Direct numerical simulation of fully developed turbulent and oscillatory pipe flows at Reτ=1440

Shear flow turbulence in oscillatory fluid motions is of theoretical interest and practical relevance, since the onset of turbulence can drastically change the transport properties and mixing efficiency. To supplement former theoretical and experimental investigations on the transition to turbulence in Sexl-Womersley (SW) flows, we perform three-dimensional direct numerical simulations (DNS) of oscillatory pipe flows at three Womersley numbers (Wo is an element of {26, 13, 5}) and one constant Reynolds number (Re-tau = 1440) based on the friction velocity and the pipe diameter. For this, the incompressible Navier-Stokes equations are solved in cylindrical coordinates using a fourth-order-accurate finite-volume method on staggered grids, motivated by Schumann's volume balance procedure. We generate a well-correlated high-Reynolds-number initial flow field for the oscillatory flows by means of a DNS of a statistically steady pipe flow at Re-tau = 1440. To underline the reliability of the DNS results for the oscillatory pipe flows, we validate the finite-volume method, the spatial resolution of the computational grid and the length of the computational domain by comparing the results for the statistically steady pipe flow with experimental data obtained by laser Doppler anemometry (LDA). Comparing the statistical moments of the velocity components up to the fourth order shows good agreement with the corresponding LDA data. When started from the turbulent initial velocity field, the oscillatory flows relaminarise or reach a conditionally or fully turbulent state, depending on Wo. The peak flow rates decrease with increasing Wo, while the relaxation phase for the initially steady flow converging to a purely oscillating flow increases with increasing Wo. For the highest Wo considered, the flow completely relaminarises and we do not find any instabilities close to the wall, as expected from former stability analyses. On the other hand, we confirm the existence of turbulent bursts and increasing turbulence intensity during the deceleration phase of the flow and relaminarisation in the acceleration phase for Wo = 13 in agreement with experimental results in the literature. By analysing selected terms of the transport equations for the mean and turbulent kinetic energy, we demonstrate the transport of turbulent kinetic energy from the axial to the radial and azimuthal velocity components during flow deceleration.