The initial stages of two-dimensional unsteady leading-edge boundary-l
ayer separation of laminar subsonic flow over a pitching NACA-0012 air
foil have been studied numerically at Reynolds number (based on airfoi
l chord length) Re(c) = 10(4), Mach number M(infinity) = 0.2, and nond
imensional pitch rate OMEGA0+ = 0.2. Computations have been performed
using two separate algorithms for the compressible laminar Navier-Stok
es equations. The first method, denoted the structured grid algorithm,
utilizes a structured, boundary-fitted C grid and employs the implici
t approximate-factorization algorithm of Beam and Warming. The second
method, denoted the unstructured grid algorithm, utilizes an unstructu
red grid of triangles and employs the flux-difference splitting method
of Roe and a discrete representation of Gauss' theorem for the invisc
id and viscous terms, respectively. Both algorithms are second-order a
ccurate in space and time and have been extensively validated through
comparison with analytical and previous numerical results for a variet
y of problems. The results show the emergence of a primary clockwise-r
otating recirculating region near the leading edge which can be traced
to a pair of critical points (a center and a saddle) that appear with
in the flowfield, followed by a secondary counter-clockwise-rotating r
ecirculating region and a tertiary clockwise-rotating recirculating re
gion. The primary and secondary recirculating regions interact with ea
ch other to give rise to the unsteady separation (''breakaway'') of th
e boundary layer.