Temporally developing direct numerical simulations (T-DNS) are performed for bypass transition of a zero pressure gradient flat plate boundary layer to understand the interplay between pressure-strain terms and flow instability mechanisms, and to propose and validate a phenomenological hypothesis for the identification of a robust transition onset marker for use in transition-sensitive Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) simulations. Results show that transition initiates at a location where the slow pressure-strain term becomes more dominant than the rapid term in the pre-transitional boundary layer region. The slow term is responsible for the transfer of turbulence energy from the streamwise component to other components, most importantly the wall-normal. The relative magnitudes of the slow and rapid terms can potentially provide a basis for the development of physically meaningful large-scale parameters that can be used as transition onset markers for Reynolds averaged Navier-Stokes (RANS) simulations.

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