178 research outputs found
Splash wave and crown breakup after disc impact on a liquid surface
No full textIn this paper we analyse the impact of a circular disc on a free surface using experiments, potential flow numerical simulations and theory. We focus our attention both on the study of the generation and possible breakup of the splash wave created after the impact and on the calculation of the force on the disc. We have experimentally found that drops are only ejected from the rim located at the top part of the splash – giving rise to what is known as the crown splash – if the impact Weber number exceeds a threshold value Wecrit?140. We explain this threshold by defining a local Bond number Botip based on the rim deceleration and its radius of curvature, with which we show using both numerical simulations and experiments that a crown splash only occurs when Botip?1, revealing that the rim disrupts due to a Rayleigh–Taylor instability. Neglecting the effect of air, we show that the flow in the region close to the disc edge possesses a Weber-number-dependent self-similar structure for every Weber number. From this we demonstrate that Botip?We, explaining both why the transition to crown splash can be characterized in terms of the impact Weber number and why this transition occurs for Wecrit?140. Next, including the effect of air, we have developed a theory which predicts the time-varying thickness of the very thin air cushion that is entrapped between the impacting solid and the liquid. Our analysis reveals that gas critically affects the velocity of propagation of the splash wave as well as the time-varying force on the disc, FD. The existence of the air layer also limits the range of times in which the self-similar solution is valid and, accordingly, the maximum deceleration experienced by the liquid rim, that sets the length scale of the splash drops ejected when We>Wecrit
Air flow in a collapsing cavity
No full textWe experimentally study the airflow in a collapsing cavity created by the impact of a circular disc on a water surface. We measure the air velocity in the collapsing neck in two ways: Directly, by means of employing particle image velocimetry of smoke injected into the cavity and indirectly, by determining the time rate of change of the volume of the cavity at pinch-off and deducing the air flow in the neck under the assumption that the air is incompressible. We compare our experiments to boundary integral simulations and show that close to the moment of pinch-off, compressibility of the air starts to play a crucial role in the behavior of the cavity. Finally, we measure how the air flow rate at pinch-off depends on the Froude number and explain the observed dependence using a theoretical model of the cavity collapse
Granular fountains: Convection cascade in a compartmentalized granular gas
No full textvan der Meer D, van der Weele K, Reimann P. Granular fountains: Convection cascade in a compartmentalized granular gas. PHYSICAL REVIEW E. 2006;73(6): 061304.This paper extends the two-compartment granular fountain [D. van der Meer, P. Reimann, K. van der Weele, and D. Lohse, Phys. Rev. Lett. 92, 184301 (2004)] to an arbitrary number of compartments: The tendency of a granular gas to form clusters is exploited to generate spontaneous convective currents, with particles going down in the well-filled compartments and going up in the diluted ones. We focus upon the bifurcation diagram of the general K-compartment system, which is constructed using a dynamical flux model and which proves to agree quantitatively with results from molecular dynamics simulations
Highly focused supersonic microjets: numerical simulations
No full textBy focusing a laser pulse inside a capillary partially filled with liquid, a vapour bubble is created that emits a pressure wave. This pressure wave travels through the liquid and creates a fast, focused axisymmetric microjet when it is reflected at the meniscus. We numerically investigate the formation of this microjet using axisymmetric boundary integral simulations, where we model the pressure wave as a pressure pulse applied on the bubble. We find a good agreement between the simulations and experimental results in terms of the time evolution of the jet and on all parameters that can be compared directly. We present a simple analytical model that accurately predicts the velocity of the jet after the pressure pulse and its maximum velocit
Supersonic air flow due to solid-liquid impact
No full textA solid object impacting on liquid creates a liquid jet due to the collapse of the impact cavity. Using visualization experiments with smoke particles and multiscale simulations, we show that in addition, a high-speed air jet is pushed out of the cavity. Despite an impact velocity of only 1??m/s, this air jet attains supersonic speeds already when the cavity is slightly larger than 1 mm in diameter. The structure of the air flow closely resembles that of compressible flow through a nozzle—with the key difference that here the “nozzle” is a liquid cavity shrinking rapidly in time
Competetive clustering in a bidisperse granular gas: experiment, molecular dynamics, and flux model
No full textA compartmentalized bidisperse granular gas clusters competitively [R. Mikkelsen, D. van der Meer, K. van der Weele, and D. Lohse, Phys. Rev. Lett. 89, 214301 (2002)]: By tuning the shaking strength, the clustering can be directed either towards the compartment initially containing mainly small particles or to the compartment containing mainly large particles. Here, the conditions under which this competitive clustering occurs are studied experimentally, numerically (by means of molecular dynamics simulations), and analytically. A minimal model is derived that quantitatively accounts for the observed phenomena
Impact of a steel ball on soft sand
No full textThe radius of the steel ball is 1.25cm,
the falling height 2.5 m.
