  Using a Navier-Stokes isotropic turbulent field numerically simulated
(Biferale et al., Phys. Fluids (2005)), we show that the probability of having
a stretching-tilting larger than twice the local enstrophy is negligible. By
using an anisotropic filter in the Fourier space we analyze these survival
statistics when the large, the small inertial or the small inertial and
dissipation scales are filtered out. The probability that the ratio (|{\omega}
\dot U|/|{\omega}|^2) is higher than a given threshold is higher in the
unfiltered isotropic field than in the fields where the large scales were
filtered out and is lower than in the fields were the small inertial and
dissipative scales are filtered out. This is due to the suppression of compact
structures in the removed ranges. The partial removal of the background of
filaments and sheets does not have a first order effect on these statistics.
These results are discussed in the light of a hypothesized relation between
vortical filaments, sheets and blobs. The study can be viewed as a test for
this idea and tries to highlight its limits. A qualitative relation in physical
space and in Fourier space can be supposed to exist for blobs only. That is for
the near isotropic structures which are sufficiently described by a single
spatial scale and do not suffer from the disambiguation problem as filaments
and sheets do. Information is also given on the filtering effect on the angles
between the strain rate tensor eigenvectors and the vorticity: eigenvector 2
reduces its alignment, while eigenvector 3 reduces its misalignment. All
filters increase the gap between the most extensional eigenvalue and the
intermediate one and the gap between this last and the contractile eigenvalue.
When the large scales are missing, eigenvalue modulus 1 and 3 become nearly
equal, similar to the modulus of the related components of the enstrophy
production.
