  Recent Chandra X-ray observations of jets in central cluster galaxies show
interesting features around the symmetry plane. We have carried out simulations
involving bipolar jets, removing the artificial boundary condition at the
symmetry plane. We use a very low jet density (IGM/jet roughly 10000) and take
into account a decreasing density profile. We find that the jet bow shock
undergoes two phases: First a nearly spherical one and second the well-known
cigar-shaped one. We propose Cygnus A to be in a transition phase, clear signs
from both phases. Due to inward growing of Kelvin Helmholtz instabilities (KHI)
between cocoon and shocked IGM, mass entrainment is observed predominantly in
the symmetry plane. We propose this mechanism to produce some of the so far
enigmatic X-ray features in the symmetry plane in Cygnus A.
