  In this fluid dynamics video prepared for the APS-DFD Gallery of Fluid Motion
we study the Kelvin-Helmholtz instability for both Newtonian and viscoelastic
jets. The nonlinear dynamics of the jet motion are slowed down by orders of
magnitude using a synchronized strobe effect coupled with precise timing
control of perturbation frequencies. Our results show that at high wave-numbers
the imposed perturbations initially grow linearly with time and the jet axis
remains straight while the Kelvin-Helmholtz wave amplitude grows and rolls up
into bags that encapsulate the central jet within themselves. At low
wave-numbers (long wave-lengths) the jet axis buckles under the action of
viscous stresses and a coupling between the Kelvin-Helmholtz instability and
bending of the jet leads to new concertina or chevron modes which grow with
time as they move downstream. Addition of viscoelasticity to the jet leads to
the pronounced inhibition of the Kelvin-Helmholtz instability as the jet
perturbation amplitude grows and large elongational stresses in the fluid
become important. For long waves, the initially-relaxed viscoelastic jet first
buckles in a manner similar to the Newtonian solvent but once again the
viscoelastic effects suppress the instability growth as they are convected
downstream.
