  We present hydrodynamical simulations of the photoevaporation of a cloud with
large-scale density gradients, giving rise to an ionized, photoevaporation
flow. The flow is found to be approximately steady during the large part of its
evolution, during which it can resemble a "champagne flow" or a "globule flow"
depending on the curvature of the ionization front. The distance from source to
ionization front and the front curvature uniquely determine the structure of
the flow, with the curvature depending on the steepness of the lateral density
gradient in the neutral cloud. We compare these simulations with both new and
existing observations of the Orion nebula and find that a model with a mildly
convex ionization front can reproduce the profiles of emission measure,
electron density, and mean line velocity for a variety of emitting ions on
scales of 10^{17} to 10^{18} cm. The principal failure of our model is that we
cannot explain the large observed widths of the [O I] 6300 Angstrom line that
forms at the ionization front.
