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Transient wall shear stress measurements and estimates at high Reynolds numbers
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0002-3349-601X
Department of Energy and Process Engineering, Norwegian University of Science and Technology.ORCID iD: 0000-0001-7599-0895
2017 (English)In: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 58, p. 112-119Article in journal (Refereed) Published
Abstract [en]

Transient wall shear stress measurements using hot-film anemometry have been performed in a large-scale laboratory setup at high Reynolds numbers. Starting from Reynolds numbers 1.7×1061.7×106 and 0.7×1060.7×106, the flow was brought to a complete rest by closing a knife gate thus replicating a pressure-time (also know as Gibson) flow rate measurement in a hydropower plant. Ensemble-averaged mean wall shear stresses obtained from 22 repeated runs have been compared with estimates obtained using the pressure-time method. The objective of the work has been to assess the accuracy of the frictional formulation entering the pressure-time integral. It is shown that both the standard method, a quasi-steady approach as well as the recently introduced unsteady method all reproduce the measured wall shear stresses quantitatively during most of the transient. The last phase, following the complete closure of the gate, which is characterized by a slow decay towards zero shear stress at the wall is, however, not captured by the available methods. In general, the unsteady formulation produces the smallest flow rate estimation error, which in turn, implies the best modeling of the frictional losses.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 58, p. 112-119
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-66149DOI: 10.1016/j.flowmeasinst.2017.10.003ISI: 000419419600014Scopus ID: 2-s2.0-85032350533OAI: oai:DiVA.org:ltu-66149DiVA, id: diva2:1149515
Note

Validerad;2017;Nivå 2;2017-11-06 (andbra)

Available from: 2017-10-16 Created: 2017-10-16 Last updated: 2018-03-06Bibliographically 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
Keyword
Turbulent flows, Pipe flow, Unsteadiness, Friction modeling
National Category
Fluid Mechanics and Acoustics
Research subject
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)
Opponent
Supervisors
Available from: 2018-03-07 Created: 2018-03-06 Last updated: 2018-04-05Bibliographically approved

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Sundström, JoelCervantes, Michel

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