Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H37: General Fluid Dynamics and Phenomena |
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Sponsoring Units: DFD Chair: Gary Williams, UCLA Room: LACC 512 |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H37.00001: Spectrum and Dynamics of Luminescence from Laser-created Bubbles in Pressurized Water Frank Lee, David Hecht, Emil A. Brujan, Gary A. Williams The properties of the luminescence pulse from laser-created bubbles in pressurized water are studied for pressures between 0.25 and 15 bars. The duration of the light pulse is linear in the maximum bubble size, but for a given bubble size it increases with pressure. The spectrum of the light is blackbody in form, with a temperature that increases somewhat with pressure, from 8100 K at 1 bar to 9400 K at 10 bars. At higher pressures the blackbody temperature drops, but this is primarily due to the rapid onset above 10 bars of a fission instability, where the bubbles split into two just before the collapse point. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H37.00002: Luminescence Spectra of Laser-induced Cavitation Bubbles Emil A. Brujan, Gary A. Williams The luminescence spectra of laser-induced cavitation bubbles near rigid boundaries are measured for various relative distances between the bubble and the boundaries. We find that the luminescence spectra of bubbles collapsing near a single boundary consist of only a blackbody continuum. Luminescence from bubbles collapsing between two parallel rigid boundaries also contains OH* emission bands similar to those found in multi-bubble sonoluminescence. In both cases, the bubble interior temperature deduced from blackbody fits decreases with the distance between bubble and boundary. The shape instabilities of the collapse near a boundary and the consequent presence of high-velocity jets inside the bubble at its minimum volume will be discussed in connection with the generation of the OH* radicals. [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H37.00003: Leidenfrost Effect of Water Drops on the Surface of Liquid Nitrogen Heetae Kim, Gary A. Williams The properties of small water drops floating on the surface of liquid nitrogen are studied, a type of Leidenfrost effect. Heat extracted from the water drop evaporates the liquid nitrogen under it, forming a gaseous dimple that the drop floats on, spinning and moving with constant speed across the nitrogen. The temperature of the drop falls with time, finally reaching a Leidenfrost temperature below 0\r{}C where the ice crystal sinks into the nitrogen. We have measured the time that the drop floats as a function of the drop size and the initial temperature of the water. The time increases linearly with both the drop size and with temperature, in agreement with a simplified force-balance theory including thermal conduction and the vapor and surface tension forces. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H37.00004: Liquid Drop Growth On A Fiber In Fibrous Filters S.V. Doiphode, A.L. Yarin, W. Liu, G.G. Chase, D.H. Reneker This paper describes and verifies a quantitative model to predict the growth by coalescence of oil droplets on a single fiber. The model considered a stream of fluid carrying many tiny droplets of a different fluid. Different capture mechanisms by which the tiny droplets are captured by a drop growing on a fiber were examined, including: interception, Brownian motion of droplets and vapor deposition by diffusion. Number average distribution and volume average distribution of particle diameters were used to characterize the tiny droplets. The comparison, of the predictions for different droplet capture mechanisms, with the experimental data showed that both droplet interception and Brownian diffusion contribute to drop growth on the fiber for droplets in the size range from 1 to 1000 nm. Brownian motion was found to be the dominant mechanism of the two. Merging of growing drops on the fiber was also modeled and experimentally observed. The experimental merging time for drops was close to that predicted. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H37.00005: Coalescence of liquid drops without singularities Stephen Decent It is now known that the usual equations for an incompressible viscous free-surface flow (conservation of mass, Navier-Stokes, normal and tangential stress boundary conditions, kinematic condition) give rise to a singularity when the equations are used to model the coalescence of two or more liquid drops in a gas if the surface tension of the free-surfaces is assumed to be constant. This singularity arises at the location and moment of impact of the liquid free-surfaces. This singularity will cause the solution to be unrealistic very close to the impact point and at times very soon after the moment of impact. Allowing the surface tension to be dynamic removes this unphysical singularity (Shikhmurzaev, Physics of Fluids, vol. 12, 2386-2396; October 2000). New computational results demonstrate the removal of the singularity from the model's solution and show how the flow close to the impact point is altered by dynamic surface tension. The results are discussed in the context of coalescing micron sized drops where these new effects are thought to be particularly important. