Abstract
We perform direct numerical simulation (DNS) for annular pipe flow and report localized structures in subcritical transition. Our flow system is the simplest fundamental annular pipe flow which is a flow between two concentric cylinders and is driven by a constant uniform pressure gradient in the axial direction. By changing Reynolds number (Re_τ = uτδ/ν) and radius ratio (η = r_<in>/r_<ou>) systematically, we found several localized intermittent turbulent states. To observe the effect of η, we choose low to high radius ratios, which are 0.1, 0.3, 0.5, and 0.8. At η ≥ 0.5, we confirmed stripe pattern similar to that in the plane Poiseuille flow. In contrast, with a low η of 0.1, localized structures corresponding to equilibrium puffs and splitting puffs arise. The moderate η of 0.3 gives rise to localized turbulence taking a form of finite (like a puff) and helical shape (like a stripe pattern).
