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Computational studies of passive vortex generators for flow control
KTH, School of Engineering Sciences (SCI), Mechanics. (Turbulence)
2009 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Many flow cases in fluid dynamics face undesirable flow separation due torising static pressure on wall boundaries. This occurs e.g. due to geometry as ina highly curved turbine inlet duct or e.g. on flow control surfaces such as wingtrailing edge flaps within a certain angle of attack range. Here, flow controldevices are often used in order to enhance the flow and delay or even totallyeliminate flow separation. Flow control can e.g. be achieved by using passiveor active vortex generators (VG) that enable momentum mixing in such flows.This thesis focusses on passive VGs, represented by VG vanes that are mountedupright on the surface in wall-bounded flows. They typically have an angle ofincidence to the mean flow and, by that, generate vortex structures that in turnallow for the desired momentum mixing in order to prevent flow separation.A statistical VG model approach, developed by KTH Stockholm and FOI,the Swedish Defence Research Agency, has been evaluated computationally.Such a statistical VG model approach removes the need to build fully resolvedthree-dimensional geometries of VGs in a computational fluid dynamics mesh.Usually, the generation of these fully resolved geometries is rather costly interms of preprocessing and computations. By applying this VG model, thecosts reduce to computations without VG effects included. Nevertheless, theVG model needs to be set up in order to define the modelled VG geometry inan easy and fast preprocessing step. The presented model has shown sensitivityfor parameter variations such as the modelled VG geometry and the VG modellocation in wall-bounded zero pressure gradient and adverse pressure gradientflows on a flat plate, in a diffuser, and on an airfoil with its high-lift systemextracted. It could be proven that the VG model qualitatively describes correcttrends and tendencies for these different applications.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , 108 p.
Series
Trita-MEK, ISSN 0348-467X ; 2009:18
Keyword [en]
passive flow control, vortex generator, statistical modelling, turbulence, separation prevention, flat plate, diffuser, high-lift design, airfoil
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-11737ISBN: 978-91-7415-503-7 (print)OAI: oai:DiVA.org:kth-11737DiVA: diva2:280761
Presentation
2009-12-16, E3, Main building, KTH campus, Osquars Backe 14, Stockholm, 10:15 (English)
Opponent
Supervisors
Available from: 2009-12-14 Created: 2009-12-11 Last updated: 2012-05-11Bibliographically approved
List of papers
1. Vortex-Generator Models for Zero- and Adverse-Pressure-Gradient Flows
Open this publication in new window or tab >>Vortex-Generator Models for Zero- and Adverse-Pressure-Gradient Flows
2012 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 50, no 4, 855-866 p.Article in journal (Refereed) Published
Abstract [en]

A computational fluid-dynamics investigation, including passive vortex generators (VGs) that generate streamwise counter-rotating vortex structures, usually requires a grid with fully resolved VG geometries and vortex structures with a corresponding large number of grid points to obtain an accurate solution. An efficient way to avoid such a setup and time-consuming process in turbulent shear-layer flows is to introduce statistics-based vortex-generator modeling. The second-order statistics of the initial vortices are computed by using a vortex model in combination with the lifting-line theory. The statistics are added as additional turbulence stress terms to the equations within a differential Reynolds stress-turbulence model. In this investigation, results from statistical VG model computations for zero- and adverse-pressure-gradient flat-plate boundary-layer flows, as well as for the flow in a plane asymmetric diffuser, are evaluated against results from fully resolved VG computations and experiments. It could be shown that the initial near-field forcing is too weak for the proposed VG model. An improved VG model description removes some drawbacks by adding additional statistical forcing terms. Results become more comparable:, resulting in improved predictions when compared to experiments and fully resolved computations.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-94047 (URN)10.2514/1.J051169 (DOI)000302277000009 ()2-s2.0-84859321971 (Scopus ID)
Funder
Swedish Research Council, 2010-6965Swedish e‐Science Research Center
Note

QC 20120508

Available from: 2012-05-08 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved
2. Evaluation of a vortex generator model in adverse pressure gradient boundary layers
Open this publication in new window or tab >>Evaluation of a vortex generator model in adverse pressure gradient boundary layers
2011 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 49, no 5, 982-993 p.Article in journal (Refereed) Published
Abstract [en]

The use of a two-dimensional statistical passive vortex generator model, applied to an adverse pressure gradient boundary-layer flow, is evaluated qualitatively against experimental and fully resolved vortex generator computations. The modeling approach taken here has the advantage of substantially reducing the complexity of including such flow separation control devices in a computational mesh, thus giving the opportunity to carry out faster parametric studies. Additional stresses, originating from the vortex generator model approach, are added as additional turbulent stresses to the mean governing equations instead of resolving vortex structures in the computational domain. The vortex generator model has been applied to allow direct comparison with prior experiments carried out at the Royal Institute of Technology Stockholm. Variations of the vortex generator streamwise position and tests of different vortex generator setups, such as co- and counter-rotational settings, are presented. Distributions of wall-pressure and skin-friction coefficients are used to evaluate the vortex generator model against fully resolved vortex generator data. It is shown that the vortex generator model successfully predicts attached and separated flow states. Moreover, the results illustrate the vortex generator model's capability to predict flow control sensitivity with respect to the streamwise position.

Keyword
LONGITUDINAL VORTICES, STREAMWISE VORTICES, SEPARATION, FLOW
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-25896 (URN)10.2514/1.J050680 (DOI)000291072000010 ()2-s2.0-79955448523 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note
QC 20101103 Uppdaterad från submitted till published (20110613).Available from: 2010-11-03 Created: 2010-11-03 Last updated: 2017-12-12Bibliographically approved

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