The flow over a NACA-0012 airfoil, impulsively set to motion from rest with a constant acceleration, at an angle of attack of 22.5 deg, was investigated experimentally and computationally. The experimental investigation relied on a continuous-field Image Correlation Velocimetry (ICV) that determines the optical flow in a sequence of images of a convected Lagrangian marker, e.g., a conserved scalar field, or particles, etc. The method extends the original ICV implementation (Tokumaru & Dimotakis 1995) by implementing a hierarchical, multi-resolution B-spline representation, imposing the desired continuity order to the inferred velocity field, also accommodating boundary conditions in the process, as appropriate (Gornowicz 1997). The flow-field measurements were performed in the mid-span plane of the NACA-0012 airfoil. The velocity field was reconstructed assuming a no-slip boundary condition on the airfoil surface. The numerical simulations relied on a spectral element method with adaptive mesh refinement. This technique builds on the formulation for unstructured spectral elements (Henderson 1994). Adaptivity is based on the local "spectrum" of the flow as determined by projecting onto an orthogonal polynomial basis, thus indicating on which subdomains the flow is poorly resolved (Henderson & Meiron 1997). The experimental and computational results are in good agreement until the first separation bubble is detected. The laboratory and simulated flows remain in good qualitative accord thereafter, tracking the emergence of a secondary and tertiary vortex with increasing time. The laboratory flow registers larger separation bubbles, however, with non-zero in-plane divergence and streamlines that spiral out of the primary separated vortices, indicating unsteady, three-dimensional in-flow into the midspan plane, despite the nominally two-dimensional flow geometry and low Reynolds number for this flow.

The experimental work and development of the ICV experimental technique derives from the thesis research of Galen Gornowicz and was supported by the Air Force Office of Scientific Research. The numerical simulations were supported by the National Science Foundation and Office of Naval Research.

GORNOWICZ, G. G. 1997 "Continuous-Field Image-Correlation Velocimetry and its Application to Unsteady Flow over an Airfoil," Aeronautical Engineer's thesis, California Institute of Technology.

TOKUMARU, P. T., and DIMOTAKIS, P. E. 1995
"Image Correlation Velocimetry,"
*Exps. in Fluids* **19**(1),1-15.

HENDERSON, R. D., 1994 "Unstructured Spectral Element Methods: Parallel Algorithms and Simulations," Ph.D. Dissertation, Princeton University.

HENDERSON, R. D., and MEIRON, D. I. 1997
"Dynamic Refinement Algorithms for Spectral Element Methods,"
*13th AIAA Fluid Dynamics Conference*, Snowmass CO.

Last Modified: January 5, 1998