The Experts below are selected from a list of 11151 Experts worldwide ranked by ideXlab platform
Detlef Lohse - One of the best experts on this subject based on the ideXlab platform.
-
oblique drop impact onto a deep liquid pool
arXiv: Fluid Dynamics, 2017Co-Authors: Marise V Gielen, Pascal Sleutel, Jos Benschop, Michel Riepen, Victoria Voronina, Claas Willem Visser, Detlef Lohse, Jacco H Snoeijer, Michel Versluis, Hanneke GelderblomAbstract:Oblique impact of drops onto a solid or liquid surface is frequently observed in nature. Most studies on drop impact and splashing, however, focus on perpendicular impact. Here we study oblique impact of 100μm drops onto a deep liquid pool, where we quantify the splashing threshold, maximum cavity dimensions and cavity collapse by high-speed imaging above and below the water surface. Gravity can be neglected in these experiments. Three different impact regimes are identified: smooth deposition onto the pool, splashing in the direction of impact only, and splashing in all directions. We provide scaling arguments that delineate these regimes by accounting for the drop impact angle and Weber Number. The angle of the axis of the cavity created below the water surface follows the impact angle of the drop irrespectively of the Weber Number, while the cavity depth and its displacement with respect to the impact position do depend on the Weber Number. Weber Number dependency of both the cavity depth and displacement is modeled using an energy argument.
-
phase diagram for droplet impact on superheated surfaces
Journal of Fluid Mechanics, 2015Co-Authors: Hendrik J J Staat, Guillaume Riboux, Jose Manuel Gordillo, A T Tran, B M Geerdink, Detlef LohseAbstract:We experimentally determine the phase diagram for impacting ethanol droplets on a smooth, sapphire surface in the parameter space of Weber Number We versus surface temperature T. We observe two transitions, namely the one towards splashing (disintegration of the droplet) with increasing We, and the one towards the Leidenfrost state (no contact between the droplet and the plate due to a lasting vapour film) with increasing T. Consequently, there are four regimes: contact and no splashing (deposition regime), contact and splashing (contact–splash regime), neither contact nor splashing (bounce regime), and finally no contact, but splashing (film–splash regime). While the transition temperature TL to the Leidenfrost state depends weakly, at most, on We in the parameter regime of the present study, the transition Weber Number WeC towards splashing shows a strong dependence on T and a discontinuity at TL. We quantitatively explain the splashing transition for T
-
microscopic structure influencing macroscopic splash at high Weber Number
Soft Matter, 2011Co-Authors: Peichun Amy Tsai, Maurice H W Hendrix, Remko R M Dijkstra, Lingling Shui, Detlef LohseAbstract:The dynamics of water drop impact at high impinging velocity onto superhydrophobic substrates is experimentally investigated. The solid substrate—comprised of regular and hydrophobic micropillars—is transparent, thereby facilitating close-up, top-or-bottom-view, high-speed imaging. With a sufficient impact velocity, instead of a completely-bouncing ‘‘Fakir’’ droplet, wetting splashing can occur, with an entrapped air bubble at the centre surrounded by a wetted area as well as an emission of satellite droplets during the advancing phase of spreading lamella. A large portion of the lamella travels upon air and subsequently recoils due to surface tension, forming a partial rebound on the central wet spot. We present and discuss quantitative results of the entrapped air bubble, the central wetted area, and the maximal spreading lamella as the impact velocity is increased. We further vary the lattice periodicity of the micro-patterns and find its profound influence on the macroscopic flow. More specifically, directional splashing can emerge, emitting secondary droplets in certain directions which are associated with the lattice. Directional splashing can be suppressed to a gentle spreading by decreasing the periodicity of the lattice and, furthermore, can be tuned to a completely-wetting splashing in the diagonal directions of the lattice by a larger periodicity, offering opportunities to control the wetting process. Finally, the elimination of directional splashing by reducing air pressure suggests that the underlying air is squeezed outwards by the falling droplet upon the solid boundary whereby the air flow is affected, leading to different splashing behavior.
