Laminar-turbulent transition for attached and separated flow

A major challenge in the design of turbomachinery components for aircraft gas turbine engines is high cycle fatigue failures due to flutter. Of particular concern is the subsonic/transonic stall flutter boundary which occurs at part speed near the stall line. At these operating conditions the incidence angle is large and the relative Mach number is high subsonic or transonic. Viscous effects dominate for high incidence angles.;In order to predict the flutter phenomena, accurate calculation of the steady and unsteady aerodynamic loading on the turbomachinery airfoils is necessary. The development of unsteady aerodynamic models to predict the unsteady forces and moments acting on turbomachine airfoils is an area of fundamental research interest. Unsteady Reynolds Averaged Navier-Stokes (RANS) models have been developed to accurately account for viscous effects. For these Reynolds averaged equations turbulence models are needed for the Reynolds stress terms. A transition model is also necessary. The transition onset location is determined by a transition onset model or specified at the suction peak. Usually algebraic, one or two-equation or Reynolds stress turbulence models are used. Since the Reynolds numbers in turbomachinery are large enough to guarantee the flow is turbulent, suitable transition and turbulence models are crucial for accurate prediction of steady and unsteady separated flow.;The viscous flow solution of compressor airfoils at off-design conditions is challenging due to flow separation and transition to turbulent flow within separation bubbles. Additional complexity arises when the airfoils are vibrating as is encountered in stall flutter. In this investigation calculations are made of a transonic compressor airfoil in steady flow and with the airfoils oscillating in a pitching motion about the mid-chord at 0° and 10° of chordal incidence angle, and correlated with experiments conducted in the NASA GRC Transonic Flutter Cascade. To model the influence of flow transition on the steady and unsteady aerodynamic flow characteristics, the Solomon, Walker, and Gostelow (SWG) transition model is utilized. The one-equation Spalart-Allmaras model is used to model turbulence. Different transition onset models including fixed onset are implemented and compared for the two incidence angle cases. At each incidence angle, the computational model is compared to the experimental data for the steady flow case and also for pitching oscillation at a reduced frequency of 0.4. The 10° incidence angle case has flow separation over front 40% of the airfoil chord. The operating conditions considered are an inlet Mach number of 0.5 and a Reynolds number of 0.9 Million.;KEYWORDS: Flutter, High Cycle Fatigue, Turbulence model, Transition model, Computational Fluid Dynamics.