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H37.00006: The Hydrodynamic Coupling of Two Rotating Micro-objects Liangyu Zhou, Yafei Wang, Tao Zhang, Gang Hu We applied emulsification techniques to encapsulate a micro- sized, optically anisotropic particle inside a liquid droplet, which is in turn freely suspended in another fluid. Using optical tweezers, we have demonstrated the trapping and rotating of the internal particle by radiation force and torque. The particle-filled liquid droplet can execute a constant rotation motion due to hydrodynamic transport from the rotating particle, and the internal flow is also coupled out of the liquid droplet. In addition, the suspended liquid droplet may show a complex rotational motion depending on the configurations of the two rotating objects even though the fluid flow is in a low Reynolds number regime. We measured the ratio of rotational speeds versus the size ratio of two rotating objects and studied experimentally the stability of the droplet rotation when the relative position of the trapped particle is varied. Further theoretical and numerical work is needed to fully understand the effects of hydrodynamic interaction. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H37.00007: Molecular Dynamics Simulations of Two-Phase Nanojets and Nanobridges Wei Kang, W.D. Luedtke, Uzi Landman The effects of the ambient environment on the properties of a nanojet (NJ) injected into it are investigated using molecular dynamics (MD) simulations. These studies reveal that the ambient gas slows the propagation of the NJ. However, the intact-length of the NJ and the dynamics of droplet formation remain similar to the case of a nanojet propagating in vacuum. Separate MD simulations of a liquid nanobridge verify the exponent occurring in the description of the pinching dynamics of nanojets [1], predicted through analysis of the stochastic lubrication equations derived in ref. [2]. \\ $[1]$ J. Eggers, Phys. Rev. Lett. 89, 084502 (2002) \\ $[2]$ M. Moseler and U. Landman, Science 289, 1165 (2000)\\ [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H37.00008: Selective Withdrawal with an Inverted Viscosity Ratio Sarah Case, Sidney Nagel In the selective withdrawal experiment, fluid is withdrawn, at rate Q, through a tube with its tip suspended a distance S above an unperturbed interface separating two immiscible fluids. For high Q, the lower fluid is entrained along with the upper one while for low Q only the upper fluid is withdrawn. We have studied the situation where the ratio of lower to the upper fluid viscosities, $\eta >1$. For low Q, the interface forms a steady-state hump and only the upper fluid is withdrawn. When Q is increased, or S is decreased, the interface undergoes a two-stage transition: first the hump forms an unsteady, thin spout which then expands into a second thicker steady-state structure with distinct flow patterns in the lower fluid. This thick-spout structure is not observed for $\eta < 1$. Near the hump to thin-spout transition, the hump curvature increases with power-law scaling similar to that seen for $\eta < 1$. [1] If Q is decreased from the thick spout regime, a steady-state thin spout can also be created within a limited hysteretic region. [1] I. Cohen and S. R. Nagel, Phys. Rev. Lett. 88, 074501 – 1- 4 (2002). [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H37.00009: Electro-viscous effect on Heat Transfer in Ionic Fluid Flow in Microchannels Abhishek Jain Electro-viscous effect, generated due to electrolytic or ionic flow plays a critical role in enhancing the heat transfer in microchannels. Fundamental understanding of electrokinetic behavior and its effect on heat transfer is important for the design and control of microfluidics and lab-on-chip devices. In the present paper an analytic modeling of electrolytic flow in parallel plate microchannel is done. The electrolyte considered is KCl. The Navier-Stokes equations have been modified to take into account the electro-viscous effect. The effect of electro-viscous phenomenon on the boundary layer development is discussed using analytical solutions. A rapid formation of the boundary layer indicates the existence of electro-viscous effect. The expressions for velocity profile in developing and the developed flow regimes were derived and compared to those with and without the electro-viscous effect. The equations for fully developed friction factor and Nusselt number for constant wall heat flux condition have been developed and the results are interpreted in the physical domain. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H37.00010: Propagation velocities of chemical reaction fronts advected by Poiseuille flow Boyd Edwards Ascending chemical wave fronts place a reacted fluid mixture below a more-dense unreacted fluid mixture, and are therefore potentially unstable to buoyancy-driven convection. Indeed, ascending fronts bounded by parallel vertical no-slip plates are unstable to buoyancy-driven convection above a critical gap width. Imposed finite-amplitude Poiseuille flows distort the shapes of such reaction fronts and alter their velocities of propagation. We have investigated these effects using the Navier-Stokes equations and the appropriate cubic reaction-diffusion-advection equation. Analytical solutions of this equation resolve the chemical concentration across the gap for narrow gaps, wide gaps, and small-amplitude flow. Numerical solutions supply a general description for fluid flow in the direction of propagation of the chemical reaction front, and for flow in the opposite direction. In contrast with previous assumptions, the propagation velocity is found to exceed the sum of the velocity of a planar front in a static fluid and the average flow velocity. General velocity results preclude the need for the reaction-diffusion-advection equation in future studies of nonlinear fingering. