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Hanifi, Ardeshir, DocentORCID iD iconorcid.org/0000-0002-5913-5431
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Publications (10 of 88) 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., 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
Shahriari, N., Hanifi, A. & Henningson, D. S. (2016). Application of biorthogonal eigenfunction system for extraction of Tollmien-Schlichting waves in acoustic receptivity simulations. KTH Royal Institute of Technology
Open this publication in new window or tab >>Application of biorthogonal eigenfunction system for extraction of Tollmien-Schlichting waves in acoustic receptivity simulations
2016 (English)Report (Other academic)
Abstract [en]

Acoustic receptivity of a two-dimensional boundary layer on a flat plate with elliptic leading edge is studied through direct numerical simulation (DNS). Sound waves are modelled by a uniform oscillation of freestream boundaries in time which results to an infinite-wavelength acoustic wave. Acoustic disturbances interact with strong streamwise gradients at the leading edge or surface non- homogeneities and create Tollmien-Schlichting (TS) waves inside the boundary layer. Measuring amplitude of TS waves created by sound waves is challenging due to presence of Stokes wave (acoustic boundary layer) with the same temporal frequency of TS waves. In this study biorthogonal eigenfunction system of local linear stability equations has been utilised to extract TS wave amplitudes. This method is based on the concept of using adjoint mode as a projector where the TS amplitude is obtained by projecting the DNS solution onto adjoint TS modes. However, the computed TS wave amplitude employing this method found to be modulated. It is shown that the modulation is due to existence of a small amplitude wave in the DNS data that is not expandable onto the basis of local linear stability equations. 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016
National Category
Mechanical Engineering Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-196874 (URN)
Note

QC 20161125

Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2016-11-25Bibliographically approved
Hosseini, S. M., Vinuesa, R., Schlatter, P., Hanifi, A. & Heninngson, D. S. (2016). Direct numerical simulation of the flow around a wing section at moderate Reynolds number. International Journal of Heat and Fluid Flow, 61, 117-128
Open this publication in new window or tab >>Direct numerical simulation of the flow around a wing section at moderate Reynolds number
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2016 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 61, p. 117-128Article in journal (Refereed) Published
Abstract [en]

Abstract A three-dimensional direct numerical simulation has been performed to study the turbulent flow around the asymmetric NACA4412 wing section at a moderate chord Reynolds number of R e c = 400 , 000 , with an angle of attack of A o A = 5 ∘ . The mesh was optimized to properly resolve all relevant scales in the flow, and comprises around 3.2 billion grid points. The incompressible spectral-element Navier–Stokes solver Nek5000 was used to carry out the simulation. An unsteady volume force is used to trip the flow to turbulence on both sides of the wing at 10% of the chord. Full turbulence statistics are computed in addition to collection of time history data in selected regions. The Reynolds numbers on the suction side reach Reτ ≃ 373 and R e Ξ = 2 , 800 with the pressure-gradient parameter ranging from β ≈ 0.0 to β ≈ 85. Similarly, on the pressure side, the Reynolds numbers reach Reτ ≈ 346 and R e Ξ = 818 while β changes from β ≈ 0.0 to β ≈ − 0.25 . The effect of adverse pressure gradients on the mean flow is consistent with previous observations, namely a steeper incipient log law, a more prominent wake region and a lower friction. The turbulence kinetic energy profiles show a progressively larger inner peak for increasing pressure gradient, as well as the emergence and development of an outer peak with stronger APGs. The present simulation shows the potential of high-order (spectral) methods in simulating complex external flows at moderately high Reynolds numbers.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Turbulent boundary layer, Vortex shedding, Wake, Incipient separation, Pressure gradient, NACA4412
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-190788 (URN)10.1016/j.ijheatfluidflow.2016.02.001 (DOI)000390745800013 ()2-s2.0-84961782297 (Scopus ID)
Note

QC 20160816

Available from: 2016-08-15 Created: 2016-08-15 Last updated: 2017-05-30Bibliographically approved
Weng, C., Bake, F., Boij, S. & Hanifi, A. (2016). Effectofmean flow shear on sound propagation in uniform circular ducts. In: ICSV 2016 - 23rd International Congress on Sound and Vibration: From Ancient to Modern Acoustics. Paper presented at 23rd International Congress on Sound and Vibration, ICSV 2016, 10 July 2016 through 14 July 2016.
Open this publication in new window or tab >>Effectofmean flow shear on sound propagation in uniform circular ducts
2016 (English)In: ICSV 2016 - 23rd International Congress on Sound and Vibration: From Ancient to Modern Acoustics, 2016Conference paper, Published paper (Refereed)
Abstract [en]

Acoustic measurements based on multi-port models are commonly applied to investigate generation or scattering behaviors of in-duct components (turbocharger, mufflers, etc.) in confined flows. These multi-port models usually require information about the sound attenuations and the mode shapes of the cut-on higher-order acoustic modes in the rigid-wall duct. Although there are well-established models for determining theses parameters, most of the models are based on a uniform-flow assumption which neglects the refraction effects of the mean flow shear on the sound propagation. To understand the importance of the refraction effects, a numerical investigation is conducted to study the influence of the transverse mean-velocity gradients on the viscothermal damping and mode shapes of both the fundamental and higher-order modes in uniform circular ducts with rigid walls. The investigation is based on the linearized Navier-Stokes equations (LNSE) formulated in the frequency domain and several prescribed shear mean-velocity profiles (one turbulent boundary layer profile and two exponential-then-uniform profiles). The numerical results are compared with a model based on the uniform-flow assumption.

Keywords
Acoustic wave propagation, Boundary layer flow, Boundary layers, Ducts, Frequency domain analysis, Navier Stokes equations, Refraction, Acoustic measurements, Higher-order modes, Linearized navier-stokes equations, Mean velocity profiles, Numerical investigations, Refraction effects, Turbulent boundary layers, Viscothermal damping, Shear flow
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-194593 (URN)2-s2.0-84987876625 (Scopus ID)9789609922623 (ISBN)
Conference
23rd International Congress on Sound and Vibration, ICSV 2016, 10 July 2016 through 14 July 2016
Note

Conference Paper. QC 20161102

Available from: 2016-11-02 Created: 2016-10-31 Last updated: 2017-06-28Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-5913-5431

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