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On the robustness of separation control by streamwise vortices
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
Vattenfall Research and Development AB.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-1146-3241
2010 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 29, no 1, 9-17 p.Article in journal (Refereed) Published
Abstract [en]

The robustness of vane-type vortex generators (VGs) for separation flow control was studied in a separating turbulent boundary layer on a flat plate. VG arrays of different sizes and streamwise positions were positioned upstream of the separation bubble and their effect on the flow field was studied with the help of particle image velocimetry (PIV). The extent of the separated region was varied by changing the pressure gradient. Three different separation bubbles were produced and their extent was approximately doubled for each increase in pressure gradient. It was found that the sensitivity of the control effect to changes in the size of the separation bubble is small within the applied range of pressure gradients. Furthermore, the importance of the relative position of the VGs with respect to the separated region is small.

Place, publisher, year, edition, pages
2010. Vol. 29, no 1, 9-17 p.
Keyword [en]
Adverse pressure gradient; Turbulent boundary layer; Flow separation; Flow control; Vortex generators
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-9837DOI: 10.1016/j.euromechflu.2009.09.001ISI: 000272928900002Scopus ID: 2-s2.0-71649098287OAI: oai:DiVA.org:kth-9837DiVA: diva2:133521
Note
QC 20100825. Uppdaterad från manuskript till artikel (20100825). Ola Lögdberg acknowledges Scania CV for the opportunity to carry out his doctoral work at KTH Mechanics within the Linné Flow Centre.Available from: 2009-01-12 Created: 2009-01-12 Last updated: 2011-01-25Bibliographically approved
In thesis
1. Turbulent Boundary Layer Separation and Control
Open this publication in new window or tab >>Turbulent Boundary Layer Separation and Control
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Boundary layer separation is an unwanted phenomenon in most technical applications, as for instance on airplane wings, ground vehicles and in internal flow systems. If separation occurs, it causes loss of lift, higher drag and energy losses. It is thus essential to develop methods to eliminate or delay separation.In the present experimental work streamwise vortices are introduced in turbulent boundary layers to transport higher momentum fluid towards the wall. This enables the boundary layer to stay attached at  larger pressure gradients. First the adverse pressure gradient (APG) separation bubbles that are to be eliminated are studied. It is shown that, independent of pressure gradient, the mean velocity defect profiles are self-similar when the scaling proposed by Zagarola and Smits is applied to the data. Then vortex pairs and arrays of vortices of different initial strength are studied in zero pressure gradient (ZPG). Vane-type vortex generators (VGs) are used to generate counter-rotating vortex pairs, and it is shown that the vortex core trajectories scale with the VG height h and the spanwise spacing of the blades. Also the streamwise evolution of the turbulent quantities scale with h. As the vortices are convected downstream they seem to move towards a equidistant state, where the distance from the vortex centres to the wall is half the spanwise distance between two vortices. Yawing the VGs up to 20° do not change the generated circulation of a VG pair. After the ZPG measurements, the VGs where applied in the APG mentioned above. It is shown that that the circulation needed to eliminate separation is nearly independent of the pressure gradient and that the streamwise position of the VG array relative to the separated region is not critical to the control effect. In a similar APG jet vortex generators (VGJs) are shown to as effective as the passive VGs. The ratio VR of jet velocity and test section inlet velocity is varied and a control effectiveness optimum is found for VR=5. At 40° yaw the VGJs have only lost approximately 20% of the control effect. For pulsed VGJs the pulsing frequency, the duty cycle and VR were varied. It was shown that to achieve maximum control effect the injected mass flow rate should be as large as possible, within an optimal range of jet VRs. For a given injected mass flow rate, the important parameter was shown to be the injection time t1. A non-dimensional injection time is defined as t1+ = t1Ujet/d, where d is the jet orifice diameter. Here, the optimal  t1+ was 100-200.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. vi, 41 p.
Series
Trita-MEK, ISSN 0348-467X ; 2008:11
Keyword
Flow control, adverse pressure gradient (APG), flow separation, vortex generators, jet vortex generators, pulsed jet vortex generators
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9821 (URN)978-91-7415-203-6 (ISBN)
Public defence
2009-01-23, F3, KTH, Lindstedtsvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20100825Available from: 2009-01-12 Created: 2009-01-09 Last updated: 2010-08-25Bibliographically approved

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Alfredsson, P. Henrik

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