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Modelling flow over rough surfaces in hydropower waterways
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.ORCID iD: 0000-0001-9426-2375
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-71104ISBN: 978-91-7790-224-9 (print)ISBN: 978-91-7790-225-6 (electronic)OAI: oai:DiVA.org:ltu-71104DiVA, id: diva2:1253422
Public defence
2018-11-23, E632, Luleå tekniska universitet, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-10-08 Created: 2018-10-04 Last updated: 2018-11-21Bibliographically approved
List of papers
1. Characterization of Flow Structures Induced by Highly Rough Surface Using Particle Image Velocimetry, Proper Orthogonal Decomposition and Velocity Correlations
Open this publication in new window or tab >>Characterization of Flow Structures Induced by Highly Rough Surface Using Particle Image Velocimetry, Proper Orthogonal Decomposition and Velocity Correlations
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2018 (English)In: Engineering, ISSN 1947-3931, Vol. 10, p. 399-416Article in journal (Refereed) Published
Abstract [en]

High Reynolds number flow inside a channel of rectangular cross section is examined using Particle Image Velocimetry. One wall of the channel has been replaced with a surface of a roughness representative to that of real hydropower tunnels, i.e. a random terrain with roughness dimensions typically in the range of ≈10% - 20% of the channels hydraulic radius. The rest of the channel walls can be considered smooth. The rough surface was captured from an existing blasted rock tunnel using high resolution laser scanning and scaled to 1:10. For quantification of the size of the largest flow structures, integral length scales are derived from the auto-correlation functions of the temporally averaged velocity. Additionally, Proper Orthogonal Decomposition (POD) and higher-order statistics are applied to the instantaneous snapshots of the velocity fluctuations. The results show a high spatial heterogeneity of the velocity and other flow characteristics in vicinity of the rough surface, putting outer similarity treatment into jeopardy. Roughness effects are not confined to the vicinity of the rough surface but can be seen in the outer flow throughout the channel, indicating a different behavior than postulated by Townsend’s similarity hypothesis. The effects on the flow structures vary depending on the shape and size of the roughness elements leading to a high spatial dependence of the flow above the rough surface. Hence, any spatial averaging, e.g. assuming a characteristic sand grain roughness factor, for determining local flow parameters becomes less applicable in this case.

Place, publisher, year, edition, pages
Scientific Research Publishing, 2018
Keywords
CFD, Validation, Hydraulic Roughness, PIV, Hydropower
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-71097 (URN)10.4236/eng.2018.107028 (DOI)
Note

Validerad;2020;Nivå 1;2020-01-07 (marisr)

Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2020-01-07Bibliographically approved
2. Localised roughness effects in non-uniform hydraulic waterways
Open this publication in new window or tab >>Localised roughness effects in non-uniform hydraulic waterways
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(English)In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079Article in journal (Refereed) Submitted
Abstract [en]

Hydropower tunnels are generally subject to a degree of rock falls. Studies explaining this are scarce and the current industrial standards offer little insight. To simulate tunnel conditions, high Reynolds number flow inside a channel with a rectangular cross-section is investigated using Particle Image Velocimetry and pressure measurements. For validation, the flow is modelled using LES and a RANS approach with k - ε turbulence model. One wall of the channel has been replaced with a rough surface captured using laser scanning. The results indicate flow-roughness effects deviating from the standard non-asymmetric channel flow and hence, can not be properly predicted using spatially averaged relations. These effects manifest as localized bursts of velocity connected to individual roughness elements. The bursts are large enough to affect both temporally and spatially averaged quantities. Both turbulence models show satisfactory agreement for the overall flow behaviour, where LES also provided information for in-depth analysis.

