On spray formation

We depict and analyse the successive steps of atomization of a liquid Jet when a fast gas stream blows parallel to its surface. Experiments performed with various liquids in a fast air flow show that the liquid destabilization proceeds from a two-stage mechanism: a shear instability first forms waves on the liquid. The transient acceleration experienced by the liquid suggests that a Rayleigh-Taylor type of instability is triggered at the wave crests, producing liquid ligaments which further stretch in the air stream and break into droplets. The primary wavelength lambda similar to delta(rho(1)/rho(2))(1/2) is set by the vorticity thickness 6,:in the fast air stream and the liquid/gas density ratio rho(1)/rho(2). The transverse corrugations of the crests have a size lambda(perpendicular to) similar to delta We(delta)(-1/3)(rho(1)/rho(2))(1/3), where We(delta) = rho(2)u(2)(2)delta/sigma is the Weber number constructed on the gas velocity u(2) and liquid surface tension sigma. The ligament dynamics gives rise, after break-up, to a well-defined droplet size distribution whose mean is given bylambda(perpendicular to). This distribution bears an exponential tail characteristic of the broad size statistics in airblast sprays.