Direct numerical simulation for puffs in tubes with various lengths

Under certain conditions, the turbulent transition can produce local turbulent structures knows as puffs. Puffs are the zones of turbulence surrounded by laminar flow; their behavior depends mainly on the Reynolds number (Re). We study the influence of initial flow conditions on puffs generation in circular tubes with different lengths using the direct numerical simulation (DNS) approach at a constant Reynolds number Re ≃ 2200. The literature data show that in the flow with Re ≃ 2000, the singled puffs are generated as metastable structures with alternatives for decay or survival, while at Re > 2300 these puffs can interact and produce more complicated and stable configurations (double and triple puffs). Puffs are classified as stable puffs if the lifetime exceeds considerably the characteristic time in the system (typically defined as the flow area length divided by the value of axial velocity in a laminar flow interval). The literature data evidences an increase in the puffs amount with increasing in the Reynolds number, but the problem of flow reproducibility for flow structure at the same Reynolds remains an open issue. The published data are important for prognosis and control of turbulence in various engineering application.