Place, publisher, year, edition, pages
Taylor & Francis
Keywords
Hydropower, CFD, Validation, Hydraulic Roughness, PIV
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-71098 (URN)
Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2018-10-04
3. Gävunda case study
Open this publication in new window or tab >>Gävunda case study
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:ltu:diva-71100 (URN)
Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2018-10-04
4. Inlet Blockage Effects in a Free Surface Channel With Artificially Generated Rough Walls
Open this publication in new window or tab >>Inlet Blockage Effects in a Free Surface Channel With Artificially Generated Rough Walls
2018 (English)In: Proceedings of the 7th IAHR International Symposium on Hydraulic Structures / [ed] Daniel Bung ; Blake Tullis, 2018, p. 723-732Conference paper, Published paper (Refereed)
Abstract [en]

When considering free surface flow in channels, it is essential to have in-depth knowledge about the inlet flow conditions and the effect of surface roughness on the overall flow field. Hence, we hereby investigate flow inside an 18m long channel by using Particle Tracking Velocimetry (PTV) and Acoustic Doppler Velocimetry (ADV). The roughness of the channel walls is generated using a diamond-square fractal algorithm and is designed to resemble the actual geometry of hydropower tunnels. Four different water levels ranging from 20 to 50cm are investigated. For each depth, the inlet is blocked by 25 and 50% at three positions each, at the centre, to the right and to the left in the flow-direction. The flow is altered for each depth to keep the flow velocity even throughout the measurements. PTV is applied to measure the velocity of the free water surface; four cameras are placed above the setup to capture the entirety of the channel. The results show a clear correlation between roughness-height and velocity distribution at depths 20-30 cm. The surface roughness proved effective in dispersing the subsequent perturbations following the inlet blockage. At 50cm, perturbations from the 50% blockage could be observed throughout the channel. However, at 20cm, most perturbations had subsided by a third of the channel length. The ADV was used to capture the velocity in a total of 375 points throughout the channel, at a depth of 50 cm with no inlet perturbations.

Keywords
Hydraulic roughness, PTV, diamond-square algorithm, free-surface flows
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-71096 (URN)10.15142/T3P644 (DOI)2-s2.0-85054178430 (Scopus ID)9780692132777 (ISBN)
Conference
7th International Symposium on Hydraulic Structures, Aachen, Germany, 15-18 May 2018
Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2018-10-12Bibliographically approved
5. Experimental Study of Head Loss over Laser Scanned Rock Tunnel
Open this publication in new window or tab >>Experimental Study of Head Loss over Laser Scanned Rock Tunnel
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2016 (English)In: Experimental Study of Head Loss over Laser Scanned Rock Tunnel: Hydraulic Structures and Water System Management, ISHS 2016, Portland, United States, 27 - 30 June 2016, Portland: Utah State University , 2016, p. 22-29Conference paper, Published paper (Refereed)
Abstract [en]

Flow in hydropower tunnels is characterized by a high Reynolds number and often very rough rock walls. Due to the roughness of the walls, the flow in the tunnel is highly disturbed, resulting in large fluctuations of velocity and pressure in both time and space. Erosion problems and even partial collapse of tunnel walls are in some cases believed to be caused by hydraulic jacking from large flow induced pressure fluctuations. The objective of this work is to investigate the effects of the rough walls on the pressure variations in time and space over the rock surfaces. Pressure measurement experiments were performed in a 10 m long Plexiglas tunnel where one of the smooth walls was replaced with a rough surface. The rough surface was created from a down-scaled (1:10) laser scanned wall of a hydraulic tunnel. The differential pressure was measured at the smooth surface between points placed at the start and end of the first four 2 m sections of the channel. 10 gauge pressure sensors where flush mounted on the rough surface; these sensors measure the magnitude and the fluctuations of the pressure on the rough surface. The measurements showed significant spatial variation of the pressure on the surface. For example, sensors placed on protruding roughness elements showed low gauge pressure but high fluctuations. The differential pressure indicated a head loss through the tunnel that was almost four times higher than a theoretical smooth channel.

Place, publisher, year, edition, pages
Portland: Utah State University, 2016
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-40310 (URN)10.15142/T360628160853 (DOI)2-s2.0-84988981565 (Scopus ID)f63f31d3-d2db-4351-8149-87a65d10ced0 (Local ID)978-1-884575-75-4 (ISBN)f63f31d3-d2db-4351-8149-87a65d10ced0 (Archive number)f63f31d3-d2db-4351-8149-87a65d10ced0 (OAI)
Conference
International Symposium on Hydraulic Structures : 27/06/2016 - 30/06/2016
Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2018-10-04Bibliographically approved

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