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Hanifi, Ardeshir, DocentORCID iD iconorcid.org/0000-0002-5913-5431
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Publications (10 of 91) Show all publications
Shahriari, N., Kollert, M. R. & Hanifi, A. (2018). Control of a swept-wing boundary layer using ring-type plasma actuators. Journal of Fluid Mechanics, 844, 36-60
Open this publication in new window or tab >>Control of a swept-wing boundary layer using ring-type plasma actuators
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 844, p. 36-60Article in journal (Refereed) Published
Abstract [en]

Application of ring-type plasma actuators for control of laminar-turbulent transition in a swept-wing boundary layer is investigated thorough direct numerical simulations. These actuators induce a wall-normal jet in the boundary layer and can act as virtual roughness elements. The flow configuration resembles experiments by Kim et al. (2016 Technical Report. BUTERFLI Project TR D3.19, http://eprints.nottingham.ac.uk/id/eprint/46529). The actuators are modelled by the volume forces computed from the experimentally measured induced velocity field at the quiescent air condition. Stationary and travelling cross-flow vortices are triggered in the simulations by means of surface roughness and random unsteady perturbations. Interaction of vortices generated by actuators with these perturbations is investigated in detail. It is found that, for successful transition control, the power of the actuators should be increased to generate jet velocities that are one order of magnitude higher than those used in the experiments by Kim et al. (2016) mentioned above.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
boundary layer control, boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-227547 (URN)10.1017/jfm.2018.195 (DOI)2-s2.0-85044771928 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20180517

Available from: 2018-05-17 Created: 2018-05-17 Last updated: 2018-05-17Bibliographically approved
Dadfar, R., Hanifi, A. & Henningson, D. S. (2018). Control of instabilities in an unswept wing boundary layer. AIAA Journal, 56(5), 1750-1759
Open this publication in new window or tab >>Control of instabilities in an unswept wing boundary layer
2018 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 5, p. 1750-1759Article in journal (Refereed) Published
Abstract [en]

Linear control theory is used to construct an output feedback controller to attenuate the amplitude of the Tollmien–Schlichting waves inside the boundary layer developing over an unswept wing. The analysis is based on direct numerical simulations. The studied scenario includes the impulse response of the system to a generic disturbance in the freestream, which triggers a Tollmien–Schlichting wave packet inside the boundary layer. The performance of a linear quadratic Gaussian controller is analyzed to suppress the amplitude of the Tollmien–Schlichting wave packet using a row of sensors and plasma actuators localized at the wall. The target of the controller is chosen as a subset of proper orthogonal decomposition modes describing the dynamics of the unstable disturbances. The plasma actuators are implemented as volume forcing. To account for the limitations of the plasma actuators concerning a unidirectional forcing, several strategies are implemented in the linear quadratic Gaussian framework. Their performances are compared with that for classical linear quadratic Gaussian controller. These controllers successfully reduced the amplitude of the wave packet.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics Inc., 2018
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-228955 (URN)10.2514/1.J056415 (DOI)000432661400005 ()2-s2.0-85046622897 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20180530

Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-06-25Bibliographically approved
Negi, P. S., Hanifi, A. & Henningson, D. S. (2018). LES of the unsteady response of a natural laminar flow airfoil. In: 2018 Applied Aerodynamics Conference: . Paper presented at 36th AIAA Applied Aerodynamics Conference, 2018, [state] GA, United States, 25 June 2018 through 29 June 2018. American Institute of Aeronautics and Astronautics
Open this publication in new window or tab >>LES of the unsteady response of a natural laminar flow airfoil
2018 (English)In: 2018 Applied Aerodynamics Conference, American Institute of Aeronautics and Astronautics, 2018Conference paper, Published paper (Refereed)
Abstract [en]

