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Hanifi, Ardeshir, DocentORCID iD iconorcid.org/0000-0002-5913-5431
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Publications (10 of 202) Show all publications
Kern, S., Blanco, D. C., Cavalieri, A. V., Negi, P., Hanifi, A. & Henningson, D. S. (2024). Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels. Journal of Fluid Mechanics, 986, Article ID A3.
Open this publication in new window or tab >>Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels
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2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 986, article id A3Article in journal (Refereed) Published
Abstract [en]

Thin airfoil dynamic stall at moderate Reynolds numbers is typically linked to the sudden bursting of a small laminar separation bubble close to the leading edge. Given the strong sensitivity of laminar separation bubbles to external disturbances, the onset of dynamic stall on a NACA0009 airfoil section subject to different levels of low-amplitude free stream disturbances is investigated using direct numerical simulations. The flow is practically indistinguishable from clean inflow simulations in the literature for turbulence intensities at the leading edge of Tu = 0.02 %. At slightly higher turbulence intensities of Tu = 0.05 %, the bursting process is found to be considerably less smooth and strong coherent vortex shedding from the laminar separation bubble is observed prior to the formation of the dynamic stall vortex (DSV). This phenomenon is considered in more detail by analysing its appearance in an ensemble of simulations comprising statistically independent realisations of the flow, thus proving its statistical relevance. In order to extract the transient dynamics of the vortex shedding, the classical proper orthogonal decomposition method is generalised to include time in the energy measure and applied to the time-resolved simulation data of incipient dynamic stall. Using this technique, the dominant transient spatiotemporally correlated features are distilled and the wave train of the vortex shedding prior to the emergence of the main DSV is reconstructed from the flow data exhibiting dynamics of large-scale coherent growth and decay within the turbulent boundary layer.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2024
Keywords
boundary layer separation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-346305 (URN)10.1017/jfm.2024.314 (DOI)001209573200001 ()2-s2.0-85192671697 (Scopus ID)
Note

QC 20240513

Available from: 2024-05-13 Created: 2024-05-13 Last updated: 2024-05-24Bibliographically approved
Blanco, D. C. P., Hanifi, A., Henningson, D. S. & Cavalieri, A. V. G. (2024). Linear and nonlinear receptivity mechanisms in boundary layers subject to free-stream turbulence. Journal of Fluid Mechanics, 979, Article ID A31.
Open this publication in new window or tab >>Linear and nonlinear receptivity mechanisms in boundary layers subject to free-stream turbulence
2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 979, article id A31Article in journal (Refereed) Published
Abstract [en]

Large-eddy simulations of a flat-plate boundary layer, without a leading edge, subject to multiple levels of incoming free-stream turbulence are considered in the present work. Within an input-output model, where nonlinear terms of the incompressible Navier-Stokes equations are treated as an external forcing, we manage to separate inputs related to perturbations coming through the intake of the numerical domain, whose evolution represents a linear mechanism, and the volumetric nonlinear forcing due to triadic interactions. With these, we perform the full reconstruction of the statistics of the flow, as measured in the simulations, to quantify pairs of wavenumbers and frequencies more affected by either linear or nonlinear receptivity mechanisms. Inside the boundary layer, different wavenumbers at near-zero frequency reveal streaky structures. Those that are amplified predominantly via linear interactions with the incoming vorticity occur upstream and display transient growth, while those generated by the nonlinear forcing are the most energetic and appear in more downstream positions. The latter feature vortices growing proportionally to the laminar boundary layer thickness, along with a velocity profile that agrees with the optimal amplification obtained by linear transient growth theory. The numerical approach presented is general and could potentially be extended to any simulation for which receptivity to incoming perturbations needs to be assessed.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2024
Keywords
boundary layer receptivity, low-dimensional models
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-343253 (URN)10.1017/jfm.2023.1035 (DOI)001143358600001 ()2-s2.0-85183918235 (Scopus ID)
Note

