  We observe oscillatory decay in the two-point, non-equal time, velocity
correlation function of homogeneous, isotropic turbulence. We found this
through a direct numerical simulation (DNS) of the three dimensional
Navier-Stokes ($3-D$ NS) equation. We give an approximate analytic theory which
explains this oscillatory behaviour. The wave-number and frequency dependent
effective viscosity turns out to be complex; the imaginary part gives rise to
the temporal oscillation. We find that, at least for the decay at short times,
data collapse occur among the inertial range velocity wave-vector modes with
the long time dynamic exponent $z=2/3$, but the time period of the temporal
oscillation is not universal.