Yujen Tseng - One of the best experts on this subject based on the ideXlab platform.
-
binary droplet collision at high Weber Number
Physical Review E, 2009Co-Authors: Kuo Long Pan, Pingchung Chou, Yujen TsengAbstract:By using the techniques developed for generating high-speed droplets, we have systematically investigated binary droplet collision when the Weber Number (We) was increased from the range usually tested in previous studies on the order of 10 to a much larger value of about 5100 for water (a droplet at 23 m/s with a diameter of 0.7 mm). Various liquids were also used to explore the effects of viscosity and surface tension. Specifically, beyond the well-known regimes at moderate We's, which exhibited coalescence, separation, and separation followed by satellite droplets, we found different behaviors showing a fingering lamella, separation after fingering, breakup of outer fingers, and prompt splattering into multiple secondary droplets as We was increased. The critical Weber Numbers that mark the boundaries between these impact regimes are identified. The specific impact behaviors, such as fingering and prompt splattering or splashing, share essential similarity with those also observed in droplet-surface impacts, whereas substantial variations in the transition boundaries may result from the disparity of the boundary conditions at impacts. To compare the outcomes of both types of collisions, a simple model based on energy conservation was carried out to predict the maximum diameter of an expanding liquid disk for a binary droplet collision. The results oppose the dominance of viscous drag, as proposed by previous studies, as the main deceleration force to effect a Rayleigh-Taylor instability and ensuing periphery fingers, which may further lead to the formations of satellite droplets.
Kuo Long Pan - One of the best experts on this subject based on the ideXlab platform.
-
Study on high-Weber-Number droplet collision by a parallel, adaptive interface-tracking method
Journal of Fluid Mechanics, 2014Co-Authors: Chih Kuang Kuan, Kuo Long Pan, Wei ShyyAbstract:We have established a parallel, adaptive interface-tracking framework in order to conduct, based on the framework, direct simulation of binary head-on droplet collision in the high-Weber-Number regime (from 200 to 1500) that exhibits complex topological changes and substantial length scale variations. The overall algorithms include a combined Eulerian and Lagrangian solver to track moving interfaces, conservative Lagrangian mesh modification and reconstruction, cell-based unstructured adaptive mesh refinement (AMR) in the Eulerian solver, and associated Eulerian and Lagrangian domain partitions to minimize communication overhead. Based on the combined computational and experimental efforts, we have resolved for the first time the free-surface instabilities of the colliding droplets at such high Weber Number. We detail the characteristics of coalescence, stretch, end pinching, fingering, free-surface movement and drop breakup. The Taylor–Culick rim is present soon after the collision. Furthermore, we observe two types of longitudinal instabilities on the rim, namely, the Rayleigh–Taylor (RT)-type instability in the initial deceleration phase of the circular sheet right after droplet coalescence, and later the Rayleigh–Plateau (RP) instabilities. As the Taylor–Culick rim disintegrates in the retraction phase, fingering effect is profound and resulting in wider droplet size distribution.
-
binary droplet collision at high Weber Number
Physical Review E, 2009Co-Authors: Kuo Long Pan, Pingchung Chou, Yujen TsengAbstract:By using the techniques developed for generating high-speed droplets, we have systematically investigated binary droplet collision when the Weber Number (We) was increased from the range usually tested in previous studies on the order of 10 to a much larger value of about 5100 for water (a droplet at 23 m/s with a diameter of 0.7 mm). Various liquids were also used to explore the effects of viscosity and surface tension. Specifically, beyond the well-known regimes at moderate We's, which exhibited coalescence, separation, and separation followed by satellite droplets, we found different behaviors showing a fingering lamella, separation after fingering, breakup of outer fingers, and prompt splattering into multiple secondary droplets as We was increased. The critical Weber Numbers that mark the boundaries between these impact regimes are identified. The specific impact behaviors, such as fingering and prompt splattering or splashing, share essential similarity with those also observed in droplet-surface impacts, whereas substantial variations in the transition boundaries may result from the disparity of the boundary conditions at impacts. To compare the outcomes of both types of collisions, a simple model based on energy conservation was carried out to predict the maximum diameter of an expanding liquid disk for a binary droplet collision. The results oppose the dominance of viscous drag, as proposed by previous studies, as the main deceleration force to effect a Rayleigh-Taylor instability and ensuing periphery fingers, which may further lead to the formations of satellite droplets.
