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An experimental investigation of fluid flow resulting from the impact of a water drop with an unyielding dry surface
Abstract:The flow of fluid associated with the impact of water drops of radius R at a speed V onto unyielding dry metal surfaces of known roughness Ra is described. Spatial dimensions of the deforming drop are normalized by transformations of the kind x' — x/R, and time scales are normalized according to t' = tV/R, to permit comparison of events where or differ. It is shown that the primary influence of the surface roughness parameter Ra is the determination of the condition for the ejection of secondary droplets by the excitation of an instability in the developing watersheet; provided Ra≪ R, it is possible to evaluate the condition to a high degree of accuracy, and for Ra = 0.84 μm it is found to be α4/3RV1.69 > 7.4, where α is the eccentricity of the drop at the moment of impact. Deceleration of the drop apex does not commence until > 0.6, contrary to the prediction of Engel (1955) but in good agreement with that of Savic & Boult (1957). Close examination of the very early stages of impact suggests strongly that the so-called watersheet originates at a moment t' — 0.01 after first contact, regardless of the absolute values of R, V or Ra; the initial normalized watersheet velocity is of order 5. Where there is ejected material, its normalized velocity at the moment of ejection is of the order of 20 % greater than that of the watersheet substrate. Simple calculations also suggest that initial fluid velocities greater than 10 are required immediately before the initiation of the watersheet (t'< 0.01). Impacts at speeds considerably greater than the appropriate terminal fall speed in air show no deviations in character from those investigated at much lower speeds. A simple subsidiary experiment also suggests that greater impact velocities are required to produce splashing on inclined targets.
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参考文献
HARNESSING RAINDROP ENERGY IN BANGLADESH
2008
摘要
© ICME2009 1 AM-29
Drop impact on a flexible fiber.
Soft matter
2015
摘要
When droplets impact fibrous media, the liquid can be captured by the fibers or contact then break away. Previous studies have shown that the efficiency of drop capture by a rigid fiber depends on the impact velocity and a threshold velocity was defined below which the drop is captured. However, it is necessary to consider the coupling of elastic and capillary effects to achieve an improved understanding of the capture process for soft substrates. Here, we study experimentally the dynamics of a single drop impacting on a thin flexible fiber. Our results demonstrate that the threshold capture velocity depends on the flexibility of fibers in a non-monotonic way. We conclude that tuning the mechanical properties of fibers can optimize the efficiency of droplet capture.
Inclined to splash: triggering and inhibiting a splash with tangential velocity
2009
摘要
When a liquid drop impacts a smooth, solid, dry surface, the drop forms a radially spreading lamella, which can lead to a splash. Previous studies have focused almost exclusively on impacts perpendicular to a surface; yet it is common for drops to impact on angled or moving surfaces. The asymmetry of such impacts leads to an azimuthal variation of the ejected rim, and under certain conditions only part of the rim breaks up to form droplets. We show that the tangential component of impact can act to enhance or suppress a splash. We develop a new model to predict when this type of splashing will occur. The model accounts for our observations of the effects of tangential velocity and agrees well with previous experimental data.
Splashing on elastic membranes: The importance of early-time dynamics
2007
摘要
We study systematically the effect of substrate compliance on the threshold for splashing of a liquid drop using an elastic membrane under variable tension. We find that the splashing behavior is strongly affected by the tension in the membrane and splashing can be suppressed by reducing this tension. The deflection of the membrane upon droplet impact is measured using a laser sheet, and the results allow us to estimate the energy absorbed by the film upon drop impact. Measurements of the velocity and acceleration of the spreading drop after impact indicate that the splashing behavior is set at very early times after, or possibly just before, impact, far before the actual splash occurs. We also provide a model for the tension dependence of the splashing threshold based on the pressure in the drop upon impact that takes into account the interplay between membrane tension and drop parameters.
Numerical studies of droplet splashing on a dry surface: triggering a splash with the dynamic contact angle
2011
摘要
We numerically investigate water droplet splashing on a dry, flat and smooth surface. Our numerical framework can reproduce a prompt splash with satellite droplets and spikes at least qualitatively. The numerical results have shown that the dynamic contact angle can trigger a prompt splash without the roughness of a solid surface. We also propose a possible mechanism for how the splash (satellite droplets and spikes) gains the vertical momentum component without the roughness based on the dynamic contact angle.
