A weakly nonlinear stability analysis of an axisymmetric viscous liquid jet is performed. The calculation is based on a small-amplitude perturbation method and restricted to second order. Contrary to the inviscid jet and the planar viscous sheet cases studied by Yuen in 1968 [1] and Yang et al. in 2013 [2], respectively, a part of the solution results from a polynomial approximation of Bessel functions. Results on interface shapes for a small wave number and initial perturbation amplitude, four different Ohnesorge numbers, taking into account the approximate part or not, are used to predict the influence of liquid viscosity on satellite drop formation and evaluate the influence of the approximation. It is observed that the liquid viscosity has a retarding effect on satellite drop formation, in agreement with previous experimental and numerical work. In addition, it is found that the approximate terms can be reasonably ignored, providing a simpler viscous weakly nonlinear model for the description of the first nonlinearity growth in liquid jets. The present work replaces the ILASS 2016 paper [3] by the authors on the same subject.
KeywordsViscous liquid jet, nonlinear capillary instability, satellite drop formation.
IntroductionA liquid jet breaks up forming main and satellite drops. The present work is concerned with the influence of both liquid viscosity and nonlinearities on satellite drop formation. The first linear stability analysis of the capillary instability of a liquid jet in an ambient medium was conducted by Rayleigh [4,5], more than a century ago. In this reference work, the liquid is assumed inviscid and the ambient medium is the vacuum. His analysis shows that, in order to destabilize the jet, the wavelength λ of a varicose surface disturbance must be greater than the circumference of the undeformed circular jet cross section, as observed initially by Savart and Plateau [6,7]. Two associated amplitude growth rates correspond to such an unstable perturbation wavelength, one being the opposite of the other, with the magnitude given by the well-known Rayleigh's linear dispersion relation for the inviscid jet in a vacuum. The effect of liquid viscosity was then investigated by Weber [8], about fifty years later. The generalization of the previous dispersion relation introduces a growth rate and viscosity dependent modified wave number making the dispersion relation transcendental, yet numerically solvable, which is turned into a closed-form expression in the long-wave approximation. Contrary to the inviscid case, there are two different real growth rates with opposite signs for each unstable wavelength, with distinct absolute values below the corresponding inviscid one. This difference leads to dispersion relation curves always lower than the inviscid ones, as expected by the classical damping effect inferred from liquid viscosity. Due to the linearity of the previous analysis, the interaction of disturbances with different wavelengths is not accounted for, and the drops produced by the jet br...