Temporally developing DNS for channel flow at Reτ = 180 and 590 are performed to understand the turbulence generation mechanism for wall bound flow transition. Simulations for Reτ = 180 were performed for initial turbulence intensities TI = 0.1%, 1%, 2.5% and 5% and for Reτ = 590 with TI = 1% and 5%. The results in the fully developed turbulent region were compared with Moser et al. (1999) DNS data to validate the predictions. The near-wall vortical structures, mean and turbulent stresses and energy spectra, and turbulent kinetic energy (TKE) and stress budget in the pre-transition, transition and turbulent regions are analyzed to understand the turbulence onset, growth and decay mechanism. Finally, molecular diffusion and pressure strain time scales in the pre-transition to turbulent regions are analyzed to evaluate the turbulence onset criteria.

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