Numerical Investigation of the Role of Free-Stream Turbulence on Boundary-Layer Separation and Separation Control

Persistent Link:
http://hdl.handle.net/10150/204289
Title:
Numerical Investigation of the Role of Free-Stream Turbulence on Boundary-Layer Separation and Separation Control
Author:
Balzer, Wolfgang
Issue Date:
2011
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Embargo:
Embargo: Release after 4/26/2012
Abstract:
The aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by laminar separation. Understanding of the physical mechanisms and hydrodynamic instabilities that are associated with laminar separation and the formation of laminar separation bubbles (LSBs) is key for the design and development of effective and efficient active flow control (AFC) devices. For the present work, laminar separation and its control were investigated numerically by employing highly-accurate direct numerical simulations (DNS).For a LSB on a curved plate, the primary and secondary instability of the uncontrolled flow were investigated. An inviscid Kelvin-Helmholtz (KH) instability was found to be responsible for the shedding of predominantly two-dimensional (2D) vortices. The onset of transition was caused by temporally-growing three-dimensional (3D) disturbances inside the separated region, which were supported by elliptical and hyperbolic secondary instabilities. The hyperbolic instability was demonstrated to be of absolute/global nature. High-amplitude forcing using pulsed vortex generator jets and 2D time-periodic blowing was found to exploit the KH instability and lead to a significant reduction in bubble size. In addition, the 2D forcing was found to suppress the secondary instabilities such that transition to turbulence was delayed.The role of free-stream turbulence (FST) in the transition process was investigated for a LSB on a flat plate. FST was shown to cause the formation of streamwise-elongated streaks inside the boundary layer. For the uncontrolled LSB, increasing the FST levels led to accelerated transition and a reduction in bubble size. The stage of linear disturbance growth due to the inviscid KH instability was not ``bypassed''. Flow control by means of 2D periodic excitation was found to remain effective, since it could exploit the KH instability and suppress secondary absolute instabilities. Transition was initiated by an interaction of the 2D wave introduced by the forcing and the streamwise boundary-layer streaks. The interaction led to a spanwise modulation of the 2D wave, which was amplified due to a convective elliptical instability.
Type:
text; Electronic Dissertation
Keywords:
boundary-layer separation; flow control; free-stream turbulence; hydrodynamic instabilities; transition to turbulence
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Aerospace Engineering
Degree Grantor:
University of Arizona
Advisor:
Fasel, Hermann F.

Full metadata record

DC FieldValue Language
dc.language.isoenen_US
dc.titleNumerical Investigation of the Role of Free-Stream Turbulence on Boundary-Layer Separation and Separation Controlen_US
dc.creatorBalzer, Wolfgangen_US
dc.contributor.authorBalzer, Wolfgangen_US
dc.date.issued2011-
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.releaseEmbargo: Release after 4/26/2012en_US
dc.description.abstractThe aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by laminar separation. Understanding of the physical mechanisms and hydrodynamic instabilities that are associated with laminar separation and the formation of laminar separation bubbles (LSBs) is key for the design and development of effective and efficient active flow control (AFC) devices. For the present work, laminar separation and its control were investigated numerically by employing highly-accurate direct numerical simulations (DNS).For a LSB on a curved plate, the primary and secondary instability of the uncontrolled flow were investigated. An inviscid Kelvin-Helmholtz (KH) instability was found to be responsible for the shedding of predominantly two-dimensional (2D) vortices. The onset of transition was caused by temporally-growing three-dimensional (3D) disturbances inside the separated region, which were supported by elliptical and hyperbolic secondary instabilities. The hyperbolic instability was demonstrated to be of absolute/global nature. High-amplitude forcing using pulsed vortex generator jets and 2D time-periodic blowing was found to exploit the KH instability and lead to a significant reduction in bubble size. In addition, the 2D forcing was found to suppress the secondary instabilities such that transition to turbulence was delayed.The role of free-stream turbulence (FST) in the transition process was investigated for a LSB on a flat plate. FST was shown to cause the formation of streamwise-elongated streaks inside the boundary layer. For the uncontrolled LSB, increasing the FST levels led to accelerated transition and a reduction in bubble size. The stage of linear disturbance growth due to the inviscid KH instability was not ``bypassed''. Flow control by means of 2D periodic excitation was found to remain effective, since it could exploit the KH instability and suppress secondary absolute instabilities. Transition was initiated by an interaction of the 2D wave introduced by the forcing and the streamwise boundary-layer streaks. The interaction led to a spanwise modulation of the 2D wave, which was amplified due to a convective elliptical instability.en_US
dc.typetexten_US
dc.typeElectronic Dissertationen_US
dc.subjectboundary-layer separationen_US
dc.subjectflow controlen_US
dc.subjectfree-stream turbulenceen_US
dc.subjecthydrodynamic instabilitiesen_US
dc.subjecttransition to turbulenceen_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineAerospace Engineeringen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorFasel, Hermann F.en_US
dc.contributor.committeememberFasel, Hermann F.en_US
dc.contributor.committeememberKerschen, Edward J.en_US
dc.contributor.committeememberTumin, Anatolien_US
dc.contributor.committeememberRestrepo, Juan M.en_US
dc.identifier.proquest11512-
dc.identifier.oclc752261376-
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