A R Masri - One of the best experts on this subject based on the ideXlab platform.
-
atomization behaviour of a hybrid air blast electrostatic atomizer for spray combustion
Fuel, 2021Co-Authors: Tushar Ahmed, Agisilaos Kourmatzis, A R MasriAbstract:Abstract An analysis of the atomization region of a series of diesel sprays generated from a hybrid (air-blast & electrostatic) atomizer is presented. The atomizer has been specifically designed to operate on single or hybrid mode to enable the delivery of a charged and/or air-assisted spray for combustion applications. High speed microscopic backlit imaging is applied over a range of operating conditions. A noticeable influence of charge on the measured droplet and ligament size and their Numbers is observed at all the tested aerodynamic Weber Numbers (We = 2.4, 5.2, 8.2, 11.2). The experimental results reveal that an increase in spray specific charge can reduce both droplet and ligament sizes while a significant increase in droplet count is noted. The influence of charge is found to vary depending on the aerodynamic Weber Number, with a more noticeable influence on the atomization behaviour for the lowest Weber Number case (We = 2.4). When operating in a hybrid mode, a minimum spray specific charge (0.14 C/m3) is required for the applied electric potential to influence the aerodynamically assisted spray.
-
air assisted atomization of liquid jets in varying levels of turbulence
Journal of Fluid Mechanics, 2015Co-Authors: Agisilaos Kourmatzis, A R MasriAbstract:Air-assisted primary atomization is investigated in a configuration where liquid is injected in a turbulent gaseous jet flow both within as well as outside of the potential core. Cases are studied where the injection point is moved within the flow to maintain a range of constant gaseous mean velocities but changing local fluctuating velocity root-mean-square (r.m.s.) levels. Over a range of mean conditions, this allows for a systematic understanding of both the effects of gas-phase turbulence and mean shear on primary break-up independently. Extensive data is obtained and analysed from laser Doppler anemometry/phase Doppler anemometry, high-speed microscopic backlit imaging and advanced image processing. It is found that the ratio of the turbulent Weber Number to the mean Weber Number is a relevant parameter as is the turbulence intensity. The primary break-up length is found to be heavily influenced not only by the mean velocity, but also by the turbulence level and the mass fuel to air ratio. Above a particular threshold intensity level the break-up time changes in proportion to the change in the integral time scale of the flow. In addition, it is found that regardless of diameter and turbulent flow conditions at the liquid jet, the final size of ligaments converges to a value which is of the order of the measured primary instability wavelength ( ). In contrast, cases of different turbulence intensity show the mean of droplet sizes diverging as the spray is advected downstream and this is because droplets are generated from ligaments, the latter of which are subjected both to Rayleigh–Taylor instabilities and turbulent fluctuations. This contribution, for the first time, examines the theoretical applicability of the Rayleigh–Taylor instability in flows where the turbulence is substantial with respect to the mean flow. It is shown that for high turbulence intensities a full theoretical reconstruction of the measured final droplet size distribution is possible from a probability density function of model Rayleigh–Taylor wavelengths ( ). In agreement with the literature (Varga et al. J. Fluid Mech., vol. 497, 2003, pp. 405–434), mean droplet sizes are found to be equal to a mean theoretical Rayleigh–Taylor wavelength normalized by a particular constant value. This, however, is only true for local turbulence intensities less than , or for ratios of the turbulent Weber Number to mean Weber Number ( ) of less than . Above this, the normalization value is no longer constant, but increases with . Finally, the instability wavelengths can be used as part of an approximation that estimates the total Number of objects formed after break-up, where the object Number is found to be dictated by a balance of both mean flow conditions and local turbulence.