QC 20240209

Available from: 2024-02-09 Created: 2024-02-09 Last updated: 2024-02-15Bibliographically approved
Kern, S., Negi, P., Hanifi, A. & Henningson, D. S. (2024). Onset of absolute instability on a pitching aerofoil. Journal of Fluid Mechanics, 988, Article ID A8.
Open this publication in new window or tab >>Onset of absolute instability on a pitching aerofoil
2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 988, article id A8Article in journal (Refereed) Published
Abstract [en]

A global transient linear stability analysis of the three-dimensional time-dependent flow around an aerofoil undergoing small-amplitude pitching motion is performed using the optimally time-dependent (OTD) framework. The most salient linear instabilities associated with the instantaneous basic state are computed and tracked over time. The resulting OTD modes reflect the variations in the basic state and can be used as predictors of its spatial and temporal evolution, including the formation of a laminar separation bubble and its gradual spanwise modulation via primary global instability, leading to secondary instability and finally rapid breakdown to turbulence. The study confirms and expands upon earlier stability analyses of the same case based on the local properties of spanwise averaged velocity profiles in the bubble that predicted the onset of absolute instability soon followed by rapid breakdown of the separation bubble. The three-dimensional structure of the most unstable OTD mode is extracted, which compares well with both the locally absolutely unstable mode and the evolution of the basic state itself.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2024
Keywords
absolute/convective instability, boundary layer stability
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-347625 (URN)10.1017/jfm.2024.407 (DOI)001233796800001 ()2-s2.0-85195049078 (Scopus ID)
Note

QC 20240613

Available from: 2024-06-12 Created: 2024-06-12 Last updated: 2024-06-18Bibliographically approved
Alarcón, J. F., Cavalieri, A. V. .., Hanifi, A. & Henningson, D. S. (2024). Role of streak secondary instabilities on free-stream turbulence-induced transition. Journal of Fluid Mechanics, 988, Article ID A6.
Open this publication in new window or tab >>Role of streak secondary instabilities on free-stream turbulence-induced transition
2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 988, article id A6Article in journal (Refereed) Published
Abstract [en]

We study the stability of a zero-pressure gradient boundary layer subjected to free-stream disturbances by means of local stability analysis. The dataset under study corresponds to a direct numerical simulation (DNS) of a flat plate with a sharp leading edge in realistic wind tunnel conditions, with a turbulence level of 3.45 % at the leading edge. We present a method to track the convective evolution of the secondary instabilities of streaks by performing sequential stability calculations following the wave packet, connecting successive unstable eigenfunctions. A scattered nature, in time and space, of secondary instabilities is seen in the stability calculations. These instabilities can be detected before they reach finite amplitude in the DNS, preceding the nucleation of turbulent spots, and whose appearance is well correlated to the transition onset. This represents further evidence regarding the relevance of secondary instabilities of streaks in the bypass transition in realistic flow conditions. Consistent with the spatio-temporal nature of this problem, our approach allows us to integrate directly the local growth rates to obtain the spatial amplification ratio of the individual instabilities, where it is shown that instabilities reaching an -factor in the range [2.5,4] can be directly correlated to more than 65 % of the nucleation events. Interestingly, it is found that high amplification is not only attained by modes with high growth rates, but also by instabilities with sustained low growth rates for a long time.

Place, publisher, year, edition, pages
Cambridge University Press, 2024
Keywords
absolute/convective instability, boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-347686 (URN)10.1017/jfm.2024.433 (DOI)001232840700001 ()2-s2.0-85195056445 (Scopus ID)
Note

QC 20240613

Available from: 2024-06-13 Created: 2024-06-13 Last updated: 2024-06-13Bibliographically approved
Dotto, A., Luzzi, M., Verdoya, J., Simoni, D., Hanifi, A. & Pralits, J. O. (2024). Stability of low-pressure turbine boundary layers under variable Reynolds number and pressure gradient. Physics of fluids, 36(3), Article ID 034116.
Open this publication in new window or tab >>Stability of low-pressure turbine boundary layers under variable Reynolds number and pressure gradient
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2024 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 36, no 3, article id 034116Article in journal (Refereed) Published
Abstract [en]

