Wall friction and turbulence dynamics in decelerating pipe flows

Wall shear stresses in decelerating pipe flows are studied using a well-established CFD code developed specifically for modelling unsteady turbulent flows. The suitability of the turbulence model used was demonstrated previously and re-confirmed by comparison with recent experimental data. The response of the wall shear stress to the deceleration is shown to exhibit features in common with responses in accelerating flows, but also indicates strong differences. Turbulence delays are shown to be important, causing, in effect, a frozen-turbulence response at early times. Also, flow reversal and increasing turbulence timescales prevent the development of asymptotic conditions at larger times. It is demonstrated that, at intermediate times, the instantaneous wall shear stress may be either larger or smaller than the corresponding quasi-steady value, thereby explaining features of existing experimental data that were previously regarded as conflicting. It is shown that the amplitude and timing of key features of the wall shear stress history can be correlated by means of a single non-dimensional parameter.