Understanding the structure of the turbulent mixing layer in hydrodynamic instabilities

When a heavy fluid is placed above a light fluid, tiny vertical perturbations in the interface create a characteristic structure of rising bubbles and falling spikes known as Rayleigh-Taylor instability. Rayleigh-Taylor instabilities have received much attention over the past half-century because of their importance in understanding many natural and man-made phenomena, ranging from the rate of formation of heavy elements in supernovae to the design of capsules for Inertial Confinement Fusion. We present a new approach to analyze Rayleigh-Taylor instabilities in which we extract a hierarchical segmentation of the mixing envelope surface to identify bubbles and analyze analogous segmentations of fields on the original interface plane. We compute meaningful statistical information that reveals the evolution of topological features and corroborates the observations made by scientists. We also use geometric tracking to follow the evolution of single bubbles and highlight merge/split events leading to the formation of the large and complex structures characteristic of the later stages. In particular we (i) Provide a formal definition of a bubble; (ii) Segment the envelope surface to identify bubbles; (iii) Provide a multi-scale analysis technique to produce statistical measures of bubble growth; (iv) Correlate bubble measurements with analysis of fields on the interface plane; (v) Track the evolution of individual bubbles over time. Our approach is based on the rigorous mathematical foundations of Morse theory and can be applied to a more general class of applications.     

High-current density DC magenetohydrodynamics micropump with bubble isolation and release system

One of the major challenges for integrated Lab-on-a-Chip (LOC) systems is the precise control of fluid flow in a micro-flow cell. Magnetohydrodynamics (MHD) micropumps which contain no moving parts and capable of generating a continuous flow in any ionic fluid offer an ideal solution for biological applications. MHD micropumping has been demonstrated by using both AC and direct current (DC) currents by a number of researchers with varying degrees of success. However, current MHD designs based on DC do not meet the flow rate requirements for fully automated LOC applications (>100 μl/min). In this research, we introduce a novel DC-based MHD micropump which effectively increases flow rate by limiting the effects of electrolysis generated bubbles at the electrode–electrolyte interface through isolation and a mechanism for their release. Gas bubbles, particularly, hydrogen generated by high current density at the electrodes are the main culprit in low experimental flow rate compared with theoretical values. These tiny bubbles coalesce in the flow channel thus obstructing the flow of fluid. Since hydrolysis is inevitable with DC excitation, compartmentalized electrode channels with bubble isolating and coalescence retarding mechanisms and bubble release systems are implemented to prevent the coalescence of these bubbles and minimize their effects on flow rate. In this novel design called bubble isolation and release system, flow rate of up to 325 μl/min is achieved using 1 M NaCl solution in DC mode with potentials of 5 V and current density of about 5,000 A/m² for a main channel of 800 μm × 800 μm cross-section and 6.4 mm length.     

不同起始形貌气泡在磁性液体中上升的实验与数值分析

研究了气泡在磁性液体中的上升过程.用VOF方法追踪了气泡的自由界面,并预测了气泡的变形.考虑了磁场强度对不同起始形貌的气泡的影响,并且发现气泡的最终形貌受磁场强度的影响,当磁场强度较弱时,动力学压力占主导,使气泡变扁,当磁场强度强时,磁性压力占主导,使气泡变圆.当磁场较弱时,起始形貌对气泡的最终形态有影响.     

Bubble coalescence at a microfluidic T-junction convergence: from colliding to squeezing

This paper aims at studying the coalescence of bubbles at a microfluidic T-junction convergence by using a high-speed digital camera and the VOF simulation. The microfluidic channels have uniform square cross-section with 400 μm wide and 400 μm deep. The responses of bubble collisions at the T-junction convergence have been investigated within a wide range of dimensionless bubble size and capillary number Ca. Colliding coalescence, squeezing coalescence, and non-coalescence were observed at the junction. The result showed that whatever for colliding coalescence or squeezing coalescence, the coalescence efficiency decreases with the increase in the two-phase superficial velocity for moderate liquid viscosities, and the transition from colliding to squeezing coalescence due to the increase in the two-phase superficial velocity enhances the coalescence of bubbles. The decrease in the bubble size for moderate liquid viscosities and the increase in the liquid viscosity are not conducive to bubble coalescence.     