The free-stream turbulence induced transition occurring under typical low-pressure turbine flow conditions is investigated by comparing linear stability theory with wind tunnel measurements acquired over a flat plate subjected to high turbulence intensity. The analysis was carried out, accounting for three different Reynolds numbers and four different adverse pressure gradients. First, a non-similarity-based boundary layer (BL) solver was used to compute base flows and validated against pressure taps and particle image velocimetry (PIV) measurements. Successively, the optimal disturbances and their spatial transient growth were calculated by coupling classical linear stability theory and a direct-adjoint optimization procedure on all flow conditions considered. Linear stability results were compared with experimental particle image velocimetry measurements on both wall-normal and wall-parallel planes. Finally, the sensitivity of the disturbance spatial transient growth to the spanwise wavenumber of perturbations, the receptivity position, and the location where disturbance energy is maximized were investigated via the built numerical model. Overall, the optimal perturbations computed by linear stability theory show good agreement with the streaky structures surveyed in experiments. Interestingly, the energy growth of disturbances was found to be maximum for all the flow conditions examined, when perturbations entered the boundary layer close to the position where minimum pressure occurs.

Place, publisher, year, edition, pages
AIP Publishing, 2024
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-345951 (URN)10.1063/5.0188024 (DOI)001183702100020 ()2-s2.0-85187781137 (Scopus ID)
Note

QC 20240502

Available from: 2024-05-02 Created: 2024-05-02 Last updated: 2024-05-02Bibliographically approved
De Vincentiis, L., Durovic, K., Lengani, D., Simoni, D., Pralits, J., Henningson, D. S. & Hanifi, A. (2023). Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade. Journal of turbomachinery, 145(5), Article ID 051011.
Open this publication in new window or tab >>Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade
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2023 (English)In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 145, no 5, article id 051011Article in journal (Refereed) Published
Abstract [en]

In the present work, the evolution of the boundary layer over a low-pressure turbine blade is studied using direct numerical simulations, with the aim of investigating the unsteady flow field induced by the rotor-stator interaction. The freestream flow is characterized by the high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream of the turbine blades. To include the presence of the wake without employing an ad-hoc model, we simulate both the moving bars and the stationary blades in their respective frames of reference and the coupling of the two domains is done through appropriate boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part of the blade is suppressed. Moreover, the presence of the wake introduces alternating regions in the streamwise direction of high- and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress. The analysis of the velocity fields allows the characterization of the streaky structures forced in the boundary layer by turbulence carried by upstream wakes. The breakdown events are observed once positive streamwise velocity fluctuations reach the end of the blade. Both the fluctuations induced by the migration of the wake in the blade passage and the presence of the streaks contribute to high values of the disturbance velocity inside the boundary layer with respect to a steady inflow case. The amplification of the boundary layer disturbances associated with different spanwise wavenumbers has been computed. It was found that the migration of the wake in the blade passage stands for the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the low- and high-speed regions associated with the wakes.

Place, publisher, year, edition, pages
ASME International, 2023
Keywords
boundary layer development, computationalfluiddynamics(CFD)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-326643 (URN)10.1115/1.4056108 (DOI)000964312500012 ()2-s2.0-85144290077 (Scopus ID)
Note

QC 20230509

Available from: 2023-05-09 Created: 2023-05-09 Last updated: 2023-05-09Bibliographically approved
Coelho Leite Fava, T., Lobo, B. A., Nogueira, P. A., Schaffarczyk, A. P., Breuer, M., Henningson, D. S. & Hanifi, A. (2023). Influence of free-stream turbulence on the boundary layer stability of a wind turbine airfoil and near wake. Paper presented at 8th Wake Conference 2023, Visby, Sweden, 20 - 22 June 2023. Journal of Physics, Conference Series, 2505(1), 012002-012002
Open this publication in new window or tab >>Influence of free-stream turbulence on the boundary layer stability of a wind turbine airfoil and near wake
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2023 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 2505, no 1, p. 012002-012002Article in journal (Refereed) Published
Abstract [en]