Large-eddy simulations are performed to investigate the dynamic response of a natural laminar flow airfoil undergoing harmonic pitch oscillations at a chord based Reynolds number of Rec= 750, 000. Large changes in the transition location are observed throughout the pitch cycles which leads to a non-linear response of the aerodynamic force coefficients. Preliminary results show that the evolution of the boundary layer over the airfoil can be modeled by using a simple phase-lag concept which implies that the boundary-layer evolution is quasi-steady in nature. A simple empirical model is developed based on this quasi-steady, phase-lag assumption which fits very well with the measured experimental data.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-234512 (URN)10.2514/6.2018-3824 (DOI)2-s2.0-85051738799 (Scopus ID)9781624105593 (ISBN)
Conference
36th AIAA Applied Aerodynamics Conference, 2018, [state] GA, United States, 25 June 2018 through 29 June 2018
Funder
VINNOVAEU, European Research CouncilSwedish e‐Science Research Center
Note

QC 20180907

Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2018-09-07Bibliographically approved
Sasaki, K., Morra, P., Fabbiane, N., Cavalieri, A. V. G., Hanifi, A. & Henningson, D. S. (2018). On the wave-cancelling nature of boundary layer flow control. Theoretical and Computational Fluid Dynamics, 32(5), 593-616
Open this publication in new window or tab >>On the wave-cancelling nature of boundary layer flow control
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2018 (English)In: Theoretical and Computational Fluid Dynamics, ISSN 0935-4964, E-ISSN 1432-2250, Vol. 32, no 5, p. 593-616Article in journal (Refereed) Published
Abstract [en]

This work deals with the feedforward active control of Tollmien-Schlichting instability waves over incompressible 2D and 3D boundary layers. Through an extensive numerical study, two strategies are evaluated; the optimal linear-quadratic-Gaussian (LQG) controller, designed using the Eigensystem realization algorithm, is compared to a wave-cancellation scheme, which is obtained using the direct inversion of frequency-domain transfer functions of the system. For the evaluated cases, it is shown that LQG leads to a similar control law and presents a comparable performance to the simpler, wave-cancellation scheme, indicating that the former acts via a destructive interference of the incoming wavepacket downstream of actuation. The results allow further insight into the physics behind flow control of convectively unstable flows permitting, for instance, the optimization of the transverse position for actuation. Using concepts of linear stability theory and the derived transfer function, a more efficient actuation for flow control is chosen, leading to similar attenuation of Tollmien-Schlichting waves with only about 10% of the actuation power in the baseline case.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Boundary layer control, Flow control, Instability control, LQG controllers, Inversion controllers
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-235102 (URN)10.1007/s00162-018-0469-x (DOI)000443412500003 ()2-s2.0-85049125322 (Scopus ID)
Note

QC 20180917

Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2018-09-17Bibliographically approved
Vinuesa, R., Negi, P. S., Atzori, M., Hanifi, A., Henningson, D. S. & Schlatter, P. (2018). Turbulent boundary layers around wing sections up to Re-c=1, 000, 000. International Journal of Heat and Fluid Flow, 72, 86-99
Open this publication in new window or tab >>Turbulent boundary layers around wing sections up to Re-c=1, 000, 000
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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: 2018-09-11Bibliographically approved
Negi, P. S., Vinuesa, R., Hanifi, A., Schlatter, P. & Henningson, D. S. (2018). Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil. International Journal of Heat and Fluid Flow, 71, 378-391
Open this publication in new window or tab >>Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil
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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
Abstract [en]

High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Rec=100,000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain “off-design” conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at Rec=400,000.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Dynamic-response, Laminar separation bubble, Local stability, Transition, Unsteady aerodynamics, Wall-resolved les
National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-228734 (URN)10.1016/j.ijheatfluidflow.2018.04.009 (DOI)2-s2.0-85046802460 (Scopus ID)
Funder
VINNOVA, 2014-00933EU, European Research Council, 694452-TRANSEP-ERC-2015-AdGSwedish e‐Science Research Center
Note

