Cameron Tropea

Springer handbook of experimental fluid mechanics

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Droplet-wall collisions: Experimental studies of the deformation and breakup process

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Normal impact of a liquid drop on a dry surface: model for spreading and receding

Experimental studies of liquid spray droplets impinging on a flat surface have been performed with the aim of formulating an empirical model describing the deposition and the splashing process. Monodisperse droplets with a known viscosity and surface tension, produced by a vibrating orifice generator, were directed towards a rotating disc and the impingement was visualized using an illumination synchronized with the droplet frequency. A rubber lip was used on the rotating disk to remove any film from previous depositions. The test matrix involved different initial droplet diameters (60 < d0 < 150 μm), velocities (12 < w < 18 m/s), impingement angles (4° < α < 65°), viscosities (1.0 < μ < 2.9 mPas) and surface tensions (22 < σ < 72 mN/m). The liquids used to establish the different viscosities and surface tensions were ethanol, water and a mixture of water-sucrose-ethanol. One major result from the visualization is a correlation of the deposition-splashing boundary in terms of Reynolds number and Ohnesorge number. Noteworthy is that a distinct correlation between the Re and Oh number, K = Oh · Re1.25, is only achieved if the normal velocity component of the impinging droplets is used in these dimensionless numbers. For the case of a splashing droplet, a two-component phase Doppler anemometer was used to characterize the size and velocity of the secondary droplets. The obtained droplet size distributions and correlations between droplet size and velocity around the point of impingement constitute the basis of an empirical numerical model.

Dynamic contact angle of spreading droplets: Experiments and simulations

The normal impact of a liquid drop on a dry solid surface is studied experimentally and theoretically. In this paper a strictly theoretical model is introduced, which predicts the evolution of the drop diameter. The spreading and receding phases of the impact are described by the motion of a rim appearing at the edge of the liquid film (lamella) due to the surface–tension forces. The mass and the momentum equations of the rim are considered, taking into account the effects of inertial, viscous and surface forces, and wettability. Also, simplified approximations for the maximum spreading diameter of the drop and for the velocity of the merging of the rim in the receding phase are obtained. The theoretical predictions agree well with available experimental data.

Analysis of impact of droplets on horizontal surfaces

This paper presents results of an experimental investigation of a single drop impact onto a dry, partially wettable substrate and its numerical simulation. Particularly, the drop spreading diameter and the dynamic contact angle are measured at different time instants after impact. Two surfaces, wax (low wettability) and glass (high wettability), are used to study the effect of surface wettability (static contact angle) on the impact dynamics. It is shown that existing empirical models for the dynamic contact angle (e.g., Hoffman–Voinov–Tanner law) do not predict well the change of the dynamic contact angle, especially at high capillary numbers. In addition to the experimental investigations, the drop impact was studied numerically, focusing primarily on the contact angle treatment. The singularity in the neighborhood of the moving contact line is removed from the computational domain and replaced by a local force with some dependence on the instantaneous advancing/receding contact-line velocity. The predi...

Evaporation of acoustically levitated droplets

Abstract This paper presents the results of an experimental investigation of droplets impacting on horizontal surfaces. The effects of the impact parameters on the droplet impingement are studied. The results are presented for different droplet Weber numbers, ranging from 50 to 1080 and for three liquids: water, isopropanol and glycerin. Four kinds of surfaces were used with characteristic wettability (given in terms of the contact angle): smooth glass, PVC, wax and rough glass. We studied in some detail the kinematics of the moving contact line during the spreading process. Particularly we are interested in the effects of the wettability of the wall by the liquid. The surface wettability has been observed to have a strong influence on the spreading of droplet in the later stages of the process. The results are presented in the form of charts describing the spreading diameter and apex height of droplets in terms of time.

