Modelling Laminar Separation Bubbles at High Reynolds Number

A laminar separation bubble (LSB) occurs when the flow separates in the laminar regime. The turbulence developing inside the re-circulation region enhances the momentum transport and the flow re-attaches. The laminar separation bubble is one of the main critical aspects of flows at low Reynolds number, of order of magnitude 104-105, but results to be crucial also for flows at high Reynolds numbers. In fact, very tiny laminar separation bubbles are present in airfoil used for turbine applications operating at Reynolds number of the order of magnitude of 106. In the present study, a comparison among different solvers is performed in order to assess the capabilities to simulate the onset and the development of laminar separation bubbles using transition models in RANS approach for high Reynolds number flows. Two test cases, representative of this phenomenon and experimentally documented, are analysed. The first one is relative to an adverse pressure gradient flat plate, specifically designed to resemble the OA209 airfoil flowfield. The second test case refers to the S809 airfoil at Re=2×106. The importance of using turbulence models with the transition functions has been highlighted. This has resulted to be crucial in order to achieve reliable results.