DNS study of a separation bubble in a turbulent boundary layer

Direct numerical simulations (DNSs) of a separated turbulent boundary layer have been performed in which blowing and suction are imposed at the upper boundary in order to produce a separation bubble. The inlet data are prescribed by DNSs of a zero-pressure gradient turbulent boundary layer with the rescaling-recycling method. The Reynolds numbers at the inlet are set to be $Re_\theta=300$ and 600 where $Re_\theta$ is the Reynolds number based on the free stream velocity and the momentum thickness. Particular attention is given to the difference between large and small separation bubbles and also the Reynolds-number dependence. The present results indicate that large-scale structures of velocity and pressure fluctuations are more dominant in a large separation bubble than in a small separation bubble, which becomes more apparent with increasing Reynolds number. The relationship between the large-scale structures and the oscillatory behavior of the detachment and reattachment regions will also be discussed.