scholarly journals Droplet deformation by short laser-induced pressure pulses

2017 ◽  
Vol 828 ◽  
pp. 374-394 ◽  
Author(s):  
Sten A. Reijers ◽  
Jacco H. Snoeijer ◽  
Hanneke Gelderblom
Keyword(s):  
Pulse Duration ◽  
Pressure Pulse ◽  
Liquid Droplet ◽  
Density Fluctuations ◽  
Total Momentum ◽  
Shape Evolution ◽  
Analytic Model ◽  
Droplet Deformation ◽  
Droplet Shape ◽  
Free Falling

When a free-falling liquid droplet is hit by a laser it experiences a strong ablation-driven pressure pulse. Here we study the resulting droplet deformation in the regime where the ablation pressure duration is short, i.e. comparable to the time scale on which pressure waves travel through the droplet. To this end, an acoustic analytic model for the pressure, pressure impulse and velocity fields inside the droplet is developed in the limit of small density fluctuations. This model is used to examine how the droplet deformation depends on the pressure pulse duration while the total momentum to the droplet is kept constant. Within the limits of this analytic model, we demonstrate that when the total momentum transferred to the droplet is small the droplet shape evolution is indistinguishable from an incompressible droplet deformation. However, when the momentum transfer is increased the droplet response is strongly affected by the pulse duration. In this later regime, compressed flow regimes alter the droplet shape evolution considerably.

Numerical Investigation of a Liquid Droplet Impinging on a Heated Surface

2009 ◽  
Author(s):  
Andres Diaz ◽  
Alfonso Ortega ◽  
Ryan Anderson
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Fundamental Problem ◽  
Water Drop ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Heat Removal ◽  
Droplet Spreading ◽  
Droplet Shape ◽  
Spreading Dynamics

Previous studies, most of them experimental, reveal that the cooling effectiveness of a water drop impinging on a heated surface depends on the wall temperature, droplet shape and velocity. All previous studies focus on the behavior of a droplet falling in a quiescent environment, such as still air. Evidence in the literature also shows that gas assisted droplet sprays, in which a gas phase propels the droplets, are more efficient in heat removal than sprays consisting of droplets alone. It is conjectured that this is due to an increase in the maximum droplet spreading diameter upon impact, a thinner film, and consequently an increase in the overall heat transfer coefficient. Recent experiments in the author’s group [1, 2] show that the carrier gas jet strongly influences droplet spreading dynamics by imposing normal and shear forces on the liquid surface. The heat transfer is greatly augmented in the process, compared to a free falling droplet. To date, there has been no fundamental investigation of the physics of gas assisted spray cooling. To begin to understand the complicated process, this paper reports on a fundamental problem of a single liquid droplet that impinges on a heated surface. This paper contributes a numerical investigation of the problem using the volume of fluid (VOF) technique to capture droplet spreading dynamics and heat transfer in a single drop event. The fluid mechanics is investigated and compared to the experimental data. The greatest uncertainty in the simulation is in the specification of the contact angle of the advancing or receding liquid front, and in capturing the onset of the three-dimensional fingering phenomena.

The shape evolution of liquid droplets in miscible environments

2018 ◽  
Vol 852 ◽  
pp. 422-452 ◽  
Author(s):  
Daniel J. Walls ◽  
Eckart Meiburg ◽  
Gerald G. Fuller
Keyword(s):  
Sessile Drop ◽  
Liquid Droplet ◽  
Leading Edge ◽  
Numerical Approach ◽  
Shape Evolution ◽  
Liquid Droplets ◽  
Miscible Liquids ◽  
Experimental Findings ◽  
Sessile Drops ◽  
Pendant Drops

Miscible liquids often come into contact with one another in natural and technological situations, commonly as a drop of one liquid present in a second, miscible liquid. The shape of a liquid droplet present in a miscible environment evolves spontaneously in time, in a distinctly different fashion than drops present in immiscible environments, which have been reported previously. We consider drops of two classical types, pendant and sessile, in building upon our prior work with miscible systems. Here we present experimental findings of the shape evolution of pendant drops along with an expanded study of the spreading of sessile drops in miscible environments. We develop scalings considering the diffusion of mass to group volumetric data of the evolving pendant drops and the diffusion of momentum to group leading-edge radial data of the spreading sessile drops. These treatments are effective in obtaining single responses for the measurements of each type of droplet, where the volume of a pendant drop diminishes exponentially in time and the leading-edge radius of a sessile drop grows following a power law of $t^{1/2}$ at long times. A complementary numerical approach to compute the concentration and velocity fields of these systems using a simplified set of governing equations is paired with our experimental findings.

