Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session GG: Drops IV |
Hide Abstracts |
Room: 101A |
Monday, November 24, 2008 8:00AM - 8:13AM |
GG.00001: Effect of Nozzle Passage Length on the Surface Properties of Turbulent Liquid jets in Crossflow Anu Osta, Khaled Sallam An experimental study on the effect of the injector geometry on the surface properties of turbulent liquid jets in gaseous crossflow was carried out. Nozzles with different L/D ratios were used to gain an understanding of the Nozzle L/D ratio effect on the breakup of liquid jets in cross flow. The measurements are based on laser diagnostics, including double-pulsed holography and shadowgraphy. The measurements include surface properties, breakup location of the liquid column, and the breakup regime transitions. The hologram was captured digitally on a CCD and reconstructed numerically using convolution. With increase in the injector L/D ratio increased turbulence level was introduced from the longer passage resulting in decreasing breakup lengths of turbulent liquid jet for the same injector diameter. The jet surface characteristics were influenced by the passage length of injection and exhibited a greater degree of surface activity (ligament and drop formation) for the jet from a longer passage than a shorter one. The breakup was influenced by a new dimensionless number in terms of liquid/gas momentum ratio and jet Weber number. [Preview Abstract] |
Monday, November 24, 2008 8:13AM - 8:26AM |
GG.00002: Sliding, pinch-off and detachment of a drop on a wall in shear flow Mohammad N.H. Gilani, Hang Ding, Peter D.M. Spelt We investigate the motion and deformation of a droplet on a wall in shear flow, accounting for inertial effects and contact-angle hysteresis, with emphasis on the conditions beyond the onset of motion of the droplet. A diffuse-interface method is used for this purpose. Various flow regimes are encountered, including droplets sliding in a quasi-steady state, and entrainment of a part of or almost the entire droplet. A flow regime map is provided. Results for an apparent contact angle inferred from the simulations are shown to collapse onto a single curve when plotted against the instantaneous capillary number (based on the contact-line speed), even after an entrainment event. Finally, the approach to breakup is investigated in detail. Results are presented for the growth of a ligament on a drop, and the reduction of the radius of a pinching neck. The effect of varying several parameters on this behaviour is determined. A model based on an energy argument is proposed to explain the results for the rate of elongation of ligaments. [Preview Abstract] |
Monday, November 24, 2008 8:26AM - 8:39AM |
GG.00003: Tip streaming from microfluidic drops flowing in a sheared flow Charles Baroud, Simon Molesin, Thomas Dubos, Paul Manneville Water drops in paraffin oil are formed in a microfluidic channel, in the presence of a weak concentration of surfactant. The fluid is transported in a microchannel with a constant radius of curvature, thus producing a viscous shear stress on the drops. The drop velocity begins slower than the mean velocity of the oil and initially increases slowly. As a critical value of the velocity is reached, a transition is observed during which the drop velocity rapidly increases by a factor of three. The drops also re-align themselves parallel to the channel and one or a few well defined tails are produced behind the drop. A capillary number can be written which takes into account the time evolution of the drop. The critical value of this capillary number is found to be a constant at the moment of the transition, regardless of drop size, flowrate, or surfactant concentration. We will discuss the transition between the two states, as well as the critical tip-streaming and the small drops that are thus produced. [Preview Abstract] |
Monday, November 24, 2008 8:39AM - 8:52AM |
GG.00004: Influence of viscoelasticity on drop dynamics in shear flow Yuriko Renardy, K. Verhulst, P. Moldenaers, R. Cardinaels, S. Afkhami A computational and experimental study of drop dynamics under shear is conducted for fluid pairs in which one of the liquids is a Boger fluid. The drop to matrix viscosity ratio is either 0.75 or 1.5, Deborah numbers range to 2 and capillary numbers range to above breakup conditions. The results focus on three aspects: relaxation after cessation of shear, a novel viscoelastic drop breakup scenario, and the effect of shear flow history on drop breakup. Numerical simulations are performed with our in-house 3D volume-of-fluid PROST (Paraboloid Representation of the interface in the Surface Tension force) method. [Preview Abstract] |
Monday, November 24, 2008 8:52AM - 9:05AM |
GG.00005: The Physics of Aerobreakup: Viscous Liquids Chee-Loon Ng, Vladimir Mitkin, Theo Theofanous We present results for high viscosity silicon oil drops in supersonic gas flows, extending the available data, in Ohnesorge number (Oh) space, by one order of magnitude (up to $\sim $100). Using Laser Induced Fluorescence (LIF) at exposure times down to 3 nanoseconds, including oblique angles that allow clear visualization of the frontal area of the drop, we find that the asymptotic regime at extremely high Weber numbers is not piercing by Rayleigh Taylor waves as previously thought (also known as the ``catastrophic'' regime), but rather Shear-Induced Entrainment (SIE). This previous misunderstanding was a mirage of the shadowgraph method utilized in past work. Present results are in agreement with recent results for low viscosity liquids (Oh$<$0.1) found with the same experimental facility and instrumentation (Theofanous and Li, Physics of Fluids, Vol 20, No 5, May 2008). Moreover in the present work we quantify the critical We as function of the Oh, for the onset of breakup, as well as for the breakup regime ``transition region'', which is from Rayleigh-Taylor Piercing to SIE (as in the case of low viscosity fluids). Both critical We numbers increase with increasing Oh. [Preview Abstract] |
