Turbulent Coagulation of Aerosol Particles: New Insights From Direct Numerical Simulations and Holographic Imaging Experiments

Particle collisions driven by turbulent fluctuations play a key role in such diverse problems as cloud formation, aerosol powder manufacturing and inhalation drug therapy to name a few. In all of these examples (and many others) turbulent fluctuations increase the rate of collisions relative to the background collision rate driven by Brownian motion. Furthermore, turbulence can spontaneously generate very large fluctuations in the particle concentration field. This “clustering” is caused by the inertial mismatch between the heavy particles and the lighter surrounding gas; vortices in the flow “centrifuge” the heavier particles out of vortex cores and into the straining regions that lie in between the vortices. Because collision is a binary process, concentration fluctuations further enhance the turbulent coagulation rate by as much as two orders of magnitude. An effect of this size must be accounted for in a rational model of turbulent coagulation. Sundaram & Collins (J. Fluid Mech. 1997) showed that the radial distribution function (RDF) of the particle population, evaluated at contact, precisely corrects the collision kernel for clustering. Subsequent work has explored the dependence of the RDF on the system parameters (e.g., particle size, concentration, response time and Reynolds number) using direct numerical simulations. These results have improved our understanding and ability to predict the effect of the first three parameters; however, owing to the limited range of Reynolds number that can be reached in a numerical simulation, questions remain over the scaling of the RDF with Reynolds number. This is a critical issue for high-Reynolds-number applications such as cloud physics, where values of the Reynolds number can be 1–2 orders of magnitude greater than can be simulated. We will present our highest Reynolds number simulations to date and show our attempts to resolve this issue. Recently, the ability to measure three-dimensional particle positions using holography has been realized (e.g., Meng & Pu, J. Opt. Soc. Am. 2003). With holography, the optical image that is produced contains fringes that, upon inverting the laser, reproduce the original image in three dimensions. The hologram can then be scanned using a digital camera to obtain the particle positions. An important consideration with this study is the need to differentiate individual particles. We developed a search algorithm that locates particle centers, even in the presence of optical aberations and speckle noise. The algorithm has been used to obtain the first experimental RDF measurements to date. Thus far we see good agreement between the experimentally obtained RDF and the simulations. Besides validating the simulations, experiments can span a much broader range of Reynolds numbers, providing critical data that may help resolve the open questions associated with this parameter.

Bulk Carbon Nanotubes for Micro Anemometry (Invited Paper)

We have successfully developed a process to manipulate post-growth multi-walled carbon nanotube (MWNT) by AC electrophoresis to form resistive elements and showed that these elements can potentially served as novel sensing elements for micro/nano thermal and anemometry sensing. We have measured the temperature coefficient of resistance (TCR) of these MWNT bundles and integrated them into constant current mode configuration for dynamic characterization. Preliminary experimental measurements showed that the devices could be operated in micro-watt power range for micro thermal and anemometry sensing. This operation range is three orders of magnitude lower than conventional Micro-Electro-Mechanical Systems (MEMS) polysilicon sensors in constant current (CC) mode configuration. In addition, the devices exhibited very fast frequency response (> 100 kHz) in CC mode. Based on these results, we are currently developing polymer-based MWNT embedded sensor for various micro/nano fluidic applications.

Dynamic Break-Up and Drop Formation From a Liquid Jet Spun From a Rotating Orifice: Part I — Experimental

The dynamics of the break-up of curved jets produced by the prilling process were studied. The effects of liquid dynamic viscosity, rotation rate and orifice size upon the surface tension driven instabilities were investigated. Liquid dynamic viscosity was varied by using mixtures of glycerol and water which gave dynamic viscosities ranging from 0.001 to 0.081 Pa.s at 20°C. Over the range of experimental parameters studied, four different break-up modes were identified. For each mode, considerable differences in the break-up mechanism and in the drop size distributions produced were observed. Dimensional analysis has shown that the break-up modes can be predicted from a plot of Reynolds number against Weber number. The break-up mode observed is a strong function of viscosity and highly nonlinear effects were observed with the most viscous solutions used. The effect of rotation rate on the jet break-up length was inconclusive from the experiment.

Commercial CFD Code Validation for Simulation of Heavy-Vehicle External Aerodynamics

The issue of energy economy in transportation has grown beyond traditional concerns over environment, safety and health to include new concerns over national and international security. In collaboration with the U.S. Department of Energy Office of FreedomCAR and Vehicle Technologies’ Working Group on Aerodynamic Drag of Heavy Vehicles, Argonne National Laboratory is investigating the accuracy of aerodynamic drag predictions from commercial Computational Fluid Dynamics (CFD) Software. In this validation study, computational predictions from two commercial CFD codes, Star-CD [1] and PowerFLOW [2], will be compared with detailed velocity, pressure and force balance data from experiments completed in the 7 ft. by 10 ft. wind tunnel at NASA Ames [3, 4] using a Generic Conventional Model (GCM) that is representative of typical current-generation tractor-trailer geometries.

