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Turbulent Boundary Layer Separation and Control
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
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. , p. vi, 41
Series
Trita-MEK, ISSN 0348-467X ; 2008:11
Keyword [en]
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: urn:nbn:se:kth:diva-9821ISBN: 978-91-7415-203-6 (print)OAI: oai:DiVA.org:kth-9821DiVA, id: diva2:133356
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
List of papers
1. On the scaling of turbulent separating boundary layers
Open this publication in new window or tab >>On the scaling of turbulent separating boundary layers
2008 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 7, p. 075104-1-075104-4Article in journal (Refereed) Published
Abstract [en]

This study focuses on the mean velocity distribution of turbulent boundary layers near, at, and after separation. The proper mean velocity scaling of the outer region in strong adverse pressure gradients and separated turbulent boundary layers is still under debate and over the years various different velocity scales have been proposed. Here the scaling proposed by Zagarola and Smits [J. Fluid Mech. 373, 33 (1998)] is applied to data from three different separated flows. In all cases the mean velocity defect profiles are self-similar in the region between separation and the position of maximum mean reverse flow. Downstream of the reverse flow maximum, the profiles change, but they are still self-similar within that region. It was also found that the mean velocity defect profiles of all three pressure gradients show similarity in the region between separation and the position of maximum mean reverse flow.

Keyword
Aerodynamics; Boundary layer flow; Boundary layers; Gradient methods; Hydrodynamics; Isomers; Meteorology; Pressure gradient; Separation; Turbulent flow; Adverse pressure gradients; Different velocity; Mean velocities; Mean velocity distributions; Pressure gradients; Reverse flow; Self-similar; Separated flows; Turbulent boundary layers
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9833 (URN)10.1063/1.2958317 (DOI)000258175600027 ()2-s2.0-48849110214 (Scopus ID)
Note
QC 20100825Available from: 2009-01-12 Created: 2009-01-12 Last updated: 2017-12-14Bibliographically approved
2. Streamwise evolution of longitudinal vortices in a turbulent boundary layer
Open this publication in new window or tab >>Streamwise evolution of longitudinal vortices in a turbulent boundary layer
2009 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 623, p. 27-58Article in journal (Refereed) Published
Abstract [en]

 In this experimental study both smoke visualisation and three component hotwire measurements have been performed in order to characterize the streamwise evolution of longitudinal counter-rotating vortices in a turbulent boundary layer. The vortices were generated by means of vortex generators (VGs) in different configurations. Both single pairs and arrays in a natural setting as well as in yaw have been considered. Moreover three different vortex blade heights h, with the spacing d and the distance to the neighbouring vortex pair D for the array con guration, were studied keeping the same d / h and D / h ratios. It is shown that the vortex core paths scale with h in the streamwise direction and with D and h in the spanwise and wall-normal directions, respectively. A new peculiar "hooklike" vortex core motion, seen in the cross-ow plane, has been identi ed in the far region, starting around 200h and 50h for the pair and the array con guration, respectively. This behaviour is explained in the paper. Furthermore the experimental data indicate that the vortex paths asymptote to a prescribed location in the cross-ow plane, which rst was stated as a hypothesis and later veri ed. This observation goes against previously reported numerical results based on inviscid theory. An account for the important viscous e ects is taken in a pseudo-viscous vortex model which is able to capture the streamwise core evolution throughout the measurement region down to 450h. Finally, the e ect of yawing is reported, and it is shown that spanwiseaveraged quantities such as the shape factor and the circulation are hardly perceptible. However, the evolution of the vortex cores are di erent both between the pair and the array con guration and in the natural setting versus the case with yaw. From a general point of view the present paper reports on fundamental results concerning the vortex evolution in a fully developed turbulent boundary layer.

Keyword
Array configurations; Counter-rotating vortices; Cross flows; Experimental data; Experimental studies; Hot-wire measurements; Inviscid theory; Longitudinal vortices; Numerical results; Shape factor; Smoke visualization; Streamwise directions; Three-component; Turbulent boundary layers; Viscous effect; Vortex cores; Vortex generators; Vortex model; Vortex pair; Vortex paths; Wall-normal direction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9835 (URN)10.1017/S0022112008004825 (DOI)000264507000002 ()2-s2.0-65649153763 (Scopus ID)
Note
QC 20100825. Uppdaterad från accepted till published (20100825).Available from: 2009-01-12 Created: 2009-01-12 Last updated: 2017-12-14Bibliographically approved
3. On the robustness of separation control by streamwise vortices
Open this publication in new window or tab >>On the robustness of separation control by streamwise vortices
2010 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 29, no 1, p. 9-17Article 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.

Keyword
Adverse pressure gradient; Turbulent boundary layer; Flow separation; Flow control; Vortex generators
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9837 (URN)10.1016/j.euromechflu.2009.09.001 (DOI)000272928900002 ()2-s2.0-71649098287 (Scopus ID)
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: 2017-12-14Bibliographically approved
4. Separation control by an array of vortex generator jets: Part 1. Steady jets
Open this publication in new window or tab >>Separation control by an array of vortex generator jets: Part 1. Steady jets
(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. A adverse pressure gradient on a at plate causes the turbulent boundary layer to separate. The jets are originating from orifices in the wall and are directed 45° from the wall and 90° from the mean flow 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 produces. An effect maximum is found for a jet velocity that is 5 times the test section inlet velocity. Maxima based on volume flow effciency and energy effciency are also found at lower jet velocities. Furthermore, it is shown that the highest possible effect of the jet array is comparable to that of a vane-type vortex generator array. In sidewind, the jet array is shown to be effective at yaw angles up to 40°.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-9841 (URN)
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
5. Separation control by an array of vortex generator jets: Part 2. Pulsed jets
Open this publication in new window or tab >>Separation control by an array of vortex generator jets: Part 2. Pulsed jets
(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:nbn:se:kth:diva-9839 (URN)
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

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