Droplet impact on a thin liquid film: anatomy of the splash
Journal of Fluid Mechanics
2015
摘要
We investigate the dynamics of drop impact on a thin liquid film at short times in order to identify the mechanisms of splash formation. Using numerical simulations and scaling analysis, we show that it depends both on the inertial dynamics of the liquid and the cushioning of the gas. Two asymptotic regimes are identified, characterized by a new dimensionless number $J$ : when the gas cushioning is weak, the jet is formed after a sequence of bubbles are entrapped and the jet speed is mostly selected by the Reynolds number of the impact. On the other hand, when the air cushioning is important, the lubrication of the gas beneath the drop and the liquid film controls the dynamics, leading to a single bubble entrapment and a weaker jet velocity.
Breakup of a droplet at high velocity impacting a solid surface
2010
摘要
We have studied the collision between a droplet of different liquids with high impact energy and a solid plate with varied surface roughness, which is characterized by a dimensionless Weber number (We, defined as the impact inertia of the droplet normalized by its surface force) extending up to 12,000 for water. To make such collision, a technique was developed to generate a single droplet with speed up to 42 m/s, which was initially driven by upstream air flow through a nozzle and accelerated to nearly the same velocity of the high-speed flow downstream. Via a high-speed photographing system, the various splashing mechanisms were investigated and a specific prompt splash on a smooth plate was found at sufficiently high We, which was different somehow from the conventionally defined one that was generally believed to occur only on a rough surface. The radius when multiple secondary droplets were shed out of the rim of the expanding lamella was found to scale almost invariantly with We at large values, whereas the coupled effect of liquid viscosity might affect the ultimate value.
Influence of Dynamic Surface Tension on the Spreading of Surfactant Solution Droplets Impacting onto a Low-Surface-Energy Solid Substrate
Journal of colloid and interface science
1997
摘要
We have investigated the impact of single droplets of various surfactant solutions on a low-surface-energy solid substrate using a high-frequency visualization technique (one picture every 100 μs). Whatever the surfactant, the drop spreads and retracts in about 1 s under the action of inertia and capillarity, respectively. During retraction, the capillary waves can be amplified and, in some cases, even yield droplet bouncing. Then, the droplet may slowly spread again due to gravity and the unbalanced capillary forces at the contact line between the droplet and the substrate. During the fast spreading process (2-3 ms), the droplet surface increases by almost one order of magnitude since its shape changes from a sphere to a flat pancake; this causes a strong deviation from thermodynamic equilibrium. The relevant surface property is therefore the dynamic surface tension which we have evaluated using a maximum bubble pressure apparatus. We have shown that droplet retraction is drastically influenced by the adsorption kinetics of the surfactant which limits the return to equilibrium surface tension.
Droplets splashing upon films of the same fluid of various depths
2006
摘要
We explore the effects of fluid films of variable depths on droplets impacting into them. Corresponding to a range of fluid “film” depths, a non-dimensional parameter—H*, defined as the ratio of the film thickness to the droplet diameter—is varied in the range 0.1≤H*≤10. In general, the effect of the fluid film imposes a dramatic difference on the dynamics of the droplet–surface interaction when compared to a similar impact on a dry surface. This is illustrated by the size distribution and number of the splash products. While thin fluid films (H*≈0.1) promote splashing, thicker films (1≤H*≤10) act to inhibit it. The relative roles of surface tension and viscosity are investigated by comparison of a matrix of fluids with low and high values of these properties. Impingement conditions, as characterized by Reynolds and Weber numbers, are varied by velocity over a range from 1.34 to 4.22 m/s, maintaining a constant droplet diameter of 2.0 mm. The dependence of splashing dynamics, characterized by splash product size and number, on the fluid surface tension and viscosity and film thickness are discussed.
Microdroplet impact at very high velocity
2012
摘要
Water microdroplet impact at velocities up to 100 m s?1 for droplet diameters ranging from 12 to 100 ?m is studied. This parameter range covers the transition from capillary-limited to viscosity-limited spreading of the impacting droplet. Splashing is absent for all measurements; the droplets always gently spread over the surface. The maximum spreading radius is compared to several existing models. The model by Pasandideh-Fard et al. agrees well with the measured data, indicating the importance of a thin boundary layer just above the surface, in which most of the viscous dissipation in the spreading droplet takes place. As explained by the initial air layer under the impacting droplet, a contact angle of 180 degrees is used as the model input.
