Nasser Ashgriz

Coalescence and separation in binary collisions of liquid drops

An extensive experimental investigation of the binary collision dynamics of water drops for size ratios of 1. 0.75, and 0.5, for the Weber-number range of 1 to 100, and for all impact parameters is reported. Two different types of separating collisions, namely reflexive and stretching separations, are identified. Reflexive separation is found to occur for near head-on collisions, while stretching separation occurs for large-impact-parameter collisions. The boundaries between both of the separating collisions and coalescence collision are found experimentally. Theoretical models for predictions of the reflexive and stretching separation are also given.

FLAIR: fluz line-segment model for advection and interface reconstruction

Abstract A computational technique for solving fluid problems with free surfaces and interfaces is presented. The conventional cell volume fraction approach is employed for tracking the interfaces. However, for surface advection and its reconstruction, a new and more accurate FLAIR (flux line-segment model for advection and interface reconstruction) algorithm is developed. The surface is approximated by a set of line segments fitted at the boundary of every two neighboring computational cells. A criterion is developed for identifying the line-segment orientation by inspecting the cell volume fractions. The new cell volume fraction field is obtained by integrating the advected area underneath the interface line-segment. As an example, this technique is applied to the capillary driven viscous flow of an initially elliptic, two-dimensional fluid zone. The problem is posed mathematically as a solution of the Navier-Stokes equations with moving free surface boundary conditions. The damping motion of the fluid zone is observed through transport of the free surface, which is related to the instantaneous internal velocity field under the influence of surface tension and viscous forces.

Handbook of Atomization and Sprays

This chapter deals with capillary instability of straight free liquid jets moving in air. It begins with linear stability theory for small perturbations of Newtonian liquid jets and discusses the unstable modes, characteristic growth rates, temporal and spatial instabilities and their underlying physical mechanisms. The linear theory also provides an estimate of the main droplet size emerging from capillary breakup. Formation of satellite modes is treated in the framework of either asymptotic methods or direct numerical simulations. Then, such additional effects like thermocapillarity, or swirl are taken into account. In addition, quasi-one-dimensional approach for description of capillary breakup is introduced and illustrated in detail for Newtonian and rheologically complex liquid jets (pseudoplastic, dilatant, and viscoelastic polymeric liquids).

Temporal analysis of capillary jet breakup

The temporal instability of a cylindrical capillary jet is analysed numerically for different liquid Reynolds numbers Re , disturbance wavenumbers k , and amplitudes e 0 . The breakup mechanism of viscous liquid jets and the formation of satellite drops are described. The results show that the satellite size decreases with decreasing Re , and increasing k and e 0 . Marginal Reynolds numbers below which no satellite drops are formed are obtained for a large range of wavenumbers. The growth rates of the disturbances are calculated and compared with those from the linear theory. These results match for low- Re jets, however as Re is increased the results from the linear theory slightly overpredict those from the nonlinear analysis. (At the wavenumber of k = 0.9, the linear theory underpredicts the nonlinear results.) The breakup time is shown to decrease exponentially with increasing the amplitude of the disturbance. The cut-off wavenumber is shown to be strongly dependent on the amplitude of the initial disturbance for amplitudes larger than approximately

Coalescence collision of liquid drops

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Characteristics of liquid sheets formed by two impinging jets

of the initial jet radius. The stable oscillations of liquid jets are also investigated. The results indicate that liquid jets with Re ∼ O (1) do not oscillate, and the disturbances are overdamped. However, liquid jets with higher Re oscillate with a period which depends on Re and e 0 . The period of the oscillation decreases with increasing Re at small e 0 ; however, it increases with increasing Re at large e 0 . Marginal Reynolds numbers below which the disturbances are overdamped are obtained for a wide range of wavenumbers and e 0 = 0.05.

Nonlinear dynamics of capillary bridges: experiments

The coalescence collision of two liquid drops is studied using a Galerkin finite element method in conjunction with the spine-flux method for the free surface tracking. The effects of Reynolds number, impact velocity, drop size ratio, and internal circulation on the coalescence process is investigated. The long time oscillations of the coalesced drop are also studied and curves for the variations of the period and decay factor are provided as a function of the number of oscillations. In the study of non-equal-size drop collision, traces of different fluid particles are calculated to illustrate the liquid mixing during the collision. 2002 Elsevier Science Ltd. All rights reserved.

Binary collision dynamics of fuel droplets

An experimental investigation of liquid sheets formed by the impingement of two capillary liquid jets is conducted. The breakup mechanism of the sheet is categorized into two main regimes and five subregimes based on the experimental observations. Two types of Reynolds numbers (jet Reynolds number and sheet Reynolds number) are introduced to correlate with and map the reported breakup regimes. Breakup length and width are introduced, and their analytical models are derived. Both the analytical and experimental results show that the breakup length and width are linearly proportional to the Weber number of individual jets in the case of closed-rim sheets. The slopes of the two linear relations are dependent on the impinging angle. The distribution of fluid velocity in the sheet is examined and found to be in disagreement with the assumption of early models, which claims uniform velocity across the sheet. The nonuniform distribution of fluid velocity in the sheet causes a discrepancy between analytically pre...

Satellite formation and merging in liquid jet breakup

An experimental investigation of forced and free oscillations of liquid bridges positioned between two rods of equal diameter is presented. Both the resonance frequencies and damping rates for different aspect ratios of the bridge are reported. The damping rate data of the liquid bridges are obtained by high-speed videography and are the first ever reported. Resonance frequencies for the three modified Reynolds numbers of 14, 295 and 1654, and damping rates for the two modified Reynolds numbers of 14 and 295 are reported. These values of modified Reynolds numbers are generated by using ethylene glycol, distilled water, and mercury in small bridges. Gravitational effects are kept small by reducing the size of the capillary bridge. The internal flow fields of several bridges for different modified Reynolds numbers are described based on high-speed visualization. Experimental results show good agreement with results of linear and nonlinear theory.

Mixing Mechanisms in a Pair of Impinging Jets

Abstract An experimental study has been carried out in which the collision dynamics of two n -haxane fuel droplets are studied. The experiments are performed on the collision of two burning droplets, as well as two nonburning droplets, to assess the influence of the high temperature combustion environment on the dynamics of the collision. The results indicate that as the Weber number is increased, the collision type moves toward higher energy collision, and for the same Weber number, different types of collisions, depending on the local value of the collision impact parameter, may occur. In the range of the Weber numbers studied, the results show that for the nonburning droplets, the collision type can be bouncing, grazing, temporary coalescence-satellite generating, or permanent coalescence, depending on the local value of the impact parameter. For the burning droplets in the same initial Weber number range, only temporary coalescence and permanent coalescence are observed.