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H37.00011: Mesoscale modelling of fluctuating fluids: colloids, active particles and polymers in flow Thomas Ihle, Daniel Kroll, Erkan Tuzel, Martin Hecht A recently introduced stochastic model for fluid flow, called Stochastic Rotation Dynamics (SRD), is a promising tool for the coarse-grained modelling of a fluctuating solvent, particularly for colloidal and polymer suspensions. The solvent is represented by ''fluid''-particles which follow a simple dynamics and undergo efficient multiparticle collisions. SRD can be thought of a ''hydrodynamic heat bath'' which provides the correct hydrodynamic interactions between embedded particles. It is shown how exact analytical expressions for the transport coefficents of this model can be derived by means of discrete Green-Kubo relations. We will discuss several extensions of the original model such as obtaining a non-ideal equation of state. Colloids and polymers can be coupled to this fluid and are treated by Molecular Dynamics. Results for the sedimentation of colloids and the pattern formation of active particles and polymers in flow will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H37.00012: Nonlinear Analysis of Dewetting of a Two-Layer Thin Liquid Film Lael Fisher, Alexander Golovin Nonlinear analysis of dewetting of a two-layer thin liquid film on a solid substrate is performed. It is shown that, due to the coupling of the van der Waals forces in the two liquid layers the film can undergo either rupture or spinodal decomposition into drops divided by wetting layers, thus exhibiting an autophobic behavior. A system of coupled nonlinear evolution equations describing the shape of the two interfaces in the lubrication approximation is analysed and solved numerically. Numerical solutions confirm the possibility of an autophobic behavior. The effect of surfactant adsorption at the interface between the liquid layers is also analysed. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H37.00013: Jet Simulation in a Diesel Engine Zhiliang Xu We present a numerical study of the jet breakup and spray formation in a diesel engine by the Front Tracking method. The mechanisms of jet breakup and spray formation of a high speed diesel jet injected through a circular nozzle are the key to design a fuel efficient, nonpolluting diesel engine. We conduct the simulations for the jet breakup within a 2D axis-symmetric geometry. Our goal is to model the spray at a micro-physical level, with the creation of individual droplets. The problem is multiscale. The droplets are a few microns in size. The nozzle is about 0.2 mm in diameter and 1 mm in length. To resolve various physical patterns such as vortex, shock waves, vacuum and track droplets and spray, the Burger-Colella adaptive mesh refinement technique is used. To simulate the spray formation, we model mixed vapor-liquid region through a heterogeneous model with dynamic vapor bubble insertion. The formation of the cavitation is represented by the dynamic creation of vapor bubbles. On the liquid/vapor interface, a phase transition problem is solved numerically. The phase transition is governed by the compressible Euler equations with heat diffusion. Our solution is a new description for the Riemann problem associated with a phase transition in a fully compressible fluid. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H37.00014: Hydrodynamics of a cold one-dimensional fluid: the problem of strong shock waves Pablo I. Hurtado, Paul L. Krapivsky We study a shock wave induced by an infinitely massive piston propagating into a one-dimensional cold gas. The cold gas is modelled as a collection of hard rods which are initially at rest, so the temperature is zero. Most of our results are based on simulations of a gas of rods with binary mass distribution, and we partcularly focus on the case of spatially alternating masses. We find that the properties of the resulting shock wave are in striking contrast with those predicted by hydrodynamic and kinetic approaches, e.g., the flow-field profiles relax algebraically toward their equilibrium values. In addition, most relevant observables characterizing local thermodynamic equilibrium and equipartition decay as a power law of the distance to the shock layer. The exponents of these power laws depend non-monotonously on the mass ratio. Similar interesting dependences on the mass ratio also characterize the shock width, density and temperature overshoots, etc. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H37.00015: Lattice-Boltzmann Models of Ion Thruster Cathode 3D MHD Flows Jacques Richard, Prerit Shah The lattice-Boltzmann method (LBM) has been applied to modeling the flow through ion thruster optics where a linearized Boltzmann equation for a lattice is coupled to Poisson's equation for the electrostatics. LBM has also been implemented in modeling three-dimensional (3D) magneto-hydrodynamics (MHD) wherein the magnetic field is represented by a separate three-component vector distribution function corresponding to a vector kinetic equation. Discretization of the 3D phase space is based on a 19-bit scheme for the fluid model and on a 7-bit scheme for the magnetic field versus finite differencing of all of Maxwell equations. Issues that affect ion thruster operation, like the flow about the cathode assembly that reduce cathode and hence engine life, are investigated with this model. Historically, the transport of mass, momentum, energy, sub-atomic particles, etc. and the complex multi-scale physics involved in ion thrusters had been modeled mostly using Bird's Direct Simulation Monte Carlo (DSMC). While DSMC has achieved great success in EP models, their connection to the Boltzmann equation for the molecular velocity distribution function suggests alternate approaches based more directly on that equation. [Preview Abstract] |
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