Formation of fingers around the edges of a drop hitting a metal plate with high velocity
Journal of Fluid Mechanics
2004
摘要
Water droplets (0.55 or 1.3 mm diameter) were photographed as they impinged on a stainless steel surface. The droplet impact velocity (10–50 m s−1) and the average roughness (0.03 or 0.23 μm) of the test surfaces were varied. The stainless steel substrate was mounted on the end of a rotating arm, giving linear velocities of up to 50 m s−1. Different stages of droplet impact were photographed by synchronizing the ejection of a single droplet with the position of the rotating arm and triggering of a camera. Finger-shape perturbations were observed around the edges of spreading droplets. The maximum diameter to which a droplet spread and the number of fingers formed around it were measured. The size and number of fingers increased with impact velocity and droplet diameter. At sufficiently high velocities, the tips of these fingers detached, producing satellite droplets. By increasing surface roughness, both the number of fingers and the maximum extent of spreading were decreased. At high impact velocities the spreading liquid film became so thin that it ruptured in several places. A mathematical model, based on linear Rayleigh–Taylor instability theory, was used to predict the wavelength of the fastest growing perturbation around a spreading droplet. The corresponding wavenumber agreed reasonably well with the number of fingers around the droplet.
Understanding the effects of the system parameters of an ultrasonic cutting fluid application system for micro-machining
2010
摘要
Tool wear in micro-milling poses a serious limitation to increased production rate, and atomized cutting fluids have been shown to be quite effective in increasing tool life in micro-milling operations. A new compact cutting fluid application system has been designed and developed based on ultrasonic atomization. In order to understand the effects of the system input parameters on system performance, two performance measures have been defined in terms of spray characteristics and experiments have been performed to evaluate the system according to the defined performance measures. Based on the experimental results, the system parameters can be adjusted to obtain the desired spray characteristics, and areas of improvement on the design have been identified.
Experimental investigation of sub-millimetre droplet impingement on to spherical surfaces
1999
摘要
This study reports an experimental investigation of the phenomena which occur when discrete, monodisperse droplets of a water-ethanol-glycerol solution in the size and velocity ranges of 160 <D <230 μm and 6 < U < 13 m/s, respectively, impinge upon a spherical surface with diameter of the order of I mm. By altering the concentrations of the water, ethanol and glycerine, the surface tension and viscosity were varied in the ranges 58 < σ < 73 mN/m and 0.0010 < μ <0.0024 kg/ms, respectively. The boundary between droplets that deposit and droplets that reatomise has been quantified and an empirical correlation in terms of kinematic and liquid properties is reported. The target surface diameter was varied in the range 800-1300 μm and compared with a plane surface of equivalent non dimensional roughness; increasing surface curvature was seen to promote the onset of reatomisation. Stroboscopic images of the impingement process showed that the impinged droplet formed a crown which was influenced by both surface roughness, varied in the range 35 nm-40 μm, anti droplet kinematic and liquid properties
Transient fuel spray impingement at atmospheric and elevated ambient conditions
2012
摘要
This study analyses detailed temporally resolved full flow field data for pre- and post-impingement fuel sprays under atmospheric and elevated ambient conditions. A comprehensive suite of diagnostic techniques is utilised comprising of phase-resolved Phase Doppler Interferometry—employing a very fine grid and velocity signature to differentiate between pre- and post-impingement droplets—high-speed imaging (utilising Mie-scattering), high-magnification shadowgraphy and an adapted instantaneous mass-rate tube. Raised ambient conditions are achieved in a high-temperature, high-pressure constant volume rig affording large optical access. Temporal data for the free and impinging sprays is compared and analysed within three phases—early, mid and late injection—defined by the mass-rate tube data. All experimental techniques employed present consistent temporal and spatial trends at atmospheric and elevated ambient conditions, and global trends are consistent with the phenomenological flow structure originally proposed by Ozdemir and Whitelaw. Detailed analysis close to the piston surface reveals approximately a threefold increase in the D50 mean spray diameter for the post-impingement droplet size distribution at the wall spray tip. It is suggested that this is due to droplet coalescence, with some supporting evidence from high-speed imaging. Comparison of transient mass deposited on the surface with model predictions show reasonable agreement with the ‘Wet’ model assumption.
Knowledge concerning splat formation: An invited review
2004
摘要
This paper summarizes our knowledge at the beginning of 2003 about splat formation. First, the analytical and numerical models related to the impact and flattening of single particles on smooth or rough substrates with different tilting are recalled. Then, the different diagnostic methods, including imaging, are briefly described. The last part of the paper is devoted to the results and their discussion. Studies are related to the effect of various parameters on particle flattening. They include the characteristics of particles prior to impact: normal impact velocity, temperature, molten state, oxidation state, etc.; the parameters related to the substrate: tilting angle, roughness, oxide layer composition, thickness and crystallinity, desorption of adsorbates and condensates, wetting properties between impacting particle and substrate, etc.; and, finally, the parameters related to the heat exchange between the flattening particle and the substrate. They depend on previous parameters and control the propagation of the solidification front within the flattening particle, eventually modifying its liquid flow. It is obvious from this review that, if our understanding of the involved phenomena has been drastically improved during the last years, many points have still to be clarified. This is of primary importance because all the coating properties are linked to the particle flattening, splat formation, and layering.
