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Separation control by an array of vortex generator jets: Part 2. Pulsed jets
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
(English)Article in journal (Other academic) Submitted
Abstract [en]

The effect of longitudinal vortices produced by an array of steady jets on a separation bubble was examined experimentally. In the experiment an adverse pressure gradient causes the turbulent boundary layer on a at plate to separate. The jets are originating from ori ces in the wall and are directed 45° from the wall and 90° from the mean ow direction. In the centre of the separated region, particle image velocimetry (PIV) is used to measure the momentum increase near the wall that the vortices produce. The geometry was xed, but the ratio of jet velocity Ujet to the free stream velocity, the pulsing frequency and the duty cycle were varied. It was shown that to achieve maximum control e ect the injected mass ow should be as large as possible, within an optimal range of jet velocity ratios. For a given injected mass ow the important parameter was shown to be the injection time t1. A non-dimensional injection time is de ned as t+ 1 = t1Ujet /d, where d is the jet ori ce diameter. Here, the optimal t+ 1 was in the range 100-200.

National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-9839OAI: oai:DiVA.org:kth-9839DiVA: diva2:133541
Note
This work is part of a cooperative research program between KTH and Scania CV. QS 20120328Available from: 2009-01-12 Created: 2009-01-12 Last updated: 2012-03-28Bibliographically 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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