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Minimal formulation of the linear spatial analysis of capillary jets: Validity of the two-mode approach

H. González* and P. A. Vazquez

F. J. García

J. Guerrero

  • Departamento de Física Aplicada III, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos, s/n, 41092 Sevilla, Spain

  • Departamento de Física Aplicada I, Escuela Politécnica Superior, Universidad de Sevilla, c/ Virgen de África, 7, 41011 Sevilla, Spain

  • Department of Chemistry and Physics, Augusta University, 1120 15th Street, SCI W3005, Augusta, Georgia 30912, USA

  • *helio@us.es
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Phys. Rev. Fluids 3, 044802 – Published 18 April, 2018

DOI: https://doi.org/10.1103/PhysRevFluids.3.044802

Abstract

A rigorous and complete formulation of the linear evolution of harmonically stimulated capillary jets should include infinitely many spatial modes to account for arbitrary exit conditions [J. Guerrero et al., J. Fluid Mech. 702, 354 (2012)]. However, it is not rare to find works in which only the downstream capillary dominant mode, the sole unstable one, is retained, with amplitude determined by the jet deformation at the exit. This procedure constitutes an oversimplification, unable to handle a flow rate perturbation without jet deformation at the exit (the most usual conditions). In spite of its decaying behavior, the other capillary mode (subdominant) must be included in what can be called a “minimal linear formulation.” Deformation and mean axial velocity amplitudes at the jet exit are the two relevant parameters to simultaneously find the amplitudes of both capillary modes. Only once these amplitudes are found, the calculation of the breakup length may be eventually simplified by disregarding the subdominant mode. Simple recipes are provided for predicting the breakup length, which are checked against our own numerical simulations. The agreement is better than in previous attempts in the literature. Besides, the limits of validity of the linear formulation are explored in terms of the exit velocity amplitude, the wave number, the Weber number, and the Ohnesorge number. Including the subdominant mode extends the range of amplitudes for which the linear model gives accurate predictions, the criterion for keeping this mode being that the breakup time must be shorter than a given formula. It has been generally assumed that the shortest intact length happens for the stimulation frequency with the highest growth rate. However, we show that this correlation is not strict because the amplitude of the dominant mode has a role in the breakup process and it depends on the stimulation frequency.

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Phys. Rev. Fluids 3, 044802– Published 18 April, 2018
  • Received 21 February 2017
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