Particle‐based simulation of bubbles in water–solid interaction

In this paper, a particle‐based multiphase method for creating realistic animations of bubbles in water–solid interaction is presented. To generate bubbles from gas dissolved in the water on the fly, we propose an approximate model for the creation of bubbles, which takes into account the influence of gas concentration in the water, the solid material, and water–solid velocity difference. As the air particle on the bubble surface is treated as a virtual nucleation site, the bubble absorbs air from surrounding water and grows. The density and pressure forces of air bubbles are computed separately using smoothed particle hydrodynamics; then, the two‐way coupling of bubbles with water and solid is solved by a new drag force, so the generated bubbles’ flow on the surface of solid and the deformation in the rising process can be simulated. Additionally, touching bubbles merge together under the cohesion forces weighted by the smoothing kernel and velocity difference. The experimental results show that this method is capable of simulating bubbles in water–solid interaction under different physical conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Laser-generated liquid microjets: correlation between bubble dynamics and liquid ejection

Laser-based techniques provide excellent means for liquid microprinting, with several advantages over other more conventional printing techniques, such as being nozzle-free (as opposed to inkjet, for instance) or requiring minimal engineering of the liquid properties in the pre-printing stage. In such techniques, the transfer is usually mediated by liquid jets that contact a receiver substrate placed nearby the liquid source, leading to the deposition of a small droplet. The main cause of jetting lies in a laser-generated bubble produced inside the liquid, whose dynamics dictates the evolution of liquid ejection. However, the detailed relationship between the bubble and the jet is not completely understood, as the studies carried out so far have been mostly focused on the jetting dynamics taking place above the liquid free-surface, without access to the liquid interior and therefore to the behavior of the bubble. In this work, we analyze through time-resolved imaging the film-free laser printing of an aqueous solution by simultaneously visualizing both the bubble evolution and the liquid ejection dynamics, thus making possible the correlation between the two phenomena. We find that the pulsating behavior of the bubble leads to successive jetting events with different jet morphologies arising from the particular geometries that the bubble acquires during its evolution. Finally, we find good agreement between our results and those from studies analyzing the dynamics of cavitation bubbles near the free-surface of a liquid through numerical solution of the fluid dynamics equations.     

Experimental and computational study of gas bubble removal in a microfluidic system using nanofibrous membranes

This paper presents a simple and efficient method for removing gas bubbles from a microfluidic system. This bubble removal system uses a T-junction configuration to generate gas bubbles within a water-filled microchannel. The generated bubbles are then transported to a bubble removal region and vented through a hydrophobic nanofibrous membrane. Four different hydrophobic Polytetrafluorethylene membranes with different pore sizes ranging from 0.45 to 3 μm are tested to study the effect of membrane structure on the system performance. The fluidic channel width is 500 μm and channel height ranges from 100 to 300 μm. Additionally, a 3D computational fluid dynamics model is developed to simulate the bubble generation and its removal from a microfluidic system. Computational results are found to be in a good agreement with the experimental data. The effects of various geometrical and flow parameters on bubble removal capability of the system are studied. Furthermore, gas–liquid two-phase flow behaviors for both the complete and partial bubble removal cases are thoroughly investigated. The results indicate that the gas bubble removal rate increases with increasing the pore size and channel height but decreases with increasing the liquid flow rate.

