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Effect of freestream turbulence on roughness-induced crossflow instability
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)Report (Other academic)
Abstract [en]

The effect of freestream turbulence on generation of crossflow disturbances over swept wings is investigated through direct numerical simulations.  The set up follows  the  experiments  performed  by Downs  et  al.  in their  TAMU  experi- ment.  In this experiment the authors use ASU(67)-0315 wing geometry which promotes  growth  of crossflow  disturbances.   Distributed  roughness  elements are locally placed near the leading edge with a span-wise wavenumber, to ex- cite the corresponding crossflow vortices.  The response of boundary layer to external disturbances such as roughness heights, span-wise wavenumbers, Rey- nolds numbers and freestream turbulence characteristics are studied.  It must be noted that the experiments were conducted at a very low level of freestream turbulence  intensity  (T u).   In this  study,  we fully  reproduce the  freestream isotropic homogenous turbulence through a DNS code using detailed freestream spectrum data provided by the experiment. The generated freestream fields are then applied as the inflow boundary condition for direct numerical simulation of the wing. The geometrical set up is the same as the experiment along with application of distributed roughness elements near the leading edge to precipi- tate stationary crossflow disturbances.  The effects of the generated freestream turbulence are then studied on the initial amplitudes and growth of the bound- ary layer perturbations.  It appears that the freestream turbulence damps out the dominant stationary crossflow vortices.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , 13 p.
Keyword [en]
Swept-wing boundary layer, surface roughness, receptivity, freestream turbulence, crossflow instability
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-123192OAI: oai:DiVA.org:kth-123192DiVA: diva2:625311
Note

QC 20130604

Available from: 2013-06-04 Created: 2013-06-04 Last updated: 2013-06-04Bibliographically 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

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