Uniform momentum zones in low and high Reynolds number accelerating turbulent pipe flows

Abstract: This paper examines the transient response of uniform momentum zones (UMZs) in low and moderate Reynolds number accelerating turbulent pipe flow. Direct numerical simulation (DNS) datasets at both low (starting from an initial friction Reynolds number Re?0 = 170 up to a final friction Reynolds number Re?1 = 340) and moderate Reynolds numbers (starting from an initial friction Reynolds number Re?0 = 500 up to a final friction Reynolds number Re?1 = 670) were utilised in this study. A Probability Density Function (PDF) based approach was followed in detecting UMZs. The results suggest that although a similar arrangement of UMZs were detected in both the flow scenarios, a strong initial base flow Reynolds number dependency exists in UMZ characteristics for both the low and moderate Reynolds number cases during the accelerating flow excursion through their four unsteady stages. It was found that the most stable zonal configuration of UMZs during each unsteady state of the flow are initial steady Reynolds number dependent of the flow and the UMZs near the wall respond to the flow perturbation first as the flow ramps up. Finally, it was found that the timescales required for the core of the flow to recover are dependent on the initial base flow Reynolds number, based on the current acceleration rates of the data sets used in this study. Keywords: uniform momentum zones, internal shear layers, pipe flow, turbulence simulation, turbulent transition