     

Numerical study of seed bubble-triggered evaporation heat transfer in a single microtube

High boiling incipience temperature and flow instabilities in silicon-based microchannels with smooth surface are challenging issues. This work numerically investigates the seed bubble-triggered evaporation heat transfer in a microtube, with a length of 5.0 mm and diameter of 106 μm. Acetone was the working fluid. Seed bubbles were assumed to be generated periodically at the microtube upstream. The fixed grid allocation technique was proposed to successfully perform the parallel computation via a set of computer core solvers. It is found that the seed bubble-guided heat transfer consists of a start-up stage and a steady operation stage. The start-up time equals to the residence time of the first seed bubble growing and traveling in the microtube. The seed bubble frequency is a key parameter to influence the performance. Low-frequency seed bubbles cause alternative flow patterns of liquid flow and elongated bubble flow, corresponding to the apparent spatial-time oscillations of wall and bulk fluid superheats. High-frequency seed bubbles result in quasi-stable elongated bubble flow, corresponding to quasi-uniform and stable wall and fluid superheats. There is a saturation seed bubble frequency beyond which no further performance improvement can be made. There are residual fluid superheats specifying the required minimum superheats to sustain the evaporation heat transfer between the two phases. Elongated bubbles with thin liquid films are responsible for the heat transfer enhancement. Contrary to wall temperatures, the transient local Nusselt numbers are slightly changed due to the fact that heat transfer is more closely related to the dynamic elongated bubble flow evolution within millisecond timescale in the microchannel. The heat transfer coefficients can be 2.0 to 3.5 times of that for the superheated liquid flow before seed bubble injections.     

The merging of two gaseous bubbles with an application to underwater explosions

A numerical study of two gaseous bubbles merging into a single coalesced bubble as in underwater explosions is investigated in this paper. This explosive phenomenon is modeled using a boundary integral method. Two configurations, which are in-phase and out-of-phase explosions, are simulated and compared with available experimental results. The thickness of the liquid film between the two bubbles determines our coalescence criterion. Bubble shapes and periods of oscillation are predicted well, compared to those of the experiments.     

Visualizing Probabilistic Multi‐Phase Fluid Simulation Data using a Sampling Approach

Eulerian Method of Moment (MoM) solvers are gaining popularity for multi‐phase CFD simulation involving bubbles or droplets in process engineering. Because the actual positions of bubbles are uncertain, the spatial distribution of bubbles is described by scalar fields of moments, which can be interpreted as probability density functions. Visualizing these simulation results and comparing them to physical experiments is challenging, because neither the shape nor the distribution of bubbles described by the moments lend themselves to visual interpretation. In this work, we describe a visualization approach that provides explicit instances of the bubble distribution and produces bubble geometry based on local flow properties. To facilitate animation, the instancing of the bubble distribution provides coherence over time by advancing bubbles between time steps and updating the distribution. Our approach provides an intuitive visualization and enables direct visual comparison of simulation results to physical experiments.     

Use of orifice-in-throat device to make alginate scaffolds with embedded voids of sub-millimeter tunable dimensions

During the co-flow of a gas and a surrounding liquid film, the inner thread of gas breaks due to Rayleigh instability, and produces a series of bubbles in the embedding liquid. This article describes a co-flow arrangement that generates bubbles in the alginate solution to form a hydrogel scaffold. The flow arrangement utilized an “orifice in throat” configuration for a second squeeze on the bubble that resulted in further split into the bubbles of smaller size. The images under a microscope demonstrate the self-alignment of bubbles, without coalescence, or shrinkage. Application of the CaCl₂ solution as cross-linker on the scaffold resulted in the formation of a robust free-resting gel film with the embedded voids. The gelation took place within minutes. The gel film did not stick to the glass plate. The scaffold was dried under vacuum, and was imaged using a scanning electron microscope. The voids retained its alignments after the shrinkage.     

Numerical simulation of coalescence and breakup of rising droplets

Rising droplets with coalescence and breakup are numerically simulated using the lattice BGK method. It is shown for single droplet that the rising velocities are in good agreement with those obtained by two types of empirical correlations. The coalescence of droplets is shown when two droplets are placed vertically in line at different elevations. The breakup of droplet is observed in some cases after the coalescence. It is found that the breakup of coalesced droplet occurs when the Weber number at the coalescence exceeds a critical value, and the critical Weber number agrees well with that given by the empirical correlation.     

