Turbulent pipe flows subjected to temporal decelerations

Direct numerical simulations of temporally decelerating turbulent pipe flows were performed to examine effects of temporal decelerations on turbulence. The simulations were started with a fully developed turbulent pipe flow at a Reynolds number, $Re_{D}$=24380, based on the pipe radius ($R$) and the laminar centerline velocity ($U_{c0}$). Three different temporal decelerations were imposed to the initial flow with $f$=$|dU_{b}/dt|$=0.00127, 0.00625 and 0.025, where $U_{b}$ is the bulk mean velocity. Comparison of Reynolds stresses and turbulent production terms with those for steady flow at a similar Reynolds number showed that turbulence is highly intensified with increasing $f$ due to delay effects. Furthermore, inspection of the Reynolds shear stress profiles showed that strong second- and fourth-quadrant Reynolds shear stresses are greatly increased, while first- and third-quadrant components are also increased. Decomposition of streamwise Reynolds normal stress with streamwise cutoff wavelength ($\lambda_{x}$) 1$R$ revealed that the turbulence delay is dominantly originated from delay of strong large-scale turbulent structures in the outer layer, although small-scale motions throughout the wall layer adjusted more rapidly to the temporal decelerations.