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Stabilization of a swept-wing boundary layer by distributed roughness elements
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-5913-5431
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-7864-3071
2013 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 718, R1- p.Article in journal (Refereed) Published
Abstract [en]

The stabilization of a swept-wing boundary layer by distributed surface roughness elements is studied by performing direct numerical simulations. The configuration resembles experiments studied by Saric and coworkers at Arizona State University, who employed this control method in order to delay transition. An array of cylindrical roughness elements are placed near the leading edge to excite subcritical cross-flow modes. Subcritical refers to the modes that are not critical with respect to transition. Their amplification to nonlinear amplitudes modifies the base flow such that the most unstable cross-flow mode and secondary instabilities are damped, resulting in downstream shift of the transition location. The experiments by Saric and coworkers were performed at low levels of free stream turbulence, and the boundary layer was therefore dominated by stationary cross-flow disturbances. Here, we consider a more complex disturbance field, which comprises both steady and unsteady instabilities of similar amplitudes. It is demonstrated that the control is robust with respect to complex disturbance fields as transition is shifted from 45 to 65% chord.

Place, publisher, year, edition, pages
2013. Vol. 718, R1- p.
Keyword [en]
boundary layer receptivity, flow control, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-119099DOI: 10.1017/jfm.2013.33ISI: 000314643700001Scopus ID: 2-s2.0-84873631398OAI: oai:DiVA.org:kth-119099DiVA: diva2:611011
Note

QC 20130314

Available from: 2013-03-14 Created: 2013-03-07 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Stability and transition of three-dimensional boundary layers
Open this publication in new window or tab >>Stability and transition of three-dimensional boundary layers
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

A focus has been put on the stability characteristics of different flow types existing on air vehicles. Flow passing over wings and different junctions on an aircraft face numerous local features, ranging from different pressure gradients, to interacting boundary layers. Primarily, stability characteristics of flow over a wing subject to negative pressure gradient is studied. The current numerical study conforms to an experimental study conducted by Saric and coworkers, in their Arizona State University wind tunnel experiments. Within that framework, a passive control mechanism has been tested to delay transition of flow from laminar to turbulence. The same control approach has been studied here, in addition to underling mechanisms playing major roles in flow transition, such as nonlinear effects and secondary instabilities.

Another common three-dimensional flow feature arises as a result of streamlines passing through a junction, the so called corner-flow. For instance, this flow can be formed in the junction between the wing and fuselage on a plane. A series of direct numerical simulations using linear Navier-Stokes equations have been performed to determine the optimal initial perturbation. Optimal refers to a perturbation which can gain the maximum energy from the flow over a period of time. Power iterations between direct and adjoint Navier- Stokes equations determine the optimal initial perturbation. In other words this method seeks to determine the worst case scenario in terms of perturbation growth. Determining the optimal initial condition can help improve the design of such surfaces in addition to possible control mechanisms.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. viii, 20 p.
Series
Trita-MEK, ISSN 0348-467X ; 2013:14
Keyword
Receptivity, stability, optimal growth, three-dimensional boundary layers, crossflow instability, roughness control, freestream turbulence, secondary instability
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-123175 (URN)978-91-7501-808-9 (ISBN)
Presentation
2013-06-13, E3, Osquars Backe 14, KTH, Stockholm, 10:19 (English)
Opponent
Supervisors
Projects
RECEPT
Funder
EU, FP7, Seventh Framework Programme, 76274
Note

QC 20130604

Available from: 2013-06-04 Created: 2013-06-04 Last updated: 2013-06-10Bibliographically approved
2. On stability, transition and turbulence in three-dimensional boundary-layer flows
Open this publication in new window or tab >>On stability, transition and turbulence in three-dimensional boundary-layer flows
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A lot has changed since that day on December 17, 1903 when the Wright brothers made the first powered manned flight. Even though the concepts behind flying are unaltered, appearance of stat-of-the-art modern aircrafts has undergone a massive evolution. This is mainly owed to our deeper understanding of how to harness and optimize the interaction between fluid flows and aircraft bodies. Flow passing over wings and different junctions on an aircraft faces numerous local features, for instance, acceleration or deceleration, laminar or turbulent state, and interacting boundary layers. In our study we aim to characterize some of these flow features and their physical roles.

Primarily, stability characteristics of flow over a wing subject to a negative pressure gradient are studied. This is a common condition for flows over swept wings. Part of the current numerical study conforms to existing experimental studies where a passive control mechanism has been tested to delay laminarturbulent transition. The same flow type has also been considered to study the receptivity of three-dimensional boundary layers to freestream turbulence. The work entails investigation of effects of low-level freestream turbulence on crossflow instability, as well as interaction with micron-sized surface roughness elements.

Another common three-dimensional flow feature arises as a resultof stream-lines passing through a junction, the so-calledcorner-flow. For instance, thisflow can be formed in the junction between the wing and fuselage on aplane.A series of direct numerical simulations using linear Navier-Stokes equationshave been performed to determine the optimal initial perturbation. Optimalrefers to perturbations which can gain the maximum energy from the flow overa period of time. In other words this method seeks to determine theworst-casescenario in terms of perturbation growth. Here, power-iterationtechnique hasbeen applied to the Navier-Stokes equations and their adjoint to determine theoptimal initial perturbation.

Recent advances in super-computers have enabled advance computational methods to increasingly contribute to design of aircrafts, in particular for turbulent flows with regions of separation. In this work we investigate theturbulentflow on an infinite wing at a moderate chord Reynolds number of Re= 400,000 using a well resolved direct numerical simulation. A conventional NACA4412 has been chosen for this work. The turbulent flow is characterizedusing statistical analysis and following time history data in regions with interesting flow features.

In the later part of this work, direct numerical simulation has been chosen as a tool to mainly investigate the effect of freestream turbulence on the transition mechanism of flow from laminar to turbulent around a turbine blade.

 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2015. xvi, 49 p.
Keyword
Receptivity, stability, optimal growth, three-dimensional bound- ary layers, crossflow instability, roughness control, freestream turbulence, sec- ondary instability, transition, turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-177617 (URN)978-91-7595-783-8 (ISBN)
Public defence
2015-12-14, Sal F3, Lindstedsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20151125

Available from: 2015-11-25 Created: 2015-11-24 Last updated: 2015-11-25Bibliographically approved

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Hanifi, ArdeshirHenningson, Dan S.

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