Turbulent pipe flows are prevalent in industrial applications. Despite a large body of experimental and more recently numerical investigations there are still unresolved fundamental issues. Among those are the scaling of the mean velocity profile or the question whether the near-wall peak in the variance profile is Reynolds number invariant.
The situation is even more vague when it comes to non-canonical turbulent pipe flows, such as flows in bend pipe sections or swirling flows. One reason for the controversial views is due to insufficient insight into the limitations of state-of-the art measurement techniques employed in these studies. Pitot tube and hot-wire measurements have been employed for almost a century now, however, there is a renewed interest in their calibration at low velocities, their spatial and temporal resolution as well as their correction schemes.
In the present thesis hot-wire measurements were performed in swirling and non-swirling turbulent flows through straight and 90ft pipe bends in a Reynolds number range from 13000 to 34000 by keeping the viscous scaled wire length small and constant thereby avoiding ambiguity between Reynolds number and spatial resolution effects.
Additionally, spatial resolution e ffects were studied by repeating measurements at the same Reynolds number with four di fferent wire length. Besides the non-swirling pipe flows, various swirl strength in the range S =0{0.5 have been imparted on the fully developed turbulent flow pipe to study the effect of rotation on the flow in straight and bend pipe flows. Special attention was also paid to the calibration of hot-wire probes at low velocities. In particular a number of cylinder wake measurements were performed to expand the classical vortex shedding calibration of hot-wire probes.
Results indicate that the near-wall peak of the streamwise variance profile increases with Reynolds number, when keeping the wire length constants in viscous units; a result that con firms similar findings in channel and turbulent boundary layer flows, but contradicts recent findings from the Princeton Superpipe.
A superposition of weak degree of swirl up to S=0.1 has been found to have negligible effects on the statistical quantities, while with further increase the flow approaches its laminar pro file, i.e. a higher mean velocity and a lower turbulence intensity in the central region of the pipe. In bend pipe flows the addition of swirl has a stabilizing e ffect on the flow after the bend, i.e. a shorter development length is needed to recover from the bend.
Source: KTH
Author: Sattarzadeh Shirvan, Sohrab
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