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Stability and transition in pitching wings
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-3344-9686
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aeroelastic stability of airplanes is one of the most important aspects of airplane design. Flutter or divergence instabilities arising out of the interaction of fluid forces and structural elasticity must be avoided by design or through the limitation of the flight envelope. Classical unsteady theories have been established since the 1930s however, recent investigations with laminar wings and in transitional flows have found the theories to be unreliable in these regimes. The current work investigates the flow around unsteady airfoils in these flow regimes. A linear framework for the stability analysis of fluid-structure-interaction (FSI) problems is derived and validated. The derived formulation is then used to investigate the changes in the structural sensitivity of an eigenvalue for an oscillating cylinder, which is found to change significantly when the fluid and structural systems are close to resonance. The linear stability analysis is then applied to investigate the aeroelastic stability of a NACA0012 airfoil with a free pitch-deegree-of-freedom at transitional Reynolds numbers. The stability results of the coupled FSI system are found to be in good agreement with previously performed experimental results and were able to predict the onset of aeroelastic pitch-oscillations. The boundary layer evolution for a natural laminar flow airfoil undergoing forced small-amplitude pitch-oscillations is investigated at Rec = 7.5×105. Large changes in laminar-to-turbulent transition location are found throughout the pitch cycle which cause a non-linear aerodynamic force response. The origins of the non-linear unsteady aerodynamic response is explained on the basis of the phase-lagged quasi-steady evolution of the boundary layer. A simple empirical model is developed using the phase-lag concept to model the unsteady aerodynamic forces which fits the experimental data very well. On the other hand, the forced pitching investigation at Rec = 1.0×105 for the same airfoil found abrupt changes in transition during the pitch-cycle. A local stability analysis in the reverse flow region indicates that the stability characteristics of the LSB change character from convective to absolute, and it is conjectured that this change in stability characteristics may be the cause of abrupt changes inboundary-layertransition.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 54
Series
TRITA-SCI-FOU ; 2019:46
National Category
Aerospace Engineering Fluid Mechanics and Acoustics
Research subject
Aerospace Engineering; Vehicle and Maritime Engineering
Identifiers
URN: urn:nbn:se:kth:diva-262927ISBN: 978-91-7873-348-4 (print)OAI: oai:DiVA.org:kth-262927DiVA, id: diva2:1365798
Public defence
2019-11-22, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Vinnova, 2014-00933EU, European Research Council, 694452-TRANSEP-ERC-2015- AdGSwedish e‐Science Research Center
Note

QC 20191028

Available from: 2019-10-28 Created: 2019-10-25 Last updated: 2019-10-30Bibliographically approved
List of papers
1. A re-examination of filter-based stabilization for spectral-element methods
Open this publication in new window or tab >>A re-examination of filter-based stabilization for spectral-element methods
2017 (English)Report (Other academic)
Publisher
p. 19
Series
TRITA-MEK, ISSN 0348-467X
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-217972 (URN)KTH/MEK/TR-17/15-SE (ISRN)978-91-7729-591-4 (ISBN)
Note

QC 20171121

Available from: 2017-11-20 Created: 2017-11-20 Last updated: 2019-10-25Bibliographically approved
2. Global Stability of rigid-body-motion fluid-structure-interaction problems
Open this publication in new window or tab >>Global Stability of rigid-body-motion fluid-structure-interaction problems
2019 (English)Report (Other academic)
Abstract [en]

A rigorous derivation and validation for linear fluid-structure-interaction (FSI) equations for a rigid-body-motion problem is performed in an Eulerian framework. We show that the “added-stiffness” terms arising in the formulation of Fanion et al. (2000) vanish at the FSI interface in a first-order approximation. Several numerical tests with rigid-body motion are performed to show the validity of the derived formulation by comparing the time evolution between the linear and non-linear equations when the base flow is perturbed by identical small-amplitude perturbations. In all cases both the growth rate and angular frequency of the instability matches within 0.1% accuracy. The derived formulation is used to investigate the phenomenon of symmetry breaking for a rotating cylinder with an attached splitter-plate. The results show that the onset of symmetry breaking can be explained by the existence of a zero-frequency linearly unstable mode of the coupled fluid-structure-interaction system. Finally, the structural sensitivity of the least stable eigenvalue is studied for an oscillating cylinder, which is found to change significantly when the fluid and structural frequencies are close to resonance.

