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Stability and transition of three-dimensional boundary layers
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
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 [en]
Receptivity, stability, optimal growth, three-dimensional boundary layers, crossflow instability, roughness control, freestream turbulence, secondary instability
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-123175ISBN: 978-91-7501-808-9 (print)OAI: oai:DiVA.org:kth-123175DiVA: diva2:625111
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
List of papers
1. Stabilization of a swept-wing boundary layer by distributed roughness elements
Open this publication in new window or tab >>Stabilization of a swept-wing boundary layer by distributed roughness elements
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.

Keyword
boundary layer receptivity, flow control, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-119099 (URN)10.1017/jfm.2013.33 (DOI)000314643700001 ()2-s2.0-84873631398 (Scopus ID)
Note

QC 20130314

Available from: 2013-03-14 Created: 2013-03-07 Last updated: 2017-12-06Bibliographically approved
2. Effect of freestream turbulence on roughness-induced crossflow instability
Open this publication in new window or tab >>Effect of freestream turbulence on roughness-induced crossflow instability
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
Swept-wing boundary layer, surface roughness, receptivity, freestream turbulence, crossflow instability
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-123192 (URN)
Note

QC 20130604

Available from: 2013-06-04 Created: 2013-06-04 Last updated: 2013-06-04Bibliographically approved
3. Optimal initial perturbations in streamwise corner-flow
Open this publication in new window or tab >>Optimal initial perturbations in streamwise corner-flow
Show others...
2013 (English)Report (Other academic)
Abstract [en]

Localised optimal initial perturbations are studied to gain an understanding of the global stability properties of streamwise corner-flow. A self-similar and a modified base-flow are considered. The latter mimics a characteristic deviation from the self-similar solution, commonly observed in experiment. Poweriterations in terms of subsequent direct and adjoint linearised Navier-Stokes solution sweeps are employed to converge optimal solutions for two optimisation times. The optimal response manifests as a wave packet that initially gains energy through the Orr mechanism and continues growing exponentially thereafter. The study at hand represents the first global stability analysis of streamwise corner-flow and confirms key observations made in theoretical and/or experimental work on the subject. Namely, the presence of an inviscid instability mechanism in the near-corner region and a destabilising effect of the characteristic mean-flow deformation found in experiment.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 13 p.
Keyword
Corner flow, optimal initial condition, power iteration, adjoint
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-123194 (URN)
Note

QC 20130604

Available from: 2013-06-04 Created: 2013-06-04 Last updated: 2015-11-25Bibliographically approved

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