Distribution of local velocities in a circular pipe with accelerating fluid flow

Abstract

The results of experimental studies of accelerating fluid flow in a cylindrical pipe from rest are presented. With accelerated fluid flow, a delay of laminar-turbulent transition with instantaneous Re numbers, which are several orders of magnitude higher than critical Re in stationary conditions is observed. To determine the local characteristics of the unsteady flow, hot-wire equipment was used. For measuring the local velocity in the pipe, a hot-wire cone sensor was used, and for measuring shear stresses – a sensor mounted flush with the inner pipe wall. To process the experimental data, in addition to ensemble averaging, smoothing by time averaging over five adjacent points was also carried out. It turned out that in order to obtain smoother functions for the desired characteristic, it is necessary to have much more experiments the ensemble, especially in the wall area. It is found that with accelerated fluid flow from rest to the onset of turbulence, a uniform velocity distribution remains in the pipe section and velocity gradients are observed only in a thin surface layer. A sharp transition in the shear stress characteristic on the pipe wall τ 0 with the laminar-turbulent transition is also observed in the characteristics of local velocities. At the moment of transition to the turbulent regime, a turning point appears on the average velocity graph, and velocity distribution and turbulence intensity undergo significant changes compared with steady turbulent flows. Turbulence is generated in the wall area and distributed to the pipeline section center at an almost constant velocity. The front of laminar-turbulent transition with unsteady fluid flow in the pipe is distributed towards the section center at an almost constant velocity