The uniform-momentum zones and internal shear layers in turbulent pipe flows at Reynolds numbers up to Re τ = 1000

Highlights: • Uniform momentum zones and internal shear layer interfaces were analysed using DNS. • Reynolds number effect in the pipe flow was examined up to ${\mathrm{Re}}_{\tau }=1000$. • The number of internal shear layers are shown to increase with Re number. • The role of ejections and sweeps in UMZ interface contortion was scrutinised. • Hierarchical structures of UMZ and interfaces were established by conditional average. The statistical characteristics of the internal shear layers (ISL) and the coherent structures around the ISLs in turbulent pipe flows are examined using direct numerical simulation data at four Reynolds numbers $\mathrm{Re}\tau =180$, 360, 500 and 1000. The ISLs are defined by the peaks on the velocity gradients with no ad hoc filter applied on the ISL selection. This is different from the previous studies where uniform momentum zones (UMZs) are identified first using a velocity histogram and then the ISL is defined as an UMZ interface. In this study, ISLs are ranked by both their strength and location. The signature behaviours of the ISL including an abrupt streamwise velocity jump and a sharp decrease in velocity fluctuation across the ISL are confirmed. Both the positive and the negative ISLs are stronger towards the wall. The local imbalance between ejections and sweeps are observed around ISLs. Near-wall ISLs have stronger ejections while ISLs in the pipe centre have stronger sweeps. The balance between ejection and sweep is achieved approximately at $0.55-0.6$ of the pipe radius for the range of Reynolds numbers studied. The flow structures around an ISL have been identified via 3D conditional sampling. The average positive ISL is located between the high-speed streak above and the low-speed streak below. The low-speed streak is associated with a pair of counter-rotating streamwise vortices which result in a strong ejection around the ISL.