Direct numerical simulation of pulsatile rough-wall turbulent pipe flow

Direct numerical simulations (DNS) of pulsatile rough-wall turbulent pipe flow have been performed at a friction Reynolds number of 540. The rough walls have a three-dimensional sinusoidal height distribution and were resolved explicitly using a body-fitted grid. Three geometrically-scaled surfaces with a common amplitude-to-wavelength ratio have been considered. An unsteady axial pressure gradient was imposed to simulate a current-dominated pulsatile flow in the very-high-frequency regime. The resulting flow field exhibits axial-azimuthal periodicity, as well as phase dependence in time, and permits instantaneous quantities to be decomposed into four separate parts: (i) a global-averaged component; (ii) a roughness-induced component; (iii) a pulsation-induced component and (iv) the remaining turbulent fluctuation. We compare statistics of (i)-(iv) against their non-pulsatile counterparts using past results related to the current work (Chan et al., J. Fluid Mech., 771:743-777, 2015). Whilst the pulsatile and non-pulsatile data collapse well in the outer region, clear differences are evident in the near-roughness region and the region below the roughness crests. These differences will be examined in the context of phase-averaged statistics (in both space and time).