EFFECTS OF COMPRESSIBILITY, PITCH RATE, AND REYNOLDS-NUMBER ON UNSTEADY INCIPIENT LEADING-EDGE BOUNDARY-LAYER SEPARATION OVER A PITCHING AIRFOIL

The effects of compressibility, pitch rate and Reynolds number on the initial stages of two-dimensional unsteady separation of laminar subso nic flow over a pitching NACA-0012 airfoil have been studied numerical ly. The approach involves the simulation of the flow by solving the tw o-dimensional unsteady compressible laminar Navier-Stokes equations em ploying the implicit approximate-factorization algorithm of Beam & War ming and a boundary-fitted C-grid. The algorithm has been extensively validated through comparison with analytical and previous numerical re sults. The computations display several important trends for the 'birt h' of the primary recirculating region which is a principal precursor to leading-edge separation. Increasing the non-dimensional pitch rate from 0.05 to 0.2 at a fixed Reynolds number and Mach number delays the formation of the primary recirculating region. The primary recirculat ing region also forms closer to the leading edge. Increasing the Mach number from 0.2 to 0.5 at a fixed Reynolds number and pitch rate cause s a delay in the formation of the primary recirculating region and als o leads to its formation farther from the airfoil top surface. The len gth scale associated with the recirculating regions increases as well. Increasing the Reynolds number from 10(4) to 10(5) at a fixed Mach nu mber and pitch rate hastens the appearance of the primary recirculatin g region. A shock appears on the top surface at a Reynolds number of 1 0(5) along with the simultaneous formation of multiple recirculating r egions near the leading edge.