Applications of textured surfaces on bubble trapping and degassing for microfluidic devices

This paper introduces a passive degassing mechanism using textured surfaces to trap and transport bubbles, and then using hydrophobic porous membranes to vent out bubbles in a microfluidic system. The bubble trapping ability is achieved by creating nanostructures to promote bubble nucleation and coalescence on the sidewalls of KOH-etched concave pits in a silicon substrate. The substrate, which is bonded with a porous membrane, is placed in a liquid system with chemically generated CO₂ bubbles to examine the degassing ability. The results validate that the bubbles can be easily trapped on the surfaces with nanostructures, and then vented through the porous membrane. Our proposed approach possesses the advantage of simple fabrication, great structure robustness, and effective bubble trapping and removing abilities, which show their great potential as economic, passive means of preventing the gas byproducts from blocking surfaces and improving the efficiency of microfluidic systems during operations.     

Gas�Cliquid flow stability and bubble formation in non-Newtonian fluids in microfluidic flow-focusing devices

This communication describes the gas–liquid two-phase flow patterns and the formation of bubbles in non-Newtonian fluids in microfluidic flow-focusing devices. Experiments were conducted in two different polymethyl methacrylate (PMMA) square microchannels of, respectively, 600 × 600 and 400 × 400 μm. N₂ bubbles were generated in non-Newtonian polyacrylamide (PAAm) solutions of different concentrations. Slug bubble, missile bubble, annular and intermittent flow patterns were observed at the cross-junction by varying gas and liquid flow rates. Gas and liquid flow rates, concentration of PAAm solutions, and channel size were varied to investigate their effect on the mechanism of bubble formation. The bubble size was proportional to the ratio of gas/liquid flow rate for slug bubbles and could be scaled with the ratio of gas/liquid flow rate as a power–law relationship for missile bubbles under wide experimental conditions.     

Simulation of bubbles

We present a novel framework based on a continuous fluid simulator for general simulation of realistic bubbles, with which we can handle as many significant dynamic bubble effects as possible. To capture a very thin liquid film of bubbles, we have developed a regional level set method allowing multi-manifold interface tracking. Based on the definitions of regional distance and its five operators, the implementation of the regional level set method is very easy. An implicit surface of liquid film with arbitrary thickness can be reconstructed from the regional level set function. To overcome the numerical instability problem, we exploit a new semi-implicit surface tension model which is unconditionally stable and makes the simulation of surface tension dominated phenomena much more efficient. An approximated film thickness evolution model is proposed to control the bubble’s lifecycle. All these new techniques combine into a general framework that can produce various realistic dynamic effects of bubbles.     

DNS for flow separation control around an airfoil by pulsed jets

Direct numerical simulation (DNS) for flow separation and transition around a NACA-0012 airfoil with an attack angle of 4° and Reynolds number of 100,000 has been reported in our previous paper. The details of flow separation, formation of the detached shear layer, Kelvin-Helmholtz instability (inviscid shear layer instability) and vortex shedding, interaction of nonlinear waves, breakdown, and re-attachment are obtained and analyzed. The power spectral density of pressure shows the low frequency of vortex shedding caused by the Kelvin-Helmholtz instability still dominates from the leading edge to trailing edge. Based on our understanding on the flow separation mechanism, we try to reveal the mechanism of the flow separation control using blowing jets and then optimize the jets. DNS simulations for flow separation control by blowing jets (pulsed and pitched and skewed jets) are reported and analyzed. The effects of different unsteady blowing jets on the surface at the location just before the separation points are studied. The length of separation bubble is significantly reduced (almost removed) after unsteady blowing technology is applied. The mechanism of early transition caused by the blowing jets was found. A blowing jet with K-H frequency, sharp shape function (very small mass blowing), pitching and skewing obtained the best efficiency based on the increase of the ratio of lift over drag and decrease of blowing mass flow. In this work, a DNS code with high-order accuracy and high-resolution developed by the computational fluid dynamics group at University of Texas at Arlington is applied.     