Monday, November 24, 2008 9:05AM - 9:18AM |
GG.00006: Breakup of a droplet in a particulate shear flow Arezoo Ardekani, Roger Rangel A numerical simulation using a Distributed-Lagrange-Multiplier-based computational method is conducted in order to simulate the breakup of a droplet in a particulate shear flow. The results show that the presence of particles strongly affects droplet deformation and may, in some cases, facilitate droplet breakup. In this problem, in addition to the capillary number and viscosity ratio, the particle volume fraction, the droplet-to-particle diameter ratio, and the particle initial distribution are important in controlling the droplet deformation and breakup. [Preview Abstract] |
Monday, November 24, 2008 9:18AM - 9:31AM |
GG.00007: Numerical studies of microscopic oil droplets under intense turbulence Murray Snyder, Omar Knio The rise of small oil droplets in water experiencing isotropic turbulence conditions is analyzed computationally under four different turbulence intensities. The computational method combines DNS of the turbulent flow with Lagrangian tracking of the slightly buoyant droplets using a dynamical equation with buoyancy, virtual mass, pressure, drag, lift and history forces. In our recent work, Snyder {\it et al.} (2008), we showed that the puzzling behavior observed by Friedman and Katz (2002), where the rise velocity of droplets smaller than 800 $\mu$m in diameter is enhanced by turbulence whereas the rise of larger droplets is retarded, could be explained by significant drop then enhancement of the droplet drag coefficient, and corresponding drop in the virtual mass coefficient. In this study we use the same technique to explain the recent experimental results of Gopalan and Katz (2008), who also showed both suppression and enhancement of droplet rise velocities. Using drag and virtual mass coefficients which vary with Reynolds number, our computations approximate the experimental behavior observed by Gopalan and Katz in isotropic turbulence with 79, 100 and 151 $\mu$m Kolmogorov length scales. Combined with close agreement with the Friedman and Katz results, with an 88 $\mu$m Kolmogorov scale, our results provide further evidence that both the quasi-steady drag and virtual mass coefficients may be heavily modified under intense turbulence. [Preview Abstract] |
Monday, November 24, 2008 9:31AM - 9:44AM |
GG.00008: Interactions of Surfactant-covered Spherical Drops in Gravity and Shear Flow Michael Rother, Alexander Zinchenko, Robert Davis Collision efficiencies are calculated by a trajectory analysis for two contaminated spherical drops in buoyancy and shear flow at low Reynolds number with arbitrary surfactant surface coverage. The full time-dependent convective-diffusion equation for the insoluble surfactant is coupled with the solution of the hydrodynamic problem. The method includes high-order expansion in spherical harmonics and implicit time-marching for the surfactant, and Lamb's series with biconjugate-gradient iterations for the velocity field, to obtain solutions to near-contact (0.0001a) at arbitrary Peclet (Pe) and Marangoni numbers (Ma). At small Pe or large Ma, the deviation in surfactant coverage is small, and the results for the incompressible surfactant model are recovered. For large Pe or small Ma, however, the collision efficiency approaches the clean interface value, with generally intermediate results at other values of the governing parameters. The possibility of significant surfactant redistribution in the region of close approach under conditions when the surfactant concentration remains nearly uniform when the drops are well separated is also considered. [Preview Abstract] |
Monday, November 24, 2008 9:44AM - 9:57AM |
GG.00009: ABSTRACT WITHDRAWN |
Monday, November 24, 2008 9:57AM - 10:10AM |
GG.00010: The Implosion of Cylindrical Shell Structures in a High-Pressure Water Environment C. Ikeda, C. Rother, J.H. Duncan The implosion of gas-filled cylindrical shell structures was studied experimentally in a nearly spherical tank with a nominal internal diameter of 1.77 m. Models were made from two brass tubes with diameters of 1.67 cm and 2.54 cm ($D)$ and wall thicknesses of 0.34 mm. The models were sealed with end caps and the lengths of the models were chosen to achieve the same internal volume ($V)$ and implosion pressure ($P_{a})$, and, therefore, the same available energy ($P_{a}V)$. Underwater blast sensors recorded dynamic pressure waves at 13 positions in the tank and a high-speed movie camera recorded the implosions. For both models, $P_{a}$ was about 31.5 bar, while the small- and large-diameter cylinders imploded with mode two and three cross-sectional shapes, respectively. The dynamic pressure signals decrease when the implosion begins and then reach a sharp positive peak when the walls of the cylinder collide. The implosion times (\textit{$\Delta $t}) when divided by the time scale for the implosion of a bubble ($T_{b}$ = 0.5D(\textit{ $\rho $ /P}$_{a})^{0.5}$ where \textit{$\rho $} is the water density) for the small- and large-diameter models were similar, 1.57 and 1.66, respectively. However, the dimensionless peak pressures ($P_{max}$/($P_{a}-P_{atm}))$ were quite different, 0.345 and 0.166, respectively, for these models. [Preview Abstract] |
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