Free-stream turbulence (FST) alters the boundary layer of wind turbine blades, changing the hydrodynamic stability and near wake. Large-eddy simulations (LES) of a blade section with a laminar separation bubble for several turbulence intensities (TI) and a Reynolds number of 100,000 are performed. The effects of boundary-layer streaks generated by FST on Tollmien-Schlichting (TS) and Kelvin-Helmholtz (KH) instabilities are analyzed with a model based on the parabolized stability equations (PSE). Two competing effects on flow stability are identified. The spanwise-averaged mean-flow distortion stabilizes primary TS/KH modes for increasing TI. However, this contribution seems dominant only for TI ≥ 8.6%. For lower TI, the spanwise-oscillating distortion caused by streaks destabilizes the flow, and the growth rates of secondary modal instabilities increase with the streak amplitude. The destabilization occurs mainly at spanwise locations with negative streaks since the inflection point shifts away from the wall, enhancing inviscid instabilities. Inflection points in the spanwise direction formed by the streaks also contribute to the destabilization. The modal structures from PSE and LES agree. Finally, the trailing-edge near-wake coherent structures are more energetic for TI ≥ 8.6% due to the partial stabilization of modal instabilities, delaying the turbulent breakdown.

Place, publisher, year, edition, pages
IOP Publishing, 2023
National Category
Fluid Mechanics and Acoustics
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-337105 (URN)10.1088/1742-6596/2505/1/012002 (DOI)001004334300002 ()2-s2.0-85163429561 (Scopus ID)
Conference
8th Wake Conference 2023, Visby, Sweden, 20 - 22 June 2023
Funder
StandUpStandUp for Wind
Note

QC 20231030

Available from: 2023-09-25 Created: 2023-09-25 Last updated: 2023-10-30Bibliographically approved
Kleine, V., Hanifi, A. & Henningson, D. S. (2023). Non-iterative vortex-based smearing correction for the actuator line method. Journal of Fluid Mechanics, 961, Article ID A29.
Open this publication in new window or tab >>Non-iterative vortex-based smearing correction for the actuator line method
2023 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 961, article id A29Article in journal (Refereed) Published
Abstract [en]

The actuator line method (ALM) is used extensively in wind turbine and rotor simulations. However, its original uncorrected formulation overestimates the forces near the tip of the blades and does not reproduce well forces on translating wings. The recently proposed vortex-based smearing correction for the ALM is a correction based on physical and mathematical properties of the simulation that allows for a more accurate and general ALM. So far, to correct the forces on the blades, the smearing correction depended on an iterative process at every time step, which is usually slower, less stable and less deterministic than direct methods. In this work, a non-iterative process is proposed and validated. First, we propose a formulation of the nonlinear lifting line that is equivalent to the ALM with smearing correction, showing that the results are practically identical for a translating wing. Then, by linearizing the lifting line method, the iterative process of the correction is substituted by the direct solution of a small linear system. No significant difference is observed in the results of the iterative and non-iterative corrections, in both wing and rotor simulations. Additional contributions of the present work include the use of a more accurate approximation for the velocity induced by a smeared vortex segment and the implementation of a free-vortex wake model to define the vortex sheet, which contribute to the accuracy and generality of the method. The results presented here may motivate the adoption of the ALM by other communities, for example, in fixed-wing applications.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2023
Keywords
computational methods, vortex shedding
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-328432 (URN)10.1017/jfm.2023.237 (DOI)000976753400001 ()2-s2.0-85158156996 (Scopus ID)
Note

QC 20230612

Available from: 2023-06-12 Created: 2023-06-12 Last updated: 2023-06-12Bibliographically approved
Coelho Leite Fava, T., Lobo, B. A., Nogueira, P. A., Schaffarczyk, A. P., Breuer, M., Henningson, D. S. & Hanifi, A. (2023). Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence. Physics of fluids, 35(8)
Open this publication in new window or tab >>Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence
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2023 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 35, no 8Article in journal (Refereed) Published
Abstract [en]