QC 20180529

Available from: 2018-05-29 Created: 2018-05-29 Last updated: 2018-07-02Bibliographically approved
Vinuesa, R., Negi, P. S., Hanifi, A., Henningson, D. S. & Schlatter, P. (2017). High-fidelity simulations of the flow around wings at high reynolds numbers. In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017: . Paper presented at 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, Swissotel Chicago, United States, 6 July 2017 through 9 July 2017. , 2
Open this publication in new window or tab >>High-fidelity simulations of the flow around wings at high reynolds numbers
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2017 (English)In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, 2017, Vol. 2Conference paper (Refereed)
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 conducted a well-resolved large-eddy simulation of the turbulent flow around the NACA4412 airfoil at a Reynolds number based on freestream velocity and chord length of Rec = 1;000;000, with 5° angle of attack. The results of this simulation are used, together with the direct numerical simulation by Hosseini et al. (Int. J. Heat Fluid Flow 61, 2016) of the same wing section at Rec = 400;000, to characterize the effect of Reynolds number on APG TBLs subjected to the same pressure-gradient distribution (defined by the Caluser pressure-gradient parameter β). Our results indicate that the increase in inner-scaled edge velocity U+e, and the decrease in shape factor H, is lower in the APG on the wing than in zero-pressure-gradient (ZPG) TBLs over the same Reynolds-number range. This indicates that the lower-Re boundary layer is more sensitive to the effect of the APG, a conclusion that is supported by the larger values in the outer region of the tangential velocity fluctuation profile in the Rec = 400;000 wing. 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.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-218452 (URN)2-s2.0-85033231950 (Scopus ID)
Conference
10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, Swissotel Chicago, United States, 6 July 2017 through 9 July 2017
Note

QC 20171128

Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2017-11-28Bibliographically approved
Brynjell-Rahkola, M., Barman, E., Hanifi, A. & Henningson, D. S. (2017). On the stability of a Blasius boundary layer subject to localized suction.
Open this publication in new window or tab >>On the stability of a Blasius boundary layer subject to localized suction
2017 (English)Report (Other academic)
Abstract [en]

In this work the problem of premature transition in boundary layers due to localized suction is revisited. A thorough study involving nonlinear direct numerical simulations, a three-dimensional linear stability analysis, a sensitivity study and a Koopman analysis is presented. The ensemble of these different techniques enables the origins of oversuction to be studied in great detail and provides new insight into the transition process of the flow. The configuration considered consists of an infinite row of widely separated suction pipes that are mounted to the plate at right angles. For the parameter range investigated, the flow inside the pipe is seen to bifurcate at a lower suction ratio than the boundary layer and thus act as an oscillator that forces the external flow over the plate. At low levels of suction, this forcing is not enough to cause transition in the boundary layer, but as the suction level is increased beyond criticality, modes originating from the pipe and extending into the boundary layer are seen to destabilize as well. These modes enable the perturbations forced in the pipe to also amplify in the boundary layer, which leads to a rapid breakdown to turbulence in the wake of the suction hole.

Publisher
p. 25
Keywords
absolute/convective instability, boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-218167 (URN)
Note

QC 20171124

Available from: 2017-11-23 Created: 2017-11-23 Last updated: 2017-11-24Bibliographically approved
Vinuesa, R., Hosseini, S. M., Hanifi, A., Henningson, D. S. & Schlatter, P. (2017). Pressure-gradient turbulent boundary layers developing around a wing section. Flow Turbulence and Combustion, 99(3-4), 613-641
Open this publication in new window or tab >>Pressure-gradient turbulent boundary layers developing around a wing section
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2017 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, no 3-4, p. 613-641Article in journal (Refereed) Published
Abstract [en]

