In direct numerical simulations (DNS) of laminar-turbulent transition in laminar separation bubbles, the definition of a well-posed freestream boundary condition is crucial. Different, partially contradicting properties are required: first, an adverse pressure gradient has to be imposed to force separation. Moreover, oscillations at the freestream boundary caused by disturbance waves, which extend into the potential how have to be treated accurately. Finally, displacement effects of the separation bubble on the surrounding potential flow by the so-called viscous-inviscid boundary-layer interaction have to be captured. Usually, either the integration domain has to be sufficiently high, or a state-of-the-art boundary-layer interaction model based on the theory of thin airfoils can be applied. At a high Reynolds number at separation, neither possibility is applicable. Therefore, an improved model has been developed that meets the just-mentioned requirements. The method is validated by variations of the height of the integration domain. It is shown that boundary-layer interaction has not only quantitative but even qualitative impact on the separation bubble. The comparison of a highly resolved DNS with an experiment proves the applicability of the present method for separation bubbles on airfoils. References: 24
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