On Numerical Prediction of the Stability of Hypersonic Boundary-Layer Flow Over a Row of Microcavities

This paper is concerned with the structure of steady two–dimensional flow inside the viscous sublayer in hypersonic boundary–layer flow over a flat surface in which microscopic cavities (‘microcavities’) are embedded. Such a so–called Ultra Absorptive Coating (UAC) has been predicted theoretically [1] and demonstrated experimentally [2] to stabilize passively hypersonic boundary–layer flow. In an effort to further quantify the physical mechanism leading to flow stabilization, this paper focuses on the nature of the basic flows developing in the configuration in question. Direct numerical simulations are performed, addressing firstly steady flow inside a singe microcavity, driven by a constant shear, and secondly a model of a UAC surface in which the two–dimensional boundary layer over a flat plate and a minimum nontrivial of two microcavities embedded in the wall are solved in a coupled manner. The influence of flow– and geometric parameters on the obtained solutions is illustrated. Based on the results obtained, the limitations of currently used theoretical methodologies for the description of flow instability are identified and suggestions for the improved prediction of the instability characteristics of UAC surfaces are discussed.

Numerical Simulation of Supersonic Flow and Particle Motion in Aerodynamic Lenses

Airflow and particle motions in aerodynamic lenses are studied. The computational grid is generated with the use of GAMBIT code and FLUENT 5 is used in the analysis. The axisymmetric compressible form of the Navier-Stokes equation is solved and the airflow conditions are evaluated. One-way coupling is assumed in that the air transports the particles, but the effect of dilute particle concentrations on flow field is ignored. The particle equation of motion including drag, lift and Brownian forces is used and the particle trajectories in the aerodynamic a lens are analyzed. In addition, the airflow field and particles motions downstream of the nozzle are also studied. A series of sensitivity analyses on the effect of inlet flow stagnation pressure and backpressure of the nozzle on the aerodynamic performance of the lens is performed. Sample streamlines and particles trajectories in an axisymmetric plane of a combination of three aerodynamic lenses and a nozzle are shown in the figures.

Numerical and Experimental Evaluation of Turbulent Flow in Volute

The performance of centrifugal fans is considerably influenced by the design of tongue at the re-circulation port. The flow in the volute of a centrifugal fan was studied both experimentally and numerically. In this experiment, flow angle, pressure and velocity profiles were measured at a large number of locations in the volute. The flow field in the volute passage was analyzed using Computational Fluid Dynamics. The flow was assumed to be three dimensional, turbulent and steady. The numerical simulation produced qualitatively good agreement with the experimental result. The results from experiment and numerical simulation indicated that the adoption of a re-circulating flow port improved fan performance for all flow conditions. In addition, the existence of strong secondary flow was apparent at the cross-section of the volute passage.

Review of the Secondary Flows of Complex Fluids

A survey of secondary flows of viscoelastic liquids in straight tubes is given including recent work pointing at striking analogies with transversal deformations associated with the simple shearing of solid materials. The importance and implications of secondary flows of viscoelastic fluids in heat transfer enhancement are explored together with the difficulties in detecting weak secondary flows (dilute, weakly viscoelastic solutions) in a laboratory setting. Recent new work by the author and colleagues which explores for the first time the structure of the secondary flow field in the pulsating flow of a constitutively nonlinear simple fluid in straight tubes of arbitrary cross-sections is summarized. Arbitrary conduit contours are obtained through a novel approach to the concept of domain perturbation. Time averaged, mean secondary flow streamline contours are presented for the first time for triangular, square and hexagonal pipes.

Experimental Research on Rotating Skimmer

This paper provides the experimental results on skimmer and gives some detailed information useful for benchmark test of computer codes that are now able to simulate the fluid-structure interaction. For this purpose, we specially designed the injection system that imposes reproducible rotational speed and injection speed on the skipper. The effect of rotation is discussed by changing rotation speed in a wide range.

PANS Turbulence Model for Seamless Transition Between RANS and LES: Fixed-Point Analysis and Preliminary Results

Partially-averaged Navier-Stokes (PANS) approach has been recently developed as a possible bridging model between Reynolds-averaged Navier-Stokes (RANS) method and large-eddy simulations (LES). The resolution control parameters in PANS are the fractions of unresolved kinetic energy (fk) and unresolved dissipation (fε). We investigate the fixed-point behavior of PANS and present some preliminary results obtained using this model. By comparing the fixed-point behavior of PANS and URANS (unsteady Reynolds-averaged Navier-Stokes) methods, the possible advantage of the former over the latter is explained. Initial results from two-dimensional simulations of flow past square results are also presented.