Comparison of Surface Tension Models in Smoothed Particles Hydrodynamics Method

2019 ◽  
Author(s):  
Lijing Yang ◽  
Milad Rakhsha ◽  
Dan Negrut
Keyword(s):  
Surface Tension ◽  
Liquid Droplet ◽  
Surface Force ◽  
Accurate Estimation ◽  
Droplet Deformation ◽  
Two Phase ◽  
Mesh Free ◽  
Interface Curvature ◽  
Particle Force ◽  
Smoothed Particles Hydrodynamics

Abstract We compare two surface tension models to solve two-phase fluid interaction problems in the context of the mesh-free Smoothed Particles Hydrodynamics (SPH) method. The Continuum Surface Force (CSF) model (later extended to Continuum Surface Stress, CSS), originally derived from grid-based numerical methods, requires an accurate estimation of the interface curvature to express the surface tension. Unlike CSF, the Inter-Particle Force (IPF) model is more robust in this regard as it draws on a molecular dynamics foundation by considering how the pairwise interaction forces between particles within a cutoff distance act in relation to producing the surface tension. Herein, we rely on second-order consistent gradient and Laplacian operators to improve the accuracy of SPH formulations as well as on a particle shifting technique to “disorder” particles from non-differentiable interface geometries. A 3D liquid droplet deformation test is used to compare CSF and IPF in terms of their pressure field and kinetic energy dissipation accuracy.

scholarly journals Limitation of symmetry breaking by gravitational collapse: the revisit of Lin–Mestel–Shu instability

2020 ◽  
Vol 498 (1) ◽  
pp. 310-319
Author(s):  
Tirawut Worrakitpoonpon
Keyword(s):  
Gravitational Collapse ◽  
Power Law ◽  
Particle Number ◽  
Density Fluctuations ◽  
Shape Evolution ◽  
Critical Number ◽  
Initial State ◽  
Final State ◽  
Numerical Work ◽  
The Difference

ABSTRACT We revisit the topic of shape evolution during the spherical collapse of an N-body system. Our main objective is to investigate the critical particle number below which, during a gravitational collapse, the amplification of triaxiality from initial fluctuations is effective, and above which it is ineffective. To this aim, we develop the Lin–Mestel–Shu theory for a system of particles initially with isotropic velocity dispersion and with a simple power-law density profile. We first determine, for an unstable cloud, two radii corresponding to the balance of two opposing forces and their fluctuations: such radii fix the sizes of the non-collapsing region and the triaxial seed from density fluctuations. We hypothesize that the triaxial degree of the final state depends on which radius is dominant prior to the collapse phase leading to a different scheme of the self-consistent shape evolution of the core and the rest of the system. The condition where the two radii are equal therefore identifies the critical particle number, which can be expressed as the function of the parameters of initial state. In numerical work, we can pinpoint such a critical number by comparing the virialized flattening with the initial flattening. The difference between these two quantities agrees with the theoretical predictions only for the power-law density profiles with an exponent in the range [0, 0.25]. For higher exponents, results suggest that the critical number is above the range of simulated N. We speculate that there is an additional mechanism, related to strong density gradients that increases further the flattening, requiring higher N to further weaken the initial fluctuations.

Shape evolution of free falling deformed droplets

2015 ◽  
Vol 60 (10) ◽  
pp. 917-921
Author(s):  
Yuan LI ◽  
Zhen CHEN ◽  
XingGuo GENG ◽  
XiaoPeng CHEN ◽  
DuYang ZANG ◽  
...  
Keyword(s):  
Shape Evolution ◽  
Free Falling

Deformation of a Droplet in a Channel Flow

2008 ◽  
Vol 5 (4) ◽  
Author(s):  
Ebrahim Shirani ◽  
Shila Masoomi
Keyword(s):  
Surface Tension ◽  
Channel Flow ◽  
Polymer Electrolyte Membrane ◽  
Droplet Deformation ◽  
Navier Stokes Equation ◽  
Droplet Shape ◽  
Viscous Forces ◽  
Electrolyte Membrane ◽  
Wide Range ◽  
Surrounding Fluid