Publisher
p. 38
Series
TRITA-SCI-RAP ; 2019:007
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-262856 (URN)
Funder
Swedish National Infrastructure for Computing (SNIC)
Note

QC 20191025. QC 20191030

Available from: 2019-10-23 Created: 2019-10-23 Last updated: 2019-10-30Bibliographically approved
3. On the onset of aeroelastic pitch-oscillations of aNACA0012 wing at transitional Reynolds numbers
Open this publication in new window or tab >>On the onset of aeroelastic pitch-oscillations of aNACA0012 wing at transitional Reynolds numbers
(English)Manuscript (preprint) (Other academic)
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-262905 (URN)
Funder
Swedish National Infrastructure for Computing (SNIC)
Note

QC 20191025

Available from: 2019-10-23 Created: 2019-10-23 Last updated: 2019-10-25Bibliographically approved
4. Linear and non-linear response of a laminar airfoil subject to small amplitude pitch-oscillations
Open this publication in new window or tab >>Linear and non-linear response of a laminar airfoil subject to small amplitude pitch-oscillations
(English)Manuscript (preprint) (Other academic)
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-262906 (URN)
Funder
Swedish National Infrastructure for Computing (SNIC)Vinnova, 2014-00933
Note

QC 20191025

Available from: 2019-10-23 Created: 2019-10-23 Last updated: 2019-10-25Bibliographically approved
5. Unsteady aerodynamic effects in small-amplitude pitch oscillations of anairfoil
Open this publication in new window or tab >>Unsteady aerodynamic effects in small-amplitude pitch oscillations of anairfoil
Show others...
2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 71, p. 378-391Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-262907 (URN)
Funder
Vinnova, 2014-00933Swedish National Infrastructure for Computing (SNIC)
Note

QC 20191025

Available from: 2019-10-23 Created: 2019-10-23 Last updated: 2019-10-25Bibliographically approved
6. Turbulent boundary layers around wing sections up to Re-c=1, 000, 000
Open this publication in new window or tab >>Turbulent boundary layers around wing sections up to Re-c=1, 000, 000
Show others...
2018 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, p. 86-99Article in journal (Refereed) Published
Abstract [en]

Reynolds-number effects in the adverse-pressure-gradient (APG) turbulent boundary layer (TBL) developing on the suction side of a NACA4412 wing section are assessed in the present work. To this end, we analyze four cases at Reynolds numbers based on freestream velocity and chord length ranging from Re-c = 100, 000 to 1,000,000, all of them with 5 degrees angle of attack. The results of four well-resolved large-eddy simulations (LESs) are used to characterize the effect of Reynolds number on APG TBLs subjected to approximately the same pressure-gradient distribution (defined by the Clauser pressure-gradient parameter beta). Comparisons of the wing profiles with zero pressure-gradient (ZPG) data at matched friction Reynolds numbers reveal that, for approximately the same beta distribution, the lower-Reynolds-number boundary layers are more sensitive to pressure-gradient effects. This is reflected in the values of the inner-scaled edge velocity U-e(+), the shape factor H, the components of the Reynolds-stress tensor in the outer region and the outer-region production of turbulent kinetic energy. This conclusion is supported by the larger wall-normal velocities and outer-scaled fluctuations observed in the lower-Re-c cases. Thus, our results suggest that two complementing mechanisms contribute to the development of the outer region in TBLs and the formation of large-scale energetic structures: one mechanism associated with the increase in Reynolds number, and another one connected to the APG. Future extensions of the present work will be aimed at studying the differences in the outer-region energizing mechanisms due to APGs and increasing Reynolds number.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE INC, 2018
Keywords
Large-eddy simulation, Turbulent boundary layer, Pressure gradient, Wing section
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-234198 (URN)10.1016/j.ijheatfluidflow.2018.04.017 (DOI)000441488400008 ()2-s2.0-85048126226 (Scopus ID)
Note

QC 20180911

Available from: 2018-09-11 Created: 2018-09-11 Last updated: 2019-10-25Bibliographically approved

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