Transition modeling effects on viscous/inviscid interaction analysis of low Reynolds number airfoil flows involving laminar separation bubbles

Abstract

Estimating the low Reynolds number and off-design performance of axial turbomachine blades requires an accurate prediction of separation phenomena occurring on the blade surface. This paper discusses a viscous/inviscid interaction analysis of flow over a NACA 65-213 airfoil at a chord Reynolds number of 240,000 using a calculation method of Cebeci et al. The computed characteristics of a mid-chord laminar separation bubble are compared with experimental laser-doppler anemometer measurements of Hoheisel et al. Attention is focused on problems of modeling the laminar-turbulent transition zone within the viscous layer. A parametric study is undertaken to determine the location and extent of the transition zone which best models the observed separation bubble behavior. The required transition length is almost an order of magnitude smaller than that predicted from conventional transition length correlations. A physical model for this greatly reduced transition length in positive pressure gradient flows is proposed. The computational model correctly predicts most features of the separation bubble flow, but there are some significant discrepancies at reattachment which point to the need for improved turbulence modeling in this area. The inclusion of transverse pressure gradients associated with flow curvature in the viscous regions also appears very desirable for airfoils operating at Reynolds numbers around 10 5.