Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M15: Drops: Impact on Surfaces |
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Chair: J.R. Saylor, Clemson University Room: 3022/3024 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M15.00001: Drop impact on solid surface: Short time self-similarity Julien Philippi, Pierre-Yves Lagr\'ee, Arnaud Antkowiak Drop impact on a solid surface is a problem with many industrial or environmental applications. Many studies focused on the last stages of this phenomenon as spreading or splashing. In this study we are interested in the early stages of drop impact on solid surface. Inspired by Wagner theory developed by water entry community we shown the self-similar structure of the velocity field and the pressure field. The latter is shown to exhibit a maximum not near the impact point, but rather at the contact line. The motion of the contact line is furthermore shown to exhibit a transition from ``tank treading'' motion to pure sweeping when the lamella appears. We performed numerical simulations with the open-cource code Gerris which are in good agreement with theoretical predictions. Interestingly the inviscid self-similar impact pressure and velocities depend on the self-similar variable $r/\sqrt{t}$. This allows to construct a seamless uniform analytical solution encompassing both impact and viscous effects. We predict quantitatively observables of interest, such as the evolution of total and maximum viscous shear stresses and net total force. We finally demonstrate that the structure of the flow resembles a stagnation point flow unexpectedly involving $r/\sqrt{t}$. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M15.00002: Free radially expanding liquid sheet in air: time- and space-resolved measurement of the thickness field Christian Ligoure, Clara Vernay, Laurence Ramos The collision of a liquid drop against a small target results in the formation of a thin liquid sheet that extends radially until it reaches a maximum diameter. We have developed an original time- and space-resolved technique to measure the thickness field of this class of liquid sheet, based on the grey level measurement of the image of a dyed liquid sheet recorded using a fast camera. This method enables a precise measurement of the thickness in the range ($10-450$) $\mu$m, with a temporal resolution equals to that of the camera. Two asymptotic regimes for the expansion of the sheet are evidenced. The scalings of the thickness with $t$ and $r$ measured in the two regimes are those that were predicted but never experimentally measured before. Interestingly, our experimental data also evidence the existence of a maximum of the film thickness $h_{\rm{max}}(r)$ at a radial position $r_{\rm{h_{max}}}(t)$ corresponding to the crossover of these two asymptotic regimes. The maximum moves with a constant velocity of the order of the impact velocity, Hence, our data has allowed one to reconcile the two apparently inconsistent theoretical predictions found in the literature Thanks to our visualization technique, we also evidence an azimuthal thickness modulation. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M15.00003: The Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces Hendrik Staat, Hrudya Nair, Tuan Tran, Arie van Houselt, Andrea Prosperetti, Detlef Lohse, Chao Sun When a droplet impacts a smooth solid plate that is heated to a temperature above the boiling point of the liquid, the droplet will evaporate upon impact. Above a certain threshold plate temperature, a vapor film between the plate and the droplet prevents direct contact during impact due to the dynamic Leidenfrost effect. This state is unwanted in applications like spray cooling, as vapor limits the heat transfer from the solid to the liquid. We show that the dynamic Leidenfrost temperature for droplets that impact on a surface covered with carbon-nanofibers is higher than on the surface without these nanofibers. This is attributed to the cooling effect that vapor has on the superheated nanofibers. Because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, resulting in a higher dynamic Leidenfrost temperature than on smooth surfaces. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M15.00004: Role of cavitation in high-speed droplet impact problems Tomoki Kondo, Keita Ando High-speed droplet impact is found in physical cleaning using liquid jets, but its mechanisms for particle removal from target surfaces are yet unclear. In this study, we explore the possibility of having cavitation inside the droplet. The pressure evolution within a droplet colliding with a flat surface of deformable materials is determined by multicomponent Euler equations. Dynamics of cavitation bubbles heterogeneously nucleated from preexisting nuclei are determined from Rayleigh-Plesset calculations according to the pressure evolution within the droplet in one-way-coupling manner. The simulation shows that cavitation indeed occurs due to tension that arises from the water hammer shock reflection at the droplet interface. The role of cavitation including pressure emission from its collapse is to be discussed based on the one-way-coupling computations. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M15.00005: ABSTRACT WITHDRAWN |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M15.00006: Spray impact on a smooth, unheated surface: drop impact cavity diameter vs time John Kuhlman, Jonathan Taylor, Nicholas Hillen, Christopher Sommers A dense water spray impacting a smooth, unheated glass surface is studied. Average drop diameter is 90-60 microns; average axial velocity is 7.5-12 m/s for nozzle pressures of 1.4-4.2 bar gage, respectively, from PDPA data. Average spray Weber numbers are 70-130. Half the spray mass flux is due to larger drops with Weber numbers of 200-800, causing the splashed secondary drops and large impact cavities with longer lifetimes. Liquid film thickness is about 160 microns at all radii over the present range of nozzle pressures, with transient fluctuations of 30-40 microns. This thickness increases vs drop radial impact location, and decreases vs nozzle pressure. Drop impact cavity diameter from video images is 0.5 mm-1.5 mm, giving drop diameters of 100-300 microns, consistent with the PDPA data. Spray drop impact cavity growth vs time is fit approximately by ($t)^{\mathrm{0.2}}$ as seen in the literature. These results will be used to improve correlations in an existing preliminary Monte Carlo model of the complex spray impact process. It is believed that the transient thin liquid films formed beneath the droplet impact cavities are an important source of heat transfer augmentation via transient conduction. