Turbulent annular pipe flow in subcritical transition regime: Effect of radius ratio on structures

Direct numerical simulations are performed for annular Poiseuille flows with various radius ratios η (= rin/rout) ∈ [0.1, 0.8] in subcritical transition regime of Reτ ∈ [48, 150]. Because the flow system of the annular Poiseuille flow can be regarded as the Hagen-Poiseuille flow (when η ≈ 0) and the plane Poiseuille flow (η ≈ 1), the (dis-)similarity and the connection between them are discussed. We found the occurrences of localized structure like the turbulent puff in the Hagen-Poiseuille flow at low η and of helical wave like the turbulent stripe pattern in the plane Poiseuille flow at high η. The switching point between helical wave and localized structure by changing η is caught in the region η ∈ [0.1, 0.3].