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Experimental and computational studies of turbulent separating internal flows
KTH, School of Engineering Sciences (SCI), Mechanics.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The separating turbulent flow in a plane asymmetric diffuser with 8.5 degrees opening angle is investigated experimentally and computationally. The considered flow case is suitable for fundamental studies of separation, separation control and turbulence modelling. The flow case has been studied in a specially designed wind-tunnel under well controlled conditions. The average velocity and fluctuation fields have been mapped out with stereoscopic particle image velocimetry (PIV). Knowledge of all velocity components allows the study of several quantities of interest in turbulence modelling such as the turbulence kinetic energy, the turbulence anisotropy tensor and the turbulence production rate tensor. Pressures are measured through the diffuser. The measured data will form a reference database which can be used for evaluation of turbulence models and other computational investigations. Time-resolved stereoscopic PIV is used in an investigation of turbulence structures in the flow and their temporal evolution. A comparative study is made where the measured turbulence data are used to evaluate an explicit algebraic Reynolds stress turbulence model (EARSM). A discussion regarding the underlying reasons for the discrepancies found between the experimental and the model results is made. A model for investigations of separation suppression by means of vortex generating devices is presented together with results from the model in the plane asymmetric diffuser geometry. A short article on the importance of negative production-rates of turbulent kinetic energy for the reverse flow region in separated flows is presented. A detailed description of the experimental setup and PIV measurement procedures is given in a technical report.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , ix, 185 p.
Series
Trita-MEK, ISSN 0348-467X ; 2006:15
Keyword [en]
Fluid mechanics, turbulence, flow separation, turbulence modelling, asymmetric diffuser, boundary layer, PIV, vortex generator, separation control
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-4071ISBN: 91-7178-416-0 (print)OAI: oai:DiVA.org:kth-4071DiVA: diva2:10636
Public defence
2006-09-08, Salongen, Biblioteket KTH, Osquars backe 31, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100923Available from: 2006-08-09 Created: 2006-08-09 Last updated: 2010-09-23Bibliographically approved
List of papers
1. Measurements of mean flow statistics in a plane asymmetric diffuser with 8.5 degrees opening angle
Open this publication in new window or tab >>Measurements of mean flow statistics in a plane asymmetric diffuser with 8.5 degrees opening angle
(English)Manuscript (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-6045 (URN)
Note
QC 20100923Available from: 2006-08-09 Created: 2006-08-09 Last updated: 2010-09-23Bibliographically approved
2. Structural and temporal characteristics of the flow in a plane asymmetric diffuser with 8.5 degrees opening angle
Open this publication in new window or tab >>Structural and temporal characteristics of the flow in a plane asymmetric diffuser with 8.5 degrees opening angle
(English)Manuscript (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-6046 (URN)
Note
QC 20100923Available from: 2006-08-09 Created: 2006-08-09 Last updated: 2010-09-23Bibliographically approved
3. A Reynolds stress closure description of separation control with vortex generators in a plane asymmetric diffuser
Open this publication in new window or tab >>A Reynolds stress closure description of separation control with vortex generators in a plane asymmetric diffuser
2007 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 11, 115108- p.Article in journal (Refereed) Published
Abstract [en]

A way to model the effects of streamwise vortices in a turbulent flow with one homogeneous direction is presented. The Reynolds averaged Navier-Stokes equations are solved with a differential Reynolds stress turbulence model. Assuming that the vortices can be approximated with the Lamb-Oseen model, wall-normal Reynolds stress distributions are calculated, corresponding to the spanwise variances of the estimated velocity distribution downstream of the vortex generators. The Reynolds stress contributions that are due to the vortex generators are added to the Reynolds stresses from the turbulence model so as to mimic the increased mixing due to the vortex generators. Volume forces are applied also in the mean momentum equations to account for the drag of the vortex generators. The model is tested and compared with experimental data from a plane asymmetric diffuser flow which is separating without vortex generators. The results indicate that the model is able to mimic the major features of vortex generator flow control and that the flow case in question is susceptible to separation control. The model results show that the pressure recovery of the diffuser could be increased by almost 10% by applying vortex generators and that, if keeping the shape of the vortex generators fixed, their optimal position is close to the diffuser inlet. Computations also indicated that the time to re-establish the separation zone when the control suddenly is turned off is substantially longer than the time it takes to remove the separation after the control is turned on again. Some work on adapting a differential Reynolds stress turbulence model was necessary in order to make it capable of realistic predictions of the asymmetric diffuser flow in which the vortex generator model is tested. However, the main focus of the article is on the modelling of vortex generator effects.

Keyword
near-wall turbulence, flows, model, simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-6047 (URN)10.1063/1.2800877 (DOI)000251325700040 ()2-s2.0-36849006329 (Scopus ID)
Note
Uppdaterad från manuskript till artikel: 20100923. QC 20100923Available from: 2006-08-09 Created: 2006-08-09 Last updated: 2017-12-14Bibliographically approved
4. Numerical and experimental study of separated flow in a plane asymmetric diffuser
Open this publication in new window or tab >>Numerical and experimental study of separated flow in a plane asymmetric diffuser
Show others...
2004 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 25, no 3, 451-460 p.Article in journal (Refereed) Published
Abstract [en]

Computations of the turbulent flow through plane asymmetric diffusers for opening angles from 8degrees to 10degrees have been carried out with the explicit algebraic Reynolds stress model (EARSM) of Wallin and Johansson [J. Fluid Mech. 403 (2000) 89]. It is based on a two-equation platform in the form of a low-Re K - omega formulation, see e.g. Wilcox [Turbulence Modeling for CFD, DCW Industries Inc., 1993]. The flow has also been studied experimentally for the 8.5degrees opening angle using PIV and LDV. The models under-predict the size and magnitude of the recirculation zone. This is, at least partially, attributed to an over-estimation of the wall normal turbulence component in a region close to the diffuser inlet and to the use of damping functions in the near-wall region. By analyzing the balance between the production and dissipation of the turbulence kinetic energy we find that the predicted dissipation is too large. Hence, we can identify a need for improvement of the modeling the transport equation for the turbulence length-scale related quantity.

Keyword
plane asymmetric diffuser, separated flow, explicit algebraic Reynolds stress model, EARSM, automated code generation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-6048 (URN)10.1016/j.ijheatfluidflow.2004.02.012 (DOI)000222282000014 ()2-s2.0-2542557398 (Scopus ID)
Conference
3rd International Symposium on Turbulence and Shear Flow Phenomena (TSFP-3) Sendai, JAPAN, JUN 25-27, 2003
Note
QC 20100923Available from: 2006-08-09 Created: 2006-08-09 Last updated: 2017-12-14Bibliographically approved
5. A note on the influence of negative turbulence production on separating flows
Open this publication in new window or tab >>A note on the influence of negative turbulence production on separating flows
(English)Manuscript (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-6049 (URN)
Note
QC 20100923Available from: 2006-08-09 Created: 2006-08-09 Last updated: 2010-09-23Bibliographically approved
6. Experimental procedures for the investigation of the turbulent flow in a plane asymmetric diffuser by means of stereoscopic high-speed PIV
Open this publication in new window or tab >>Experimental procedures for the investigation of the turbulent flow in a plane asymmetric diffuser by means of stereoscopic high-speed PIV
(English)Manuscript (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-6050 (URN)
Note
QC 20100923Available from: 2006-08-09 Created: 2006-08-09 Last updated: 2010-09-23Bibliographically approved

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CiteExportLink to record
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Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
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  • de-DE
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  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
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