-
the influence of gas phase velocity fluctuations on primary atomization and droplet deformation
Experiments in Fluids, 2014Co-Authors: A Kourmatzis, A R MasriAbstract:The effects of grid-generated velocity fluctuations on the primary atomization and subsequent droplet deformation of a range of laminar liquid jets are examined using microscopic high-speed backlit imaging of the break-up zone and laser Doppler anemometry of the gas phase separately. This is done for fixed gas mean flow conditions in a miniature wind tunnel experiment utilizing a selection of fuels, turbulence-generating grids and two syringe sizes. The constant mean flow allows for an isolated study of velocity fluctuation effects on primary atomization in a close approximation to homogeneous decaying turbulence. The qualitative morphology of the primary break-up region is examined over a range of turbulence intensities, and spectral analysis is performed in order to ascertain the break-up frequency which, for a case of no grid, compares well with the existing literature. The addition of velocity fluctuations tends to randomize the break-up process. Slightly downstream of the break-up region, image processing is conducted in order to extract a Number of metrics, which do not depend on droplet sphericity, and these include droplet aspect ratio and orientation, the latter quantity being somewhat unconventional in spray characterization. A turbulent Weber Number \(We^{\prime}\) which takes into account gas phase fluctuations is utilized to characterize the resulting droplet shapes, in addition to a mean Weber Number . Above a \(We^{\prime}>0.05\) a clear positive relationship exists between the mean aspect ratio of droplets and the turbulent Weber Number where \(We^{\prime}\) is varied by altering all relevant variables including the velocity root mean square, the initial droplet diameter, the surface tension and the density.
Peng Zhang - One of the best experts on this subject based on the ideXlab platform.
-
bouncing coalescence and separation in head on collision of unequal size droplets
Physics of Fluids, 2012Co-Authors: Chenglong Tang, Peng ZhangAbstract:The dynamics of head-on collision of unequal-size droplets were experimentally and theoretically investigated, with emphasis on identifying distinct collision outcomes and interpreting the size-ratio dependence. A unified regime diagram in terms of bouncing, permanent coalescence, and separation after coalescence was identified for hydrocarbon and water droplets in the parameter space of the size ratio and a collision Weber Number. Experimental results show that the transition Weber Number, Web-c, that separates the bouncing and permanent coalescence regimes, weakly depends on the size ratio, while the transition Weber Number, Wec-s, that separates permanent coalescence and separation regimes, significantly increases with the size ratio. A theoretical model based on energy balance and scaling analysis was developed to explain the size-ratio dependence of Wec-s. The theoretical results show good agreement with the experimental data for tetradecane and decane droplets, with a moderate discrepancy for water ...
-
an analysis of head on droplet collision with large deformation in gaseous medium
Physics of Fluids, 2011Co-Authors: Peng Zhang, Chung K LawAbstract:A theoretical analysis was performed for the head-on collision of two identical droplets in a gaseous environment, with the attendant bouncing and coalescence outcomes, for situations in which the extent of droplet deformation upon collision is comparable to the original droplet radius, corresponding to O(1)–O(10) of the droplet Weber Number. The model embodies the essential physics that describes the substantial amount of droplet deformation, the viscous loss through droplet internal motion induced by the deformation, the dynamics and rarefied nature of the gas film between the interfaces of the colliding droplets, and the potential destruction and thereby merging of these interfaces due to the van der Waals attraction force. The theoretical model was applied to investigate collisions involving hydrocarbon and water droplets at sub- and superatmospheric pressures. The results agree well with previous experimental observations in that as the Weber Number increases in the range of O(1)–O(10), collision of hydrocarbon droplets at one atmospheric pressure results in the nonmonotonic coalescence-bouncing-coalescence transition, that while bouncing is absent for water droplets at atmospheric pressure, it occurs at higher pressures, and that while bouncing is observed for hydrocarbon droplets at atmospheric pressure, it is absent at lower pressures.