The plane symmetric sudden-expansion flow at low Reynolds numbers

The rate of heat and mass transfer at the surface of acoustically levitated pure liquid droplets is predicted theoretically for the case where an acoustic boundary layer appears near the droplet surface resulting in an acoustic streaming. The theory is based on the computation of the acoustic eld and squeezed droplet shape by means of the boundary element method developed in Yarin, Pfaenlehner & Tropea (1998). Given the acoustic eld around the levitated droplet, the acoustic streaming near the droplet surface was calculated. This allowed calculation of the Sherwood and Nusselt number distributions over the droplet surface, as well as their average values. Then, the mass balance was used to calculate the evolution of the equivalent droplet radius in time. The theory is applicable to droplets of arbitrary size relative to the sound wavelength , including those of the order of , when the compressible character of the gas flow is important. Also, the deformation of the droplets by the acoustic eld is accounted for, as well as a displacement of the droplet centre from the pressure node. The eect of the internal circulation of liquid in the droplet sustained by the acoustic streaming in the gas is estimated. The distribution of the time-average heat and mass transfer rate over the droplet surface is found to have a maximum at the droplet equator and minima at its poles. The time and surface average of the Sherwood number was shown to be described by the expression Sh= KB= p !D0, where B = A0e=(0c0) is a scale of the velocity in the sound wave (A0e is the amplitude of the incident sound wave, 0 is the unperturbed air density, c0 is the sound velocity in air, ! is the angular frequency in the ultrasonic range, D0 is the mass diusion coecient of liquid vapour in air, which should be replaced by the thermal diusivity of air in the computation of the Nusselt number). The coecientK depends on the governing parameters (the acoustic eld, the liquid properties), as well as on the current equivalent droplet radius a. For small spherical droplets with a , K = (45=4) 1=2 =1 :89, if A0e is found from the sound pressure level (SPL) dened using A0e. On the other hand, if A0e is found from the same value of the SPL, but dened using the root-mean-square pressure amplitude (prms = A0e= p

Modeling Rotating and Swirling Turbulent Flows: A Perpetual Challenge

Detailed velocity measurements and numerical predictions are presented for the flow through a plane nominally two-dimensional duct with a Symmetric sudden expansion of area ratio 1:2. Both the experiments and the predictions confirm a symmetry-breaking bifurcation of the flow leading to one long and one short Separation zone for channel Reynolds numbers above 125, based on the upstream channel height and the maximum flow velocity upstream. With increasing Reynolds numbers above this value, the short separated region remains approximately constant in length whereas the long region increases in length. The experimental data were obtained using a one-component laser-Doppler anemometer at many Reynolds number values, with more extensive measurements being performed for the three Reynolds numbers 70, 300 and 610. Predictions were made using a finite volume method and an explicit quadratic Leith type of temporal discretization. In general, good agreement was found between measured and predicted velocity profiles for all Reynolds numbers investigated.

Estimation of turbulent velocity spectra from laser Doppler data

Severaltypesofrotatingandswirlinge owsfora rangeofReynoldsnumbersandrotationratesorswirlintensities have been studied computationally, aimed at identifying specie c features that require special consideration in turbulence modeling. The e ows considered include turbulent channel e ows subjected to streamwise and spanwise rotation,withstationaryandmovingboundaries;developingandfullydevelopede owsinaxiallyrotatingpipes;and swirling e owsin combustorgeometriesand long pipes.Computationsperformed with threeversionsof thesecondmoment closure and two eddy-viscosity models show that the second-moment models are superior, especially when the equations are integrated up to the wall. Such models reproduced the main e ow parameters for all e ows considered in acceptable agreement with the available experimental data and direct numerical simulations. However,challengesstillremaininpredictingaccuratelysomespecie ce owfeatures,suchascapturingthetransition from a freevortex to solid-body rotation in a long straight pipewith a weak swirl, or reproducing the normal stress components in the core region. Also, the so-called uw anomaly in fully developed e ows with streamwise rotation remains questionable. For rotating e ows, the low-Reynolds-number models yield a somewhat premature e ow relaminarization at high rotation speeds.

On the acoustic levitation of droplets

We review the problem of spectral estimation from velocity data sampled irregularly in time by a laser Doppler anemometer (LDA) from very early estimators based on slot correlation to more refined estimators, which build upon a signal reconstruction and an equidistant re-sampling in time. The discussion is restricted to single realization anemometry, i.e. excluding multiple particle signals. We classify the techniques and make an initial assessment before describing currently used methods in more detail. An intimately related subject, the simulation of LDA data, is then briefly reviewed, since this provides a means of evaluating various estimators. Using the expectation and variance as figures of merit, the advantages and disadvantages of several estimators for varying types of turbulent velocity spectral distributions are discussed. A set of recommendations is put forward as a conclusion.