  We propose a method to visualize vortex cores based on manipulation of the
pressure field produced by isolated vortices in incompressible flow. Under
ideal conditions the function $D=2|\nabla P|/\nabla^2 P$ yields an approximate
distance to vortex centerlines. As opposed to local methods to identify
coherent structures, isosurfaces of $D$ produce a field of vortex tubes
equidistant to the vortex core center which, ideally, are independent of vortex
intensity or size. In contrast to other line-vortex identification methods,
which typically rely on algorithms to detect vortex core lines and frequently
need complex implementations, the proposed method can be computed from the
local Eulerian velocity and pressure fields as easily as vortex identification
methods such as the $Q$ and $\lambda_2$ criteria. $2|\nabla P|/\nabla^2 P$
results in the exact distance to the core center for a Rankine vortex and is in
general valid for the region of a vortex where there is pure rotation, yielding
an approximation to the distance farther from the core in other simple
one-dimensional vortex models. The methodology performs well in all tests we
attempted, though limitations are presented and discussed. The method is
demonstrated for a canonical Burgers vortex, a Bodewadt vortex, homogeneous
isotropic turbulent flow, the wake of a propeller, a heaving plate, a turning
containership and the airwake of a surface combatant. The proposed method helps
to better visualize vortical flow fields by displaying vortex cores,
complementing methods like $Q$ and $\lambda_2$ which display vortical volumes.
