  Kelvin-Helmholtz instabilities are common in astrophysical systems, ranging
from jet black holes up to protoplanetary accretion disk. An astrophysical
object with strong characteristics of the Kelvin-Helmholtz instability is
Caraguejo Nebula, in which the material expansion was caused by the explosion
of a supernova about 1000 years ago. This instability occurs at the boundary
between two fluids of different densities when one of the fluids accelerated
with respect to the other. In order to study this instability, we performed a
simulation with the code ATHENA Eulerian mesh. For this simulation, we consider
a square domain with periodic boundaries on the sides, and reflecting on the
boundary of the top and bottom. The upper box is filled with a gas density
{\rho}=1.0, pressure P1 = 1.0, adiabatic index {\gamma}=5/3, and velocity
u1=0.03 in the x direction (to the right). The lower portion has a density
{\rho}=2.0, the same pressure, velocity, and adiabatic index, only in the
opposite direction to the left. Speed is defined as a sinusoidal function,
which creates the initial disturbance. As a result, we observe the principle of
instability and the formation of vortices, with well-defined ridges. The
distinctness of the boundary between the material of high and low density is
well preserved due to the relatively low diffusion algorithm. We also note that
the simulation evolving vortices formed from the turmoil merge.