Heat and Fluid Flow in an Optical Switch Bubble

The Agilent all-optical bubble switch uses bubbles in an organic fluid index matched to a silica planar lightwave circuit. The bubble is created and sustained by heaters that are deposited on an attached silicon substrate. Testing of the bubble shows how heater power and ambient pressure affect bubble shape, size, and optical reflection characteristics. Heat and fluid flow in the bubble were modeled in 2D and 3D using the homogeneous bubble model in the Flow3D modeling software. Fluid condensing on the trench wall causes a dimple on the bubble and hence nonoptimum optical reflection. To aid understanding, the bubble, silica walls, and heaters were also modeled as a thermal resistance network. Because the pressure drop across the bubble wall is fixed, the bubble size is determined by P_res/DeltaT_t , where P_res is the heater power and DeltaT_t is the temperature difference between the bubble and the substrate. Heating the trench walls beyond the bubble temperature with heaters located underneath the trench wall will dry out the trench wall and give a stable optical reflection. As DeltaT_t approaches zero, a bubble is sustained without any heater power and with zero fluid flow. This "static" bubble provides for a very stable optical reflection     

A lattice Boltzmann method for immiscible two-phase Stokes flow with a local collision operator

We propose a lattice Boltzmann model for immiscible two-phase Stokes flow with a local collision operator. The model is based on two different lattice Boltzmann automata, one for the flow field and one for an indicator function for the two different phases. The model is described in detail and verified by the following test-cases: a static bubble for the surface tension, a closed capillary tube for the contact angle and two phase flow in a concentric annulus for the viscosity ratio. In the appendix an asymptotic analysis for the derivation of the two-phase Stokes equation is given.     

Numerical Simulation of growth and collapse of a bubble induced by a pulsed microheater

A three-dimensional numerical analysis of the growth and collapse of a bubble on a microheater is presented. SIMULENT code, which solves the full Navier-Stokes equations with surface tension effects, is used in these simulations. A volume of fluid (VOF) interface tracking algorithm is used to track the evolution of the free surface flow. A one-dimensional heat conduction model is used to consider the energy transfer between the bubble and the surrounding liquid, as well as the temperature distribution in the liquid layer. Details of the velocity and pressure distribution in the liquid during the growth and collapse of the vapor bubble are obtained. Numerical results for the growth and the collapse of the bubbles are compared with those of experiments under similar conditions. Comparisons show that the volume evolution of the vapor bubble is well predicted by the numerical model.     

Numerical investigation of formation and dissolution of CO<Subscript>2</Subscript> bubbles within silicone oil in a cross-junction microchannel

In this paper, a numerical investigation is conducted to study the formation and dissolution process of CO₂ bubbles within silicone oil in a cross-junction microchannel. A coupled multiphase–multicomponent computational fluid dynamics model based on the volume-of-fluid method is used, which is able to capture the physics of the multiphase bubble formation, dissolution mass transfer, and the tracking of the dissolved CO₂ species. The computational model is firstly validated with experimental results where good agreement is attained. Next, the model is used to investigate the bubble formation process at the cross-junction in the presence of dissolution and also the bubble evolution as it is transported along the downstream channel. It is revealed that during bubble formation, there is a high concentration of CO₂ solute around the cross-junction walls, as silicone oil flow to this region is minimal. As the CO₂ bubble travels downstream, the transport of the CO₂ solute is largely driven by the local flow currents of the silicone oil within the vicinity of the bubble. An extensive parametric study is also conducted, looking at the effects of varying the surface tension, diffusion coefficient and flow rates. The results demonstrate that the initial CO₂ bubble length and period of bubble formation are most affected by the flow rate, while the mass transfer is most strongly governed by the diffusion coefficient.