Experimental study of the impact of an ink-jet printed droplet on a solid substrate
2004
摘要
This paper reports on an experimental study of the impact of water droplets on a solid substrate, with a droplet radius between 18 and 42 μm. We optically measured the interface shape during impact. The measured impact sequences show the impact phenomenology, droplet radius as a function of time, and oscillation behavior in the later stages of impact. The measured radius during impact is compared with existing models, and some of the deficiencies of common models are shown. The measured oscillation frequency in the later stage of impact compares well with an available analytical model. In addition, we measured the volume of the small bubble, which forms in the initial impact stage, as a function of impact velocity. The measured volume compares reasonably well with an approximate model based on air entrapment.
Experimental investigation of the transient impact fluid dynamics and solidification of a molten microdroplet pile-up
2003
摘要
This paper presents an original systematic experimental investigation of the transient transport phenomena occurring during the pile-up of molten, picoliter-size liquid metal droplets. The prevailing physical mechanisms of the pile up process are identified and quantified experimentally. In terms of relevant dimensionless groups the following ranges are covered: Re ¼ 281–453, We ¼ 2:39–5:99, Ste ¼ 0:187–0:895. This corresponds to molten solder droplets impinging at velocities ranging between 1.12 and 1.74 m/s having an average diameter of 78 lm. The impact fluid dynamics, cooling and subsequent solidification of the second (top) droplet in the pile-up is strongly influenced by the geometry of the first, already solidified droplet, upon which it impinges. The solidification time depends, in addition to the thermal contact resistances at the interfaces, on the transport of heat through the solid structures above the flat wafer substrate. The total solidification time of the second droplet depends non-monotonically on the substrate temperature, initially increasing with decreasing substrate temperature. The impact velocities affect strongly the final shapes of the observed pile up structures. For decreasing Stefan number (i.e. higher substrate temperature) an increasing importance of wetting phenomena is observed. 2002 Elsevier Science Ltd. All rights reserved.
An experimental and computational strategy for an increased understanding of two-phase flow of natural gas
2007
摘要
This paper presents a research strategy aimed towards understanding transient two-phase flow in compact lowtemperature equipment, with emphasis on heat exchangers. The strategy is based on the hypotesis that to meet the many two-phase challenges occurring in compact process equipment, detailed knowledge about basic flow phenomena is needed. To provide this knowledge, a relevant computer simulation model and experimental setups are under development. The computer model is based on the level-set method, which is suitable for detailed two-phase flow calculations, and which has been shown to be able to reproduce flow phenomena observed in small channels. Two experimental setups are presented, one for measuring details regarding flow topology, such as film behaviour, droplet tear-off, droplet-droplet and droplet-film interaction, and a second for measuring integral quantities like pressure drop and heat transfer during condensation of natural gas mixtures in microtubes. The tubes range from 0.25‐1 mm in diameter and the experiments will be performed at low temperatures. This setup will provide data to design compact heat exchangers.
Harvesting of Kinetic Energy of the Raindrops
2014
摘要
This paper presents a methodology to harvest the kinetic energy of the raindrops using piezoelectric devices. In the study 1m×1m PVDF (Polyvinylidene fluoride) piezoelectric membrane, which is fixed by the four edges, is considered for the numerical simulation on deformation of the membrane due to the impact of the raindrops. Then according to the drop size of the rain, the simulation is performed classifying the rainfall types into three categories as light stratiform rain, moderate stratiform rain and heavy thundershower. The impact force of the raindrop is dependent on the terminal velocity of the raindrop, which is a function of diameter. The results were then analyzed to calculate the harvestable energy from the deformation of the piezoelectric membrane. Keywords—Raindrop, piezoelectricity, deformation, terminal velocity.
Fluid dynamics topics in bloodstain pattern analysis: comparative review and research opportunities.
Forensic science international
2013
摘要
This comparative review highlights the relationships between the disciplines of bloodstain pattern analysis (BPA) in forensics and that of fluid dynamics (FD) in the physical sciences. In both the BPA and FD communities, scientists study the motion and phase change of a liquid in contact with air, or with other liquids or solids. Five aspects of BPA related to FD are discussed: the physical forces driving the motion of blood as a fluid; the generation of the drops; their flight in the air; their impact on solid or liquid surfaces; and the production of stains. For each of these topics, the relevant literature from the BPA community and from the FD community is reviewed. Comments are provided on opportunities for joint BPA and FD research, and on the development of novel FD-based tools and methods for BPA. Also, the use of dimensionless numbers is proposed to inform BPA analyses.