  The effect of viscous drag and surface tension on the nonlinear two fluid
inter facial structures induced by Rayleigh -Taylor instability and
Richtmyer-Meshkov instability are investigated.Viscosity and surface tension
play important roles on the fluid instabilities. It is seen that the magnitude
of the suppression of the terminal growth rate of the tip of the bubble height
depends only on the viscous coefficient of the upper (denser) fluid through
which the bubble rises and surface tension of the interface. But in regard to
spike it is shown that in an inviscid fluid spike does not remain terminal but
approaches a free fall as the Atwood number A increases. In this respect there
exits qualitative agreement with simulation result as also with some earlier
theoretical results. Viscosity reduces the free fall velocity appreciably and
with increasing viscosity tends to make it terminal. Results obtained from
numerical integration of the relevant nonlinear equations describing the
temporal development of the spike support the foregoing observations.
