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The Reynolds number effects and scaling on response and recovery of flow over square bar roughness elements are numerically examined at a range of Reynolds numbers between 5 × 103 and 1.56 × 105. The square bar roughness element has a height of 0.05D, where D is the pipe diameter. The response is examined using streamline plots and reattachment lengths. An asymptotic trend is observed in reattachment lengths with increasing the Reynolds number. The recovery is examined quantitatively by tracing the transport of Reynolds shear stress downstream of the roughness element. While the overall trend for recovery is similar for all Reynolds numbers, the collapse of stresses toward the wall appears earlier at lower Reynolds numbers. The recovery trends follow a power-law of diffusion toward the centerline. The point of initial response, that is, the point of collapse, appears independent from the effects of Reynolds numbers at Re ≥ 5.0 × 104.

The influence of viscoelastic polymer additives on response and recovery of turbulent pipeflow over square bar roughness elements was examined using Direct Numerical Simulations at a Reynolds number of 5×103. Two different bar heights for the square bar roughness elements were examined, h/D=0.05 and 0.1. A Finitely Extensible Non-linear Elastic-Peterlin (FENE-P) rheological model was employed for modeling viscoelastic fluid features. The rheological parameters for the simulation corresponded to a high concentration polymer of 160 ppm. Recirculation regions formed behind the bar elements by the viscoelastic fluid were shorter than those associated with Newtonian fluid, which was attributed to mixed effects of viscous and elastic forces due to the added polymers. The recovery of the mean viscoelastic flow was faster. The pressure losses on the surface of the roughness were larger compared to the Newtonian fluid, and the overall contribution to local drag was reduced due to viscoelastic effects.

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