to capture the rapid velocity decay of the jumping droplet as it moves away from the surface into the viscous fluid. The results suggest the possibility of experimentally observing coalescence-induced droplet jumping in liquid-liquid systems that may be potentially exploited for microfluidic applications.Copyright © 2015 by ASME