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On stability, transition and turbulence in three-dimensional boundary-layer flows
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
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 [en]
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: urn:nbn:se:kth:diva-177617ISBN: 978-91-7595-783-8 (print)OAI: oai:DiVA.org:kth-177617DiVA: diva2:873675
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
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
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect of freestream turbulence on generation and breakdown of crossflow disturbances over a swept wing is investigated through direct numerical simu- lations. The setup of simulations follows the experiments performed by Downs et al. which were conducted at a very low freestream turbulence level. The stationary cross flow vortices are generated by a row of uniformly distributed roughness elements. Here, the isotropic freestream turbulence are numerically generated through separate simulations. The generated freestream fields are then added to the inflow boundary condition for simulation of flow over the swept wing. Different levels of freestream turbulence and roughness heights are considered. It was observed that low freestream turbulence level could play a major role in triggering transition of the flow dominated by stationary cross- flow vortices. Similar to the observations in the experiment, increasing the turbulence level moved the transition location further upstream. Moreover, it was found that slight increment in the height of critically spaced roughness elements had an stabilizing effect and delayed transition to turbulence. The ob- servations made in the present study may explain the counter-intuitive results of the recent flight experiment by Saric et al. (2015), where transition occurred further downstream on the wing model with painted surface compared to the model with polished surface. 

Publisher
31 p.
National Category
Mechanical Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-177607 (URN)
Note

QS 2015

Available from: 2015-11-24 Created: 2015-11-24 Last updated: 2015-11-25Bibliographically 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
4. Direct numerical simulation of the flow around a wing section at moderate Reynolds number
Open this publication in new window or tab >>Direct numerical simulation of the flow around a wing section at moderate Reynolds number
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-177610 (URN)10.1016/j.ijheatfluidflow.2016.02.001 (DOI)
Note

QC 20160525

Available from: 2015-11-24 Created: 2015-11-24 Last updated: 2016-05-25Bibliographically approved
5. Direct numerical simulation of flow around a turbine blade: A transition study
Open this publication in new window or tab >>Direct numerical simulation of flow around a turbine blade: A transition study
2015 (English)Report (Other academic)
Publisher
13 p.
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-177611 (URN)
Note

QC 20151125

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

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