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Direct numerical simulations of stratified open channel flows
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-9627-5903
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-9819-2906
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2011 (English)In: 13th European Turbulence Conference (ETC13): Wall-Bounded Flows And Control Of Turbulence, 2011, 022009- p.Conference paper, Published paper (Refereed)
Abstract [en]

We carry out numerical simulations of wall-bounded stably stratified flows. We mainly focus on how stratification affects the near-wall turbulence at moderate Reynolds numbers, i.e. Re-tau = 360. A set of fully-resolved open channel flow simulations is performed, where a stable stratification has been introduced through a negative heat flux at the lower wall. In agreement with previous studies, it is found that turbulence cannot be sustained for h/L values higher than 1.2, where L is the so-called Monin-Obukhov length and h is the height of the open channel. For smaller values, buoyancy does not re-laminarize the flow, but nevertheless affects the wall turbulence. Near-wall streaks are weakly affected by stratification, whereas the outer modes are increasingly damped as we move away from the wall. A decomposition of the wall-normal velocity is proposed in order to separate the gravity wave and turbulent flow fields. This method has been tested both for open channel and full channel flows. Gravity waves are likely to develop and to dominate close to the upper boundary (centerline for full channel). However, their intensity is weaker in the open channel, possibly due to the upper boundary condition. Moreover, the presence of internal gravity waves can also be deduced from a correlation analysis, which reveals (together with spanwise spectra) a narrowing of the outer structures as the stratification is increased.

Place, publisher, year, edition, pages
2011. 022009- p.
Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 318
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-93030DOI: 10.1088/1742-6596/318/2/022009ISI: 000301292300009Scopus ID: 2-s2.0-84856351989OAI: oai:DiVA.org:kth-93030DiVA: diva2:514613
Conference
13th European Turbulence Conference (ETC) SEP 12-15, 2011 Warsaw, Poland
Note
QC 20120410Available from: 2012-04-10 Created: 2012-04-10 Last updated: 2012-07-03Bibliographically approved
In thesis
1. Numerical Investigation of Rotating and Stratified Turbulence
Open this publication in new window or tab >>Numerical Investigation of Rotating and Stratified Turbulence
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Atmospheric and oceanic flows are strongly affected by rotation and stratification. Rotation is exerted through Coriolis forces which mainly act in horizontal planes whereas stratification largely affects the motion along the vertical direction through buoyancy forces, the latters related to the vertical variation of the fluid density. Aiming at a better understanding of atmospheric and oceanic processes, in this thesis the properties of turbulence in rotating and stably stratified flows are studied by means of numerical simulations, with and without the presence of solid walls.                                                                                                                                                                                           A new code is developed in order to carry out high-resolution numerical simulations of geostrophic turbulence forced at large scales. The code was heavily parallelized with MPI (Message Passing Interface) in order to be run on massively parallel computers. The main problem which has been investigated is how the turbulent cascade is affected by the presence of strong but finite rotation and stratification. As opposed to the early theories in the field of geostrophic turbulence, we show that there is a forward energy cascade which is initiated at large scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales where leads to a shallowing of the energy spectrum. Despite the idealized set-up of the simulations, two-point statistics show remarkable agreement with measurements in the atmosphere, suggesting that this process may be an important mechanism for energy transfer in the atmosphere.                                                                                                                                                                                                                                                                               The effect of stratification in wall-bounded turbulence is investigated by means of direct numerical simulations of open-channel flows. An existing full-channel code was modified in order to optimize the grid in the vertical direction and avoid the clustering of grid points at the upper boundary, where the solid wall is replaced by a free-shear condition. The stable stratification which results from a cooling applied at the solid wall largely affects the outer structures of the boundary layer, whereas the near-wall structures appear to be mostly unchanged. The effect of gravity waves is also studied, and a new decomposition is introduced in order to separate the gravity wave field from the turbulent field.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. v, 34 p.
Series
TRITA-MEK, ISSN 0348-467X ; 12/14
Keyword
Geostrophic turbulence, stable stratification, rotation, wall-bounded turbulence, gravity waves, atmospherical dynamics, direct numertical simulations
National Category
Mechanical Engineering
Research subject
SRA - E-Science (SeRC)
Identifiers
urn:nbn:se:kth:diva-98683 (URN)978-91-7501-415-9 (ISBN)
Presentation
2012-06-15, Seminarierrummet, KTH, Brinellvägen 32, Stockholm, 10:00
Opponent
Supervisors
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Note

QC 20120703

Available from: 2012-07-03 Created: 2012-06-29 Last updated: 2015-03-04Bibliographically approved

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Schlatter, PhilippBrethouwer, Gert

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