For more information, see:
Lohse, D., Bergmann, R.P.H.M., Mikkelsen, R., Zeilstra, C., Meer, D. van der, Versluis, M. , Weele, J.P. van der, Hoef, M.A. van der, & Kuipers, J.A.M.
Impact on soft sand: Void collapse and jet formation.
Phys. Rev. Lett., 93 (19), 198003-1-198003-4. (2004).The movies shows the impact of a steel ball on soft granular matter
Highly focused supersonic microjets
No full textThis paper describes the production of thin, focused microjets with velocities of up to 850??m/s by the rapid vaporization of a small mass of liquid in an open liquid-filled capillary. The vaporization is caused by the absorption of a low-energy laser pulse. A likely explanation of the observed phenomenon is based on the impingement of the shock wave caused by the nearly instantaneous vaporization on the free surface of the liquid. We conduct an experimental study of the dependence of the jet velocity on several parameters and develop a semiempirical relation for its prediction. The coherence of the jets and their high velocity, good reproducibility, and controllability are unique features of the system. A possible application is to development of needle-free drug-injection systems that would be of great importance for health care worldwide
Compartmentalized granular gases: flux model results
No full textvan der Meer D, van der Weele K, Reimann P, Lohse D. Compartmentalized granular gases: flux model results. JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT. 2007;2007(07):P07021.A review is given of our previous work on the clustering phenomenon for vibrofluidized granular matter in an array of connected compartments, being a prime example of spontaneous pattern formation in amany- body system far from thermodynamic equilibrium. Experiments show that when the shaking strength is reduced below a certain critical level, the grains cluster together: first into a subset of the compartments and ultimately, on a much longer timescale, into a single compartment. These experimental observations are explained qualitatively and quantitatively by a dynamical flux model. We discuss several variations on the original system, altering the openings between the compartments, in such a way that the clustering induces convective patterns and directed transport. Here the bifurcational structure becomes more intricate, but is again fully explained by the corresponding flux model
The impact of raindrops on sand
No full textWhen a raindrop hits a sand bed, it leaves behind a small crater with a mixture of liquid and grains located at the center. This event is frequently observed in nature, but when absent, sprinklers may artificially produce these impacting drops to facilitate irrigation. Also in industry, the interaction between liquid and grains is important in for instance, a pharmaceutical production process called wet granulation. In order to understand and influence this interplay between liquid and sand, we investigate the physics of this subtle and complex phenomenon, by studying experimentally the early stage of rain impacting on sand, i.e., the impact of a single drop on a still dry bed of grains. As soon as a drop impacts on a granular bed, both the drop and the target start to deform, and for wettable grains mixing occurs. In this thesis we succeeded in disentangling these three processes by using high-speed laser profilometry in combination with traditional high-speed imaging, such that we were able to capture both the deformation of the sand bed and the droplet. With this work, we managed to understand the effect of the mixing of liquid and sand on the maximum droplet spreading diameter and to take significant steps in understanding the influence of the deformability of an intruder on the crater formation
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