A direct numerical simulation database of the flow around a NACA4412 wing section at R e (c) = 400,000 and 5(ay) angle of attack (Hosseini et al. Int. J. Heat Fluid Flow 61, 117-128, 2016), obtained with the spectral-element code Nek5000, is analyzed. The Clauser pressure-gradient parameter beta ranges from ae integral 0 and 85 on the suction side, and from 0 to - 0.25 on the pressure side of the wing. The maximum R e (oee integral) and R e (tau) values are around 2,800 and 373 on the suction side, respectively, whereas on the pressure side these values are 818 and 346. Comparisons between the suction side with zero-pressure-gradient turbulent boundary layer data show larger values of the shape factor and a lower skin friction, both connected with the fact that the adverse pressure gradient present on the suction side of the wing increases the wall-normal convection. The adverse-pressure-gradient boundary layer also exhibits a more prominent wake region, the development of an outer peak in the Reynolds-stress tensor components, and increased production and dissipation across the boundary layer. All these effects are connected with the fact that the large-scale motions of the flow become relatively more intense due to the adverse pressure gradient, as apparent from spanwise premultiplied power-spectral density maps. The emergence of an outer spectral peak is observed at beta values of around 4 for lambda (z) ae integral 0.65 delta (99), closer to the wall than the spectral outer peak observed in zero-pressure-gradient turbulent boundary layers at higher R e (oee integral) . The effect of the slight favorable pressure gradient present on the pressure side of the wing is opposite the one of the adverse pressure gradient, leading to less energetic outer-layer structures.

Place, publisher, year, edition, pages
Springer, 2017
Keywords
Turbulent boundary layer, Pressure gradient, Wing section, Direct numerical simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-220718 (URN)10.1007/s10494-017-9840-z (DOI)000416838200005 ()2-s2.0-85027373953 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationSwedish e‐Science Research Center
Note

QC 20180104

Available from: 2018-01-03 Created: 2018-01-03 Last updated: 2018-01-04Bibliographically approved
Brynjell-Rahkola, M., Shahriari, N., Schlatter, P., Hanifi, A. & Henningson, D. S. (2017). Stability and sensitivity of a cross-flow-dominated Falkner-Skan-Cooke boundary layer with discrete surface roughness. Journal of Fluid Mechanics, 826, 830-850
Open this publication in new window or tab >>Stability and sensitivity of a cross-flow-dominated Falkner-Skan-Cooke boundary layer with discrete surface roughness
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2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 826, p. 830-850Article in journal (Refereed) Published
Abstract [en]

With the motivation of determining the critical roughness size, a global stability and sensitivity analysis of a three-dimensional Falkner-Skan-Cooke (FSC) boundary layer with a cylindrical surface roughness is performed. The roughness size is chosen such that breakdown to turbulence is initiated by a global version of traditional secondary instabilities of the cross-flow (CF) vortices instead of an immediate flow tripping at the roughness. The resulting global eigenvalue spectra of the systems are found to be very sensitive to numerical parameters and domain size. This sensitivity to numerical parameters is quantified using the epsilon-pseudospectrum, and the dependency on the domain is analysed through an impulse response, structural sensitivity analysis and an energy budget. It is shown that while the frequencies remain relatively unchanged, the growth rates increase with domain size, which originates from the inclusion of stronger CF vortices in the baseflow. This is reflected in a change in the rate of advective energy transport by the baseflow. It is concluded that the onset of global instability in a FSC boundary layer as the roughness height is increased does not correspond to an immediate flow tripping behind the roughness, but occurs for lower roughness heights if sufficiently long domains are considered. However, the great sensitivity results in an inability to accurately pinpoint the exact parameter values for the bifurcation, and the large spatial growth of the disturbances in the long domains eventually becomes larger than can be resolved using finite-precision arithmetic.

Place, publisher, year, edition, pages
Cambridge University Press, 2017
Keywords
absolute/convective instability, boundary layer stability, transition to turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-214322 (URN)10.1017/jfm.2017.466 (DOI)000407571200038 ()2-s2.0-85029412275 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20170914

Available from: 2017-09-14 Created: 2017-09-14 Last updated: 2017-11-23Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-5913-5431

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