1. Objective Minimally invasive percutaneous dilatational techniques of tracheostomy (PDT) are increasingly preferred to conventional surgical methods.

2. For Compressible fluids, the relationship between stress and deformation is modified to be (Birdetal.) τ =−µγ˙ + 2 3 µ−κ (∇·v)I NewtonianConstitutive equationfor Compressiblefluids (3.186) The parameter κ is the dilatational or bulk viscosity, a coefficient that expresses viscous