  It has been recently found that the local fluctuations of the QSO's
Ly$\alpha$ absorption spectrum transmitted flux show spiky structures. This
implies that the mass fields of the intergalactic medium (IGM) is intermittent.
This feature cannot be explained by the clustering evolution of cosmic mass
field in the linear regimes and is also difficult to incorporate into the
hierarchical clustering scenario. We calculate the structure functions and
intermittent exponent of the IGM and HI for full hydrodynamical simulation
samples. The result shows the intermittent features of the Ly$\alpha$
transmitted flux fluctuations as well as the mass field of the IGM. We find
that within the error bars of current data, all the intermittent behavior of
the simulation samples are consistent with the observation. This result is
different from our earlier result (Pando et al 2002), which shows that the
intermittent behavior of samples generated by pseudo-hydro simulation cannot be
fitted with observed data. One difference between the pseudo-hydro and full
hydro simulations is in treating the dynamical relation between the IGM (or HI)
and dark matter fields. The former assumes that the IGM density distribution
traces the underlying dark matter point-by-point on scales larger than the
Jeans length in either the linear or nonlinear regimes. However, hydrodynamic
studies have found that a statistical discrepancy between the IGM field and
underlying dark matter in nonlinear regime is possible. We find that the
point-by-point correlation between the IGM density perturbations and dark
matter become weaker on comoving scales less than 2 h$^{-1}$ Mpc (in LCDM
model), which is larger than the IGM Jeans length.
