Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session R9: General Fluid Dynamics: Instability, Surfaces and Cavitation |
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Chair: Michelle Driscoll, New York University Room: B117 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R9.00001: Shape-based separation of microparticles with magnetic fields Cheng Wang, Ran Zhou Precise manipulations, e.g., sorting and focusing, of nonspherical micro-particles in fluidic environment has important applications in the fields of biology sciences and biomedical engineering. However, non-spherical microparticles are hard to manipulate because they tumble in shear flows. Most of existing techniques, including traditional filtration and centrifugation, and recent microfluidic technology, have difficulty in separating microparticles by shape. We demonstrate a novel shape-based separation technique by combining external magnetic fields with pressure-driven flows in a microchannel. Due to the magnetic field, prolate ellipsoidal particles migrate laterally at different speeds than the spherical ones, leading to effective separation. Our experimental investigations reveal the underlying physical mechanism of the observed shape-dependent migration. We find that the magnetic field breaks the rotational symmetry of the nonspherical particles, and induces shape-dependent lift force and migration velocity. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R9.00002: Decontamination of chemical tracers in droplets by a submerging thin film flow. Julien R. Landel, Harry McEvoy, Stuart B. Dalziel We investigate the decontamination of chemical tracers contained in small viscous drops by a submerging falling film. This problem has applications in the decontamination of hazardous chemicals, following accidental releases or terrorist attacks. Toxic droplets lying on surfaces are cleaned by spraying a liquid decontaminant over the surface. The decontaminant film submerges the droplets, without detaching them, in order to neutralize toxic chemicals in the droplets. The decontamination process is controlled by advection, diffusion and reaction processes near the drop-film interface. Chemical tracers dissolve into the film flow forming a thin diffusive boundary layer at the interface. The chemical tracers are then neutralized through a reaction with a chemical decontaminant transported in the film. We assume in this work that the decontamination process occurs mainly in the film phase owing to low solubility of the decontaminant in the drop phase. We analyze the impact of the reaction time scale, assuming first-order reaction, in relation with the characteristic advection and diffusion time scales in the case of a single droplet. Using theoretical, numerical and experimental means, we find that the reaction time scale need to be significantly smaller than the characteristic time scale in the diffusive boundary layer in order to enhance noticeably the decontamination of a single toxic droplet. We discuss these results in the more general case of the decontamination of a large number of droplets. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R9.00003: Chemical decontamination of fa\c{c}ade cracks Merlin A. Etzold, Julien R. Landel, Stuart B. Dalziel The problem of cleaning and decontamination of buildings arises in the context of chemical spillages, terrorist attacks, industrial applications and in day-to-day situations such as the removal of graffiti. A common feature of all buildings is the existence of cracks and fissures, which act as contaminant traps. This contribution reports experiments and modelling of the removal of a water-soluble contaminant from the bottom of an idealised V-shaped crack. The contaminant is dissolved in a polymer thickened droplet. The surface washing techniques commonly used in industrial decontamination induce a flow in the crack which is mostly controlled by the crack geometry. Rinsing with pure water is compared against the situation in which a neutralising chemical is present. The cleaning process is modelled by solving the time-dependent diffusion equation within the droplet coupled to the steady state advection-diffusion equation outside the droplet. This approach is similar to the work of Landel et al. on decontaminating plane surfaces beneath falling films [JFM (2016), vol. 789, pp. 630-668]. Our results indicate that the proposed model describes successfully the earlier stages of decontamination. In later stages the dissolution of the thickened matrix may contribute to the process. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R9.00004: Flow visualization of the trapping induced by vortex breakdown at a junction Daniele Vigolo, Marco Riccomi, Federico Alberini, Elisabetta Brunazzi, Jesse Ault, Howard Stone Here we present experimental investigations of the vortex breakdown happening at a T-, Y- or ``arrow'' shaped junction responsible for the trapping of light material suspended in solution. Considering the ubiquitous nature of T-junctions and bifurcation in general, in industrial as well as biological environments, it is extremely interesting to better understand how this trapping phenomenon happens. In particular, we observed the flow profiles at different sections in order to perform a three-dimensional study of complex structures, such as vortices and recirculation zones, that develop at a bifurcation. We explored Reynolds number ranging from 50 to about 500 for different milli-fluidic devices. Thus we compared standard micro-PIV and a novel optical technique, the Ghost Particle Velocimetry (GPV), that was recently introduced, to investigate the onset of vortex breakdown. Moreover, the experimental results were compared with single-phase OpenFoam numerical simulations performed in the same flow conditions. Finally, we studied the mutual influence of a trapped particle on the flow field inside the recirculation zone by fully exploiting the capability of GPV to produce 3D flow field with a spatial resolution of few tens of microns. [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R9.00005: An experimental and computational study of large bubble coalescence in stagnant highly viscous liquids Sara Mohammed, Ezekiel Agunlejika, Zhihua Xie, Buddhika Hewkandamby, Barry Azzopardi, Omar Matar The coalescence of two, and sometimes, three large bubbles rising in columns of viscous, stagnant liquids is studied experimentally and computationally. Two cases are considered: a 38 mm diameter column with a 0.12 