Direct numerical simulation of low Reynolds number turbulent swirling pipe flows

Direct numerical simulation of swirling pipe flows is performed to investigate the effects of swirl on turbulence statistics as well as the inertial regime. The swirling motion is imposed via a constant azimuthal body force coupled with a body force in the axial direction that drives the flow. The friction Reynolds number is Re τ ≈ 170 with a pipe length of 8 π δ (where δ is the pipe radius). The simulations are performed at various swirl numbers. The swirling motion introduces additional drag into the flow and reduces the bulk flow rate for the swirl strengths investigated in this study. We also show from our data as well as via the mean equations that similar to the total axial stress, the total azimuthal stress when normalized by the azimuthal friction velocity follows a linear decrease from wall to the pipe centerline independent of swirl strength. In the inertial regime, with negligible viscous effects, the linear relationship holds for the turbulent shear stresses. We derive the modified axial mean momentum equation, which when studied alongside the azimuthal momentum equation shows that increasing the swirl strength increases the inertial region in the pipe by pushing the beginning of the inertial region closer to the wall. This effect is governed by the axial viscous forces rather than the azimuthal onesRey C. Chin and Jimmy Phili