The interaction of several instabilities and the influence of free-stream turbulence on laminar-turbulent transition on a 20% thick wind-turbine blade section with a laminar separation bubble (LSB) are investigated with wall-resolved large-eddy simulations (LES). Turbulence intensities (TI) of 0%, 2.2%, 4.5%, 8.6%, and 15.6% at chord Reynolds number 100,000 are considered. Linear receptivity occurs for the most energetic disturbances; high-frequency perturbations are excited via non-linear mechanisms for  TI≥8.6%⁠. Unstable Tollmien–Schlichting (TS) waves appear in the inflectional flow region for  TI≤4.5%⁠, shifting to inviscid Kelvin–Helmholtz (KH) modes upon separation and forming spanwise rolls. Sub-harmonic secondary instability occurs for  TI=0%⁠, with rolls intertwining before transition. Streaks spanwise modulate the rolls and increase their growth rates with TI for  TI≤4.5%⁠, reducing separation and shifting transition upstream. The  TI=4.5% case presents the highest perturbations, leading to the smallest LSB and most upstream transition. Earlier inception of TS/KH modes occurs on low-speed streaks, inducing premature transition. However, for  TI=8.6%⁠, the effect of the streaks is to stabilize the attached mean flow and front part of the LSB. This occurs due to the near-wall momentum deficit alleviation, leading to the transition delay and larger LSB than  TI=4.5%⁠. This also suppresses separation and completely stabilizes TS/KH modes for  TI=15.6%⁠. Linear stability theory predicts well the modal evolution for  TI≤8.6%⁠. Optimal perturbation analysis accurately computes the streak development upstream of the inflectional flow region but indicates higher amplification than LES downstream due to the capture of low-frequency, oblique modal instabilities from the LSB. Only low-amplitude [ O(1%)] streaks displayed exponential growth in the LES since non-linearity precludes the appearance of these modes.

Place, publisher, year, edition, pages
AIP Publishing, 2023
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-337104 (URN)10.1063/5.0159783 (DOI)001041183200006 ()2-s2.0-85166950584 (Scopus ID)
Funder
StandUp for WindStandUp
Note

QC 20230926

Available from: 2023-09-25 Created: 2023-09-25 Last updated: 2023-09-26Bibliographically approved
Kleine, V., Hanifi, A. & Henningson, D. S. (2023). Simulating Airplane Aerodynamics with Body Forces: Actuator Line Method for Nonplanar Wings. AIAA Journal, 61(5), 2048-2059
Open this publication in new window or tab >>Simulating Airplane Aerodynamics with Body Forces: Actuator Line Method for Nonplanar Wings
2023 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 61, no 5, p. 2048-2059Article in journal (Refereed) Published
Abstract [en]

Two configurations typical of fixed-wing aircraft are simulated with the actuator line method (ALM): a wing with winglets, and a T tail. The ALM is extensively used in rotor simulations to model the blades by body forces, which are calculated from airfoil data and the relative flow velocity. This method has not been used to simulate airplane aerodynamics, despite its advantage of allowing coarser grids. This may be credited to the failure of the uncorrected ALM to accurately predict forces near the tip of the wings, even for simple configurations. The recently proposed vortex-based smearing correction shows improved results, suggesting those limitations are part of the past. For the nonplanar configurations studied in this work, differences between the ALM with the original smearing correction and a nonlinear lifting line (LL) method are observed near the intersection of surfaces because the circulation generated in the numerical simulation differs from the calculated corrected circulation. A vorticity magnitude correction is proposed, which improves the agreement between the ALM and the LL method. This second-order correction resolves the ambiguity in the velocity used to define the lift force. The good results indicate that the improved ALM can be used for airplane aerodynamics, with an accuracy similar to the LL method.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics (AIAA), 2023
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-330912 (URN)10.2514/1.J062398 (DOI)000937607700001 ()2-s2.0-85152562998 (Scopus ID)
Note

QC 20230705

Available from: 2023-07-05 Created: 2023-07-05 Last updated: 2023-07-05Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-5913-5431

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