Pa s liquid (aqueous solution of glycerol/potassium chloride); a 290 mm diameter column with a silicone oil of 330 P s viscosity. High-speed videos are taken of the coalescence process, which are characterised by the acceleration of the trailing bubble into the rear of the leading one. There is significant penetration of the trailing bubble into the leading one, with a noticeable delay prior to rupture of the thin film separating the bubbles. The velocities of the individual bubbles, as well as the bubble shapes are measured accurately. Numerical simulations of the bubble rise and coalescence process are also carried out using the parallelised, control-volume, finite-element code, {\it Fluidity}, which uses adaptive, unstructured meshing. The numerical results are compared with the experimental observations in terms of single bubble shape and speed, as well as the entire dynamics of two-bubble coalescence process; particular attention is focused on bubble penetration and the final stages of coalescence for the very large viscosity ratio case. [Preview Abstract] |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R9.00006: Cavitation during wire brushing. Bo Li, Jun Zou, Chen Ji In our daily life, brush is often used to scrub the surface of objects, for example, teeth, pots, shoes, pool, etc. And cleaning rust and stripping paint are accomplished using wire brush. Wire brushes also can be used to clean the teeth for large animals, such as horses, crocodiles. By observing brushing process in water, we capture the cavitation phenomenon on the track of moving brush wire. It shows that the cavitation also can affect the surface. In order to take clear and entire pictures of cavity, a simplified model of one stainless steel wire brushing a boss is adopted in our experiment. A transparent organic tank filled with deionized water is used as a view box. And a high speed video camera is used to record the sequences. In experiment, ambient pressure is atmospheric pressure and deionized water temperature is kept at home temperature. An obvious beautiful flabellate cavity zone appears behind the moving steel wire. The fluctuation of pressure near cavity is recorded by a hydrophone. More movies and pictures are used to show the behaviors of cavitation bubble following a restoring wire. Beautiful tracking cavitation bubble cluster is captured and recorded to show. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R9.00007: A purely elastic upstream instability in channel flows Boyang Qin, Paulo Arratia In this talk, the flow of a viscoelastic fluid is experimentally investigated using particle velocimetry methods in a microfluidic device. The microchannel is 100-micron wide and deep and consists of a long (3-cm) straight region followed by a linear array of cylinders. Velocimetry measurements show the presence of flow instabilities far upstream of the linear array of cylinders. Velocity fluctuations are found up to 10 channel widths upstream the perturbation (linear array of cylinders); they extend far beyond the unsteady vortex that develops immediately in front the cylinder and are excited at many time scales. These fluctuations increase as the Weissenberg number (or flow rate) is increased. However, beyond a certain value of Wi, these fluctuations sharply decrease. We believe that the observed decrease in velocity fluctuations at high Wi correspond to a regime in which the flow speed (convection) is larger than the elastic wave speed, that is, the elastic Mach number exceeds unity. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R9.00008: Thermally aware, durable nanoengineered surfaces with high speed liquid impalement resistance Manish Tiwari, Chaoyi Peng, Zhuyang Chen Highly hydrophobic nanoengineered surfaces delaying freezing down to -20 degrees Centigrade for a day, sustaining dropwise steam condensation under high rate steam shear for several days, sustaining mechanical abrasion and high strains have attracted strong interest recently. Particularly, anti-icing and dropwise condensation promotion require thermally conductive surfaces with careful nucleation control -- of ice germs or droplets, respectively -- using precise surface nanotexture. Scalability of surface manufacture is an additional challenge. In the current presentation, we will demonstrate a pathway to address these needs. Anodisation of metallic substrate is first used to obtain nanotextured surfaces with a precision of approx. 200 nm. Next, rationally formulated nanocomposites comprising solution processed fluorinated copolymers and nanoparticle dispersions were spray coated on the anodized metals. The resulting nanocomposite coatings were superhydrophobic with approx. 20 nm precision in surface texture. The surface durability is assessed using tape peel, sand abrasion, and droplet and water jet impact tests up to 30 m/s. High speed jet splashing is recorded at speeds \textgreater 10 m/s to demonstrate the influence of jet diameter on splashing characteristics. [Preview Abstract] |
Tuesday, November 22, 2016 3:14PM - 3:27PM |
R9.00009: Predicting the onset of high-frequency self-excited oscillations in a channel with an elastic wall Thomas Ward, Robert Whittaker Flow-induced oscillations of fluid-conveying elastic-walled channels arise in many industrial and biological systems including the oscillation of the vocal cords during phonation. We derive a system of equations that describes the wall displacement in response to the steady and oscillatory components of the fluid pressure derived by Whittaker et. al. (2010). We show that the steady pressure component results in a base state deformation assumed to be small in magnitude relative to the length of the channel. The oscillation frequency of the elastic wall is determined by an eigenvalue problem paramterised by the shape of the base state deformation, the strength of axial tension relative to azimuthal bending, $\mathcal F$, and the size of non-linear stretching effects from the wall's initial deformation, $\mathcal K$. We determine the slow growth or decay of the normal modes in each by considering the energy budget of the system. The amplitude of the oscillations grow or decay exponentially with a growth rate $\Lambda$, which may be expressed in terms of a critical Reynolds number $Re_c$. We use numerical simulations to identify three distinct regions in parameter regimes space and determine the stability of oscillations in each. [Preview Abstract] |
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