A Laminar Separation Bubble (LSB) is crated under the influence of an adverse (positive) pressure gradient along a wall by laminar separation, laminarturbulent transition and turbulent re-attachment of the flow to the wall. Direct Numerical Simulations (DNS) of the flow in an airfoil boundary layer which are based on solving the full Navier-Stokes equations exhibit good quantitative agreement of the mean-flow data with wind-tunnel experiments, but only examinatins of the unsteady results are able t o revela the underlying physics. Thus, a hitherto unknown temporal amplification of three-dimensional small-amplitude disturbances is observed and explained by the entrainment of three-dimensional fluctuations by the roll-up of the detached bound ary layer. Once the three-dimensional distrubances become satureated this mechanism leads to a rapid breakdown of the laminar flow into regions of small-scale turbulence which are organized in a quasi two-dimensional coherent manner.
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