Turbulent annular pipe flow in subcritical transition regime: Direct numerical simulation of a large domain

We performed direct numerical simulations of annular Poiseuille flow (APF) with a radius ratio of η (= r_in/r_out) = 0.8, in order to investigate the subcritical transition scenario from the developed turbulent state to the laminar state. In previous studies on annular Poiseuille flow, the flows at high Reynolds numbers were well examined and various turbulence statistics were obtained for several η, because of their dependence on η. Since the transitional APF is still unclear, we investigate annular Poiseuille flows in the transitional regime through the large-domain simulations in a range of the friction Reynolds number from Re_τ = 150 down to 56. At a transitional Reynolds number, weak-fluctuation regions occur intermittently and regularly in the flow field, and the localized turbulence appears in the form of banded patterns same as in plane Poiseuille flow (PPF). The flow system of APF with a high radius ratio η ≈ 1 can be regarded as PPF and, hence, the transition regime in high radius-ratio of APF and in PPF should be analogous. However, in APF, the banded structure takes on helical shape around the inner cylinder, since APF is a closed system in the spanwise (azimuthal) direction. In this paper, the (dis-)similarity between APF and PPF is discussed.