Laminar separation is always associated with considerable unsteadiness. This unsteadiness is caused by large coherent structures that are a consequence of hydrodynamic instability mechanisms of the mean flow. The mean-flow topology and unsteady behavior of laminar separation bubbles (LSB) is in fact mainly governed by instability and transition. In this paper, laminar separation bubbles, which are generated on a flat plate by imposing a streamwise adverse pressure gradient, are investigated by means of Direct Numerical Simulations (DNS). The streamwise pressure gradient for the DNS is chosen such that the inviscid wall pressure distribution, as reported in the Gaster experimental series Ⅰ, case Ⅳ, is closely matched. This case was classified as a "short" laminar separation bubble. The time-averaged flow field obtained from the DNS with no external disturbances introduced (no free-stream turbulence), reveals that the bubble is longer than observed in the experiments. In fact, the bubble obtained in the simulations appeared to be a "long" bubble. This was confirmed by comparing the simulation results with the measurements by Gaster for a long bubble. The discrepancy between the numerical simulations and experiments is possibly due to an earlier onset of transition in the experiments. In the present simulations, instead of forcing with random disturbances to promote transition, isotropic grid turbulence, which was modeled using a superposition of eigenmodes from the continuous spectrum of the Orr-Sommerfeld and Squire operators is introduced at the inflow boundary. It was observed that as the free-stream turbulence (FST) intensity was increased, the bubble became smaller. The separation bubble was in fact shortened from both sides (separation and reattachment sides) in the presence of free-stream turbulence. Comparing the wall pressure distribution for 0.2% free-stream turbulence with Gaster experiment revealed that then the bubble could be classified as a "short" bubble. Based on the simulations performed, FST can change a separation bubbles form "long" to "short". In order to investigate bubble "bursting", the development of bubble, that had became short due to FST, was simulated after the FST was turned-off. The short bubble grew for a short period of time. Surprisingly however, it did not return to the original, state without FST.
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