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Computational fluid-dynamics investigations of vortex generators for flow-separation control
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Many flow cases in fluid dynamics face undesirable flow separation due to ad-verse pressure gradients on wall boundaries. This occurs, for example, due togeometrical reasons as in a highly curved turbine-inlet duct or on flow-controlsurfaces such as wing trailing-edge flaps within a certain angle-of-attack range.Here, flow-control devices are often used in order to enhance the flow and delayor even totally eliminate flow separation. Flow control can e.g. be achieved byusing passive or active vortex generators (VGs) for momentum mixing in theboundary layer of such flows. This thesis focusses on such passive and activeVGs and their modelling for computational fluid dynamics investigations.

First, a statistical VG model approach for passive vane vortex genera-tors (VVGs), developed at the Royal Institute of Technology Stockholm andthe Swedish Defence Research Agency, was evaluated and further improvedby means of experimental data and three-dimensional fully-resolved computa-tions. This statistical VVG model approach models those statistical vortexstresses that are generated at the VG by the detaching streamwise vortices.This is established by means of the Lamb-Oseen vortex model and the Prandtllifting-line theory for the determination of the vortex strength. Moreover, thisansatz adds the additional vortex stresses to the turbulence of a Reynolds-stresstransport model. Therefore, it removes the need to build fully-resolved three-dimensional geometries of VVGs in a computational fluid dynamics mesh. Usu-ally, the generation of these fully-resolved geometries is rather costly in termsof preprocessing and computations. By applying VVG models, the costs arereduced to that of computations without VVGs. The original and an improvedcalibrated passive VVG model show sensitivity for parameter variations suchas the modelled VVG geometry and the VVG model location on a flat plate inzero- and adverse-pressure-gradient flows, in a diffuser, and on an airfoil withits high-lift system extracted. It could be shown that the passive VG modelqualitatively and partly quantitatively describes correct trends and tendenciesfor these different applications.

In a second step, active vortex-generator jets (VGJs) are considered. They were experimentally investigated in a zero-pressure-gradient flat-plate flow atTechnische Universitä̈t Braunschweig, Germany, and have been re-evaluated for our purposes and a parameterization of the generated vortices was conducted. Dependencies of the generated vortices and their characteristics on the VGJsetup parameters could be identified and quantified. These dependencies wereused as a basis for the development of a new statistical VGJ model. This modeluses the ansatz of the passive VVG model in terms of the vortex model, theadditional vortex-stress tensor, and its summation to the Reynolds stress ten-sor. Yet, it does not use the Prandtl lifting-line theory for the determinationof the circulation but an ansatz for the balance of the momentum impact thatthe VGJ has on the mean flow. This model is currently under developmentand first results have been evaluated against experimental and fully-resolvedcomputational results of a flat plate without pressure gradient.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , viii, 51 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:04
Keyword [en]
flow-separation control, vane vortex generator, vortex generator jet, zero-pressure-gradient turbulent boundary layer, adverse-pressure-gradient turbulent boundary layer, statistical modelling, turbulence, Reynolds stress- transport model, computational fluid dynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-94879ISBN: 978-91-7501-331-2 (print)OAI: oai:DiVA.org:kth-94879DiVA: diva2:526287
Public defence
2012-05-16, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20120511

Available from: 2012-05-11 Created: 2012-05-11 Last updated: 2013-06-10Bibliographically 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
3. Evaluation and Parameterization of Round Vortex Generator Jet Experiments for Flow Control
Open this publication in new window or tab >>Evaluation and Parameterization of Round Vortex Generator Jet Experiments for Flow Control
Show others...
2012 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 50, no 11, 2508-2524 p.Article in journal (Refereed) Published
Abstract [en]

Particle image velocimetry experiments on the application of vortex generator jets (VGJs) with single and pair configurations in a zero-pressure gradient flat plate boundary-layer flow have been carried out at the Technische Universitat Braunschweig. The experiments were analyzed at the Royal Institute of Technology by means of a velocity triple decomposition that enables the extraction of vortex velocity fields. The overall aim of this contribution is to identify and classify the influence of the different VGJ parameters on the generated vortex structures so that conclusions may be drawn for efficient flow separation control configurations. The spanwise-averaged second-order statistics (ViVj) over bar (y) of the vortex velocity field V-i(y, z) are computed in order to evaluate the influence of the VGJ parameters. It could be shown that the velocity ratio lambda and the skew angle beta have a major influence. Furthermore, the chosen single VGJ data are compared to corresponding VGJ pair results of a similar actuator setup. It was found that the VGJ pairs could be statistically well described by means of single VGJ vortices. Moreover, VGJ arrays should give very similar results for the configurations examined, facilitating VGJ modeling based on a statistical ansatz.

Keyword
Turbulent-Boundary-Layer, Streamwise Vortices, Separation Control, Evolution, Optimization
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-94888 (URN)10.2514/1.J051694 (DOI)000310559000018 ()2-s2.0-84869102213 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20121210. Updated from accepted to published.

Available from: 2012-05-11 Created: 2012-05-11 Last updated: 2017-12-07Bibliographically approved
4. Statistical Vortex-Generator-Jet Model for Turbulent Flow Separation Control
Open this publication in new window or tab >>Statistical Vortex-Generator-Jet Model for Turbulent Flow Separation Control
2013 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 51, no 5, 1119-1129 p.Article in journal (Refereed) Published
Abstract [en]

This contribution describes the development and evaluation of a new statistical modeling approach for active vortex generator jets. Previous experiments from the Technische Universitat Braunschweig and their subsequent evaluation by the present authors showed that the induced flowfield can be reasonably well represented by two-dimensional Lamb-Oseen vortices. Based on that, an analytical expression for the Lamb-Oseen-vortex-model maximum circulation Gamma(max) was derived in terms of the freestream velocity U-infinity, the jet-to-freestream velocity ratio lambda, and the jet skew angle beta, as well as the actuator diameter Phi(VGJ). Based on the parameterized results, universal values for the Lamb-Oseen-vortex-model parameters at the actuator position were determined for the development of the statistical vortex-generator-jet model. The idea behind the statistical modeling approach is that the vortices are represented by their spanwise-averaged velocity correlations, or vortex stresses, that are added to the turbulence stresses in a Reynolds stress turbulence model. The spanwise-averaged vortex stresses are derived by computing the spanwise-averaged second-order statistics of the vortex flowfleld. These vortex-generator-jet model results were compared to the spanwise-averaged vortex stresses from experiments and from fully resolved computational fluid dynamics investigations, and reasonable qualitative as well as quantitative agreement was found.

Keyword
Boundary-Layer, Evolution, Vortices
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-94889 (URN)10.2514/1.J051987 (DOI)000318260700009 ()2-s2.0-84878335974 (Scopus ID)
Note

QC 20130610. Updated from submitted to published.

Available from: 2012-05-11 Created: 2012-05-11 Last updated: 2017-12-07Bibliographically approved
5. Application of a Statistical Vortex Generator Model on the Short-Chord Flap of aThree-Element Airfoil
Open this publication in new window or tab >>Application of a Statistical Vortex Generator Model on the Short-Chord Flap of aThree-Element Airfoil
2009 (English)Conference paper, Published paper (Refereed)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-94890 (URN)
Conference
the KATnet II conference on Key Aerodynamic Technologies 12-14 May 2009, Bremen, Germany
Note

QC 20120511

Available from: 2012-05-11 Created: 2012-05-11 Last updated: 2012-10-18Bibliographically approved

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