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Characteristics of the wall shear stress in pulsating wall-bounded turbulent flows
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0002-3349-601x
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Energy and Process Engineering, Norwegian University of Science and Technology.ORCID iD: 0000-0001-7599-0895
2018 (English)In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 96, p. 257-265Article in journal (Refereed) Published
##### Abstract [en]

A pulsating turbulent pipe flow has been investigated experimentally using hot-film anemometry and particle image velocimetry. Particularly, a `paradoxical phenomenon' that is known to occur for a range of forcing frequencies and time-averaged Reynolds numbers has been investigated in detail. The paradoxical phenomenon is that the oscillating component of the wall shear stress exhibits a smaller amplitude in a turbulent flow compared to in a laminar flow exposed to the same oscillation in the pressure gradient. In here, the phenomenon is explained by splitting the response of the wall shear stress into one contribution resulting from the imposed pressure gradient $\widetilde{\tau}_p$, and a second contribution resulting from the oscillating Reynolds shear stress,$\widetilde{\tau}_t$ At the conditions of maximum reduction of the wall shear stress amplitude, $\widetilde{\tau}_{t}$ and $\widetilde{\tau}_{p}$ are nearly 136 degrees out of phase. The contributions are thus interfering destructively, this being the ultimate reason for the reduced amplitude. It is also shown that the level of reduction is dependent on the imposed forcing amplitude, this in turn residing from a dependence of the time-development of the oscillating Reynolds shear stress on the forcing amplitude.

##### Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 96, p. 257-265
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
ISI: 000432235300025Scopus ID: 2-s2.0-85044007965OAI: oai:DiVA.org:ltu-67866DiVA, id: diva2:1188184
##### Note

Available from: 2018-03-06 Created: 2018-03-06 Last updated: 2018-06-11Bibliographically approved
##### In thesis
1. Studies of Transient and Pulsating flows with application to Hydropower
Open this publication in new window or tab >>Studies of Transient and Pulsating flows with application to Hydropower
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Alternative title[sv]
Studier av transient och pulserande strömning med applikation inom vattenkraft
##### Abstract [en]

The rotational motion of a hydraulic turbine runner makes pulsating flows ubiquitous in different locations of the machine. The cyclic loading thus induced may generate large pressure forces acting periodically on both stationary and rotating parts. In addition to the presence of pulsating flows in a turbine runner, transient flows are encountered at an increasingly higher rate due to the continual installation of intermittent sources of renewable energy, such as wind and solar power. To mitigate the imbalance that these unpredictable sources induce on the frequency of the electrical grid, hydropower turbines are enforced to regulate their power production, and consequently flow rate, thus leaving them to operate under transient conditions. In terms of wear and fatigue, a startup or shutdown of a hydraulic turbine corresponds to 10-20 hours of steady state operation at the design point. Transient operation of a hydraulic machine can, however, also be used in favour for measuring the discharge through the turbine using the pressure-time method. A better understanding of pulsating and transient flows thus has the potential both to mitigate problems associated with them, and to increase the accuracy with which the turbine flow rate can be measured; two great merits for the hydropower community. In light of this observation, the following work constitutes a fundamental investigation of transient and pulsating flows performed in a straight pipe.Studies have been performed experimentally using particle image velocimetry, hot-film anemometry, laser Doppler velocimetry and pressure sensors.

A chief finding is that the time-development of the wall shear stress and near-wall turbulence fields exhibit significant similarity between transient and pulsating flows, despite the different conditions of the mean flow. Whereas the former is initiated from a statistically steady state, the latter is constantly subjected to a time-varying forcing. Both types of unsteady flows have previously been investigated in detail; however, any potential similarity between them has, largely, been unexplored. An important implication of this finding, then, is that knowledge acquired in one type of unsteady flow can be used, if not interchangeably, at least as a guidance for the expected behaviour in the other type of flow. An example is the development of unsteady turbulence models. Another important finding is that the frictional losses arising during the late stage of a pressure-time flow rate measurement can be accurately modelled using an analytical laminar formulation of the wall shear stress, despite the bulk of the flow being turbulent. The formulation of the wall shear stress has potential to be further improved by incorporating a damping-function.

##### Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018
##### Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
##### Keywords
Turbulent flows, Pipe flow, Unsteadiness, Friction modeling
##### National Category
Fluid Mechanics and Acoustics
Fluid Mechanics
##### Identifiers
urn:nbn:se:ltu:diva-67865 (URN)978-91-7790-065-8 (ISBN)978-91-7790-066-5 (ISBN)
##### Public defence
2018-04-20, E231, Luleå, 09:30 (English)
##### Supervisors
Available from: 2018-03-07 Created: 2018-03-06 Last updated: 2018-04-05Bibliographically approved

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#### Authority records BETA

Sundström, L.R. JoelCervantes, Michel

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Cite
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