The planar flow of incompressible fluid past a blunt obstacle mounted on a
flat (horizontal) fixed solid surface of infinite extent is examined in the
presence of an incident linear velocity profile, modelling the fluid behav
iour close to a small surface roughness for instance. The motion is taken t
o be steady and laminar. The obstacle is blunt in the sense that its typica
l surface slopes are not small, a feature which here always induces flow se
paration both upstream and downstream of the obstacle. Computations and non
linear theory are applied, together with comparisons. The direct computatio
ns of the Navier-Stokes equations, using for example a higher order upwind-
difference scheme, deal with a moderate range of Reynolds numbers up to 200
, based on the obstacle height and the incident uniform shear. In addition
the accuracy is necessarily limited as the Reynolds number increases. The t
heory is for large Reynolds numbers and is based on viscous-inviscid reason
ing, backpressure effects from the obstacle and slender-layer separation lo
cally, among other influences. The comparisons nevertheless yield encouragi
ngly close agreement, for the present computed cases of a vertical flap or
a rectangular block. This is both quantitatively, in terms of the upstream
separation and downstream reattachment positions in particular, and general
ly, in terms of the separating flow structure, even at the notably moderate
Reynolds numbers covered accurately by the computations.