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M15.00007: Drop Impingement Induced Dispersal of Microorganisms and Contaminants Within Porous Media Young Soo Joung, Zhifei Ge, Cullen Buie We investigate migration of chemicals and microbes with aerosol generated by drop impingement on porous media. In our previous work we found that aerosol generation from droplets hitting porous media within a specific range of the Weber number (We) and a modified Pelect number (Pe). We and Pe reflect the impact condition of droplets and the wetting properties of porous media, respectively. The relationship between We and Pe can be expressed by a third dimensionless group, the Washburn Reynolds number (Re$_{W}=$We/Pe). In a specific range of Re$_{W}$, hundreds of aerosol particles can be generated within milliseconds of drop impingement. In this work we investigate if microbes such as \textit{Corynebacterium glutamicum}, a soil bacterium, \quad and chemicals such as Rhodamine B can be dispersed by aerosols generated from droplet impact. Experimentally, \textit{C. glutamicum} and Rhodamine B are permeated into porous media. Then drop impingements are conducted on the porous media with different We and Pe in an airflow tunnel. We quantitatively investigate the volume and speed of aerosol migration as a function of Re$_{W}$ of the drop impingement and Re of the airflow. Results of this study will shed light upon the dispersal of elemental compounds and microbes within soils due to aerosol generated by rainfall. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M15.00008: Experimental characterization and numerical simulation of crown propagation induced by impingement of droplet train Taolue Zhang, Jorge Alvarado, Anoop Kanjirakat, Reza Sadr In this combined experimental and numerical study, hydrodynamics of single stream of HFE-7100 droplets striking a pre-wetted solid surface was investigated. ANSYS Fluent CFD software was employed to simulate this process numerically. Experimentally, single stream of mono-dispersed droplets were produced using a piezoelectric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet velocity. A high speed camera system was used to capture the liquid crown propagation process given the high frequency of droplet impingement. Low-Weber number droplet impingements resulted in smooth spreading of the liquid crown while splashing (i.e. the emergence of secondary droplets from the rim of the crown) was observed at high Weber number cases. The dynamics of the crown propagation was analyzed and a correlation that takes into account non-dimensional crown diameter ($d^{\ast })$ and non-dimensional time ($t^{\ast })$ has been postulated. The correlation has a mathematical form of $d^{\ast }=K\cdot (t^{\ast })^{1/2}$, where $K$ is a constant. Comparison of the dynamics of crown propagation between experiments and numerical simulations yielded reasonable agreement. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M15.00009: Drop Impact on to Moving Liquid Pools Beatriz Natividad Mu\~noz-S\'anchez, Jos\'e Rafael Castrej\'on-Pita, Alfonso Arturo Castrej\'on-Pita, Ian M. Hutchings The deposition of droplets on to moving liquid substrates is an omnipresent situation both in nature and industry. A diverse spectrum of phenomena emerges from this simple process. In this work we present a parametric experimental study that discerns the dynamics of the impact in terms of the physical properties of the fluid and the relative velocity between the impacting drop and the moving liquid pool. The behaviour ranges from smooth coalescence (characterized by little mixing) to violent splashing (generation of multiple satellite droplets and interfacial vorticity). In addition, transitional regimes such as bouncing and surfing are also found. We classify the system dynamics and show a parametric diagram for the conditions of each regime. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M15.00010: Impact of Small Raindrops on Crude Oil Slicks David Morra, Nourah Almashan, David Murphy, Joseph Katz The impact of millimeter size water droplets falling near terminal velocity (e.g. rainfall) on a pool is known to produce air bubbles at the bottom of the splash cavity. These bubbles produce noise and contribute to marine aerosol production. Layers of crude oil resulting from oil spills alter air-sea interfacial properties. Our high speed observations examine the effect of oil layer thickness on the entrainment of air and oil as small raindrops impact the surface. They reveal that layers in the 10-400 $\mu$m range suppress bubble entrainment, likely due to the reduction of air-liquid surface tension (from 72 to 28 mN/m). For ``low energy'' impacts (droplets $<$2 mm and speed $<$2.5 m/s) and $<$200 $\mu$m layers, rupture of the film in less than 1 ms causes rapid retraction of the oil layer across the subsurface cavity and formation of oil droplets on the cavity side. Subsequently, as the cavity collapses, a vortex ring develops at the bottom of this cavity and forces these droplets downward. Impact on thicker oil layers results initially in accumulation of the drop fluid at the cavity base. When the drop subsequently penetrates the layer, it creates multiphase vesicles, i.e. drops of freshwater coated by a thin oil film, which migrate down into the bulk seawater. [Preview Abstract] |
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