Navier-Stokes Airfoil Computations Including Transitional Flow Regions

Kurzfassung

Based on a Navier-Stokes approach, the flow around a lamiar airfoil is described together with transition prediction based on the e*-method. The main emphasis is placed on the computation of the transitional flow region, i.e. the range between fully laminar and fully turbulent flow. The extension and intermittency of the transition region are predicted on the basis of available models originating from the turbulent spot theory. A comprehensive computational study is performed as the transition length differs considerably depending on the model used. The influence of the transition length, varying from zero to 20 per cent chord, is investigated in a strong adverse and in a zero pressure gradient airfoil flow region. Based on the outcome of the study a conventional transition length model is proposed, which is applicable in flow regions where transition is predicted well upstream of the laminar separation location. Furthermore, a special transition length model is given, typically for low Reynolds number flows, where the boundary layer stays laminar up to separation. In this case transition is fixed right upstream of the laminar separation point, as the problem of laminar separation bubbles is not considered in the present context. The present investigation concerns the flow over the DoAL3 laminar airfoil at 2° incidence. For that particular case, the e*-method predicts transition at 8 per cent chord on the upper surface. Along the lower surface, the computed N-factors are lower than the limiting N-factor up to laminar flow separation.