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Publications (10 of 85) Show all publications
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
Keyword
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
Keyword
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
Keyword
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
Keyword
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.

Keyword
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
Shahriari, N. & Hanifi, A. (2016). Interaction of acoustic waves and micron-sized surface roughness elements in a swept-wing boundary layer. KTH Royal Institute of Technology
Open this publication in new window or tab >>Interaction of acoustic waves and micron-sized surface roughness elements in a swept-wing boundary layer
2016 (English)Report (Other academic)
Abstract [en]

E↵ect of acoustic waves on the control performance of distributed micron-sized roughness elements in a swept-wing boundary layer is investigated through direct numerical simulations. The flow configuration conforms to experiments by Kachanov et al. (2015) who observed either no significant influence of acoustic waves on the transition location or small stabilisation e↵ect. In this work, a base set up for natural transition scenario is first established by introducing unsteady background noise in the boundary layer. The natural transition is then delayed using control roughness elements. Introduction of acoustic waves to the controlled flow promotes the transition location. In all these flow cases, stationary primary crossflow vortices dominate the disturbance environment and unsteady disturbances experience an explosive growth prior to transition. The spatial distribution of the energy production associated with z-type modes shows an increase in the local transfer of energy from the modified mean flow to perturbations. Simulation of flow with control roughness elements and acoustic waves as the only source of unsteady disturbances shows no influence of acoustic wave in transition to turbulence. 

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-196875 (URN)
Note

QC 20161125

Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2016-11-25Bibliographically approved
Weng, C., Boij, S. & Hanifi, A. (2016). Numerical and theoretical investigation of pulsatile turbulent channel flows. Journal of Fluid Mechanics, 792, 98-133
Open this publication in new window or tab >>Numerical and theoretical investigation of pulsatile turbulent channel flows
2016 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 792, p. 98-133Article in journal (Refereed) Published
Abstract [en]

A turbulent channel flow subjected to imposed harmonic oscillations is studied by direct numerical simulation (DNS) and theoretical models. Simulations have been performed for different pulsation frequencies. The time- and phase-averaged data have been used to analyse the flow. The onset of nonlinear effects during the production of the perturbation Reynolds stresses is discussed based on the DNS data, and new physical features observed in the DNS are reported. A linear model proposed earlier by the present authors for the coherent perturbation Reynolds shear stress is reviewed and discussed in depth. The model includes the non-equilibrium effects during the response of the Reynolds stress to the imposed periodic shear straining, where a phase lag exists between the stress and the strain. To validate the model, the perturbation velocity and Reynolds shear stress from the model are compared with the DNS data. The performance of the model is found to be good in the frequency range where quasi-static assumptions are invalid. The viscoelastic characteristics of the turbulent eddies implied by the model are supported by the DNS data. Attempts to improve the model are also made by incorporating the DNS data in the model.

Place, publisher, year, edition, pages
Cambridge University Press, 2016
Keyword
turbulence simulation, turbulent boundary layers, wave-turbulence interactions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-184951 (URN)10.1017/jfm.2016.73 (DOI)000371400400007 ()2-s2.0-84959386828 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20160407

Available from: 2016-04-07 Created: 2016-04-07 Last updated: 2017-11-30Bibliographically approved
Brynjell-Rahkola, M., Shahriari, N., Schlatter, P., Hanifi, A. & Henningson, D. S. (2016). Stability and sensitivity of a crossflow-dominated Falkner–Skan–Cooke boundary layer with discrete surface roughness. Journal of Fluid Mechanics
Open this publication in new window or tab >>Stability and sensitivity of a crossflow-dominated Falkner–Skan–Cooke boundary layer with discrete surface roughness
Show others...
2016 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
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 crossflow (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 "-pseudospectrum, and the dependency on the domain is analysed through an impulse response and an energy budget. It is shown that 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 su ciently 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 what can be resolved using finite precision arithmetics. 

Place, publisher, year, edition, pages
Cambridge University Press, 2016
National Category
Mechanical Engineering Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-196877 (URN)
Note

QC 20161125

Available from: 2016-11-24 Created: 2016-11-24 Last updated: 2017-11-29Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-5913-5431

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