A computational study of wall friction and turbulence dynamics in accelerating pipe flows

A CFD model of turbulent flow in a smooth pipe accelerating uniformly from steady state is used to study the influence of turbulence and inertia on wall shear stresses. A low-Reynolds-number k-ε turbulence model is used in conjunction with a finite volume/finite difference discretization scheme. It is shown that the wall shear stress initially overshoots the corresponding quasi-steady value and this is attributed to inertial causes. Thereafter, the wall shear stress is shown to undershoot the quasi-steady value because inertial effects are more than counterbalanced by the cumulative influence of delays in the response of turbulence to flow changes. The dependence of the flow behaviour on the geometry, the fluid properties, the Reynolds number and the acceleration is studied and is shown to correlate well with a non-dimensional parameter based on the turbulence production timescale. The durations of the initial overshoots and the amplitudes of the overshoots and undershoots are smaller at high Reynolds numbers than at low ones.