Formation of droplets especially in microchannels, micro-electro-mechanical systems (MEMS) and polymer electrolyte membrane fuel cells and their effects on the performance of these devises, as well as scientific aspect of the droplet behavior in the fluid flow motion, makes the subject of the droplet deformation and motion an attractive problem. In this work, we numerically simulate the deformation of a drop of water attached to the wall of a channel flow using full two-dimensional Navier–Stokes equation and the volume-of-fluid method for capturing the interface. The effects of channel inlet velocity, the density and viscosity of the surrounding fluid, and the surface tension coefficient on the flow structures both inside and outside of the droplet as well as the deformation of the droplets are examined. Several test cases, which cover rather wide range of the Reynolds and capillary numbers, based on the surrounding fluid properties and the diameter of the droplet are performed. The Reynolds number, Re, range is from 24 to 1800 and the capillary number, Ca, is from 0.014 to 0.219. It is found that the droplet shape changes and depending on the capillary and Reynolds numbers, it eventually reaches an equilibrium state when there is balance between the surface tension, inertia, and the viscous forces. It is also found that the deformation of the droplet does not depend on the capillary numbers, when Ca is small, but it is a strong function of Ca, when it is large.

The transient force profile of low-speed droplet impact: measurements and model

2019 ◽  
Vol 867 ◽  
pp. 300-322 ◽  
Author(s):  
Benjamin R. Mitchell ◽  
Joseph C. Klewicki ◽  
Yannis P. Korkolis ◽  
Brad L. Kinsey
Keyword(s):  
High Speed ◽  
Liquid Droplet ◽  
Time Profile ◽  
Momentum Transport ◽  
Droplet Shape ◽  
Lumped Model ◽  
Low Speed ◽  
Long Time ◽  
Force Time ◽  
Transient Force

The transient force exerted by a low-speed liquid droplet impinging onto a flat rigid surface is investigated experimentally. The measurements employ a high-sensitivity piezo-electric sensor, along with a high-speed camera, and cover four decades in droplet Reynolds number and greater than two decades in Weber number. Across these ranges, the peak of individual force profiles span from 3 mN to over 1300 mN. Once normalised, the force–time profiles support the existence of an inertially dominated self-similar regime. Within this regime, previous numerical and theoretical studies predict a $\sqrt{t}$ dependence of impact normal force during the initial pre-peak rise. While our measurements confirm this finding, they also indicate that, after the peak force the profiles exhibit an exponential decay. This long-time decay law suggests treatment of the momentum transport from the droplet using a lumped model. An observed linear dependence between the force and force decay rate supports this approach. The reason for the efficacy of treating this system via a lumped model apparently connects to the physics right at the surface that limit the rate of momentum transport from the droplet to the surface. This is explored by estimating the momentum transfer by solely using the deforming droplet shape, but under the condition of negligible momentum gradients within the droplet. The short- and long-time solutions are combined and the resulting model equation is shown to accurately cover the entire force–time profile.

Experimental and Model Studies of Various Size Water Droplet Impacting On a Hydrophobic Surface

2021 ◽  
Author(s):  
Abba Abdulhamid Abubakar ◽  
Bekir Sami Yilbas ◽  
Hussain Al-Qahtani ◽  
Ghassan Hassan ◽  
Mubarak Yakubu ◽  
...  
Keyword(s):  
High Speed ◽  
Hydrophobic Surface ◽  
Sample Surface ◽  
Liquid Pressure ◽  
Droplet Deformation ◽  
Droplet Motion ◽  
Droplet Shape ◽  
Model Studies ◽  
High Speed Data ◽  
Droplet Liquid

Abstract Impacting droplet on a hydrophobic surface is investigated and droplet size effect on impacting properties is examined. Liquid pressure variation inside droplet is numerically simulated in the impacting and rebounding periods. Droplet motion on impacted hydrophobic surface is monitored using a high-speed recording system. We showed that predictions and high-speed data for droplet shape and geometric features appear to be almost identical in the spreading and retraction of the droplet on sample surface. Increased volume of droplet gives rise to the peak pressure enhancement in droplet liquid during impact. The maximum droplet height remains larger for large volume droplets in both spreading and retraction cycles. Increasing size of droplet enlarges the wetting diameter on the impacted surface during droplet deformation on sample surfaces. The rate of peak velocity of the spreading surface of the droplet is faster for small droplets as compared to that corresponding to large droplets. The ratio of spreading period over the retraction period of the droplet becomes small for droplets with small size.

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