Large Eddy Simulation of Natural Low-Frequency Oscillations of Separating-Reattaching Flow Near Stall Conditions

Almutairi, Jaber H.; AlQadi, Ibraheem

doi:10.2514/6.2012-2698

Cited by 5 publications

(8 citation statements)

References 16 publications

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“…The unsteady low-frequency/large-amplitude limit cycle captured here with the fully turbulent URANS model has many features in common with low frequency oscillations described in various experimental and numerical studies (Zaman et al 1987;Bragg & Zaman 1994;Broeren & Bragg 1998;Almutairi & AlQadi 2013;Hristov & Ansell 2018;ElJack & Soria 2018). By experimentally studying several airfoils at several Reynolds numbers, Broeren & Bragg (1998) established a correlation between the stall type and the characteristics of the low-frequency flow oscillation.…”

Section: Discussionmentioning

confidence: 61%

“…This would require accurate simulation of the following: separation of the laminar boundary layer; transition of the shear layer leading to the formation of a laminar separation bubble; subsequent development of the turbulent boundary layer, which may separate close to the trailing edge (Mary & Sagaut 2002); and a computational domain of several chords in the spanwise direction, so as to capture the stall cells. For transitional flow regimes (Re ∼ 2.5 × 10 4 -10 5 ), direct numerical or large eddy simulations of the Navier-Stokes equations have been used in the last decade to investigate the dynamics of laminar separation bubbles near the onset of stall (Rinoie & Takemura 2004;Almutairi, Jones & Sandham 2010;Almutairi, AlQadi & ElJack 2015;AlMutairi, ElJack & AlQadi 2017) and its interplay with low-frequency flow oscillations (Almutairi & AlQadi 2013;ElJack & Soria 2018, 2020. The results obtained with these methods compare well with experiments (Ohtake, Nakae & Motohashi 2007).…”

Section: Introductionmentioning

confidence: 99%

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Bifurcation scenario for a two-dimensional static airfoil exhibiting trailing edge stall

et al. 2021

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We numerically investigate stalling flow around a static airfoil at high Reynolds numbers using the Reynolds-averaged Navier–Stokes equations (RANS) closed with the Spalart–Allmaras turbulence model. An arclength continuation method allows to identify three branches of steady solutions, which form a characteristic inverted S-shaped curve as the angle of attack is varied. Global stability analyses of these steady solutions reveal the existence of two unstable modes: a low-frequency mode, which is unstable for angles of attack in the stall region, and a high-frequency vortex shedding mode, which is unstable at larger angles of attack. The low-frequency stall mode bifurcates several times along the three steady solutions: there are two Hopf bifurcations, two solutions with a two-fold degenerate eigenvalue and two saddle-node bifurcations. This low-frequency mode induces a cyclic flow separation and reattachment along the airfoil. Unsteady simulations of the RANS equations confirm the existence of large-amplitude low-frequency periodic solutions that oscillate around the three steady solutions in phase space. An analysis of the periodic solutions in the phase space shows that, when decreasing the angle of attack, the low-frequency periodic solution collides with the unstable steady middle-branch solution and thus disappears via a homoclinic bifurcation of periodic orbits. Finally, a one-equation nonlinear stall model is introduced to reveal that the disappearance of the limit cycle, when increasing the angle of attack, is due to a saddle-node bifurcation of periodic orbits.

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Section: Discussionmentioning

confidence: 61%

Section: Introductionmentioning

confidence: 99%

Bifurcation scenario for a two-dimensional static airfoil exhibiting trailing edge stall

et al. 2021

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“…Grid-1 (637 Â 320 Â 86) is an improved mesh distribution of the grid used by Almutairi and Alqadi. 21 The grid points in the wall-normal direction were changed to intensify grid points near the aerofoil surface and across the free shear layer. Grid-2 (780 Â 320 Â 125) is a refined version of Grid-1 that was used by Eljack.…”

Section: Computational Setupmentioning

confidence: 99%

“…They increased the domain in the spanwise direction which resulted in matching results with the DNS results. Afterwards, Almutairi and Alqadi 21 investigated the phenomenon of natural LFO using the method used by Almutairi et al 19 The authors studied numerical simulations of the flow field around a NACA-0012 aerofoil at Reynolds number of 5 Â 10 4 , Mach number of 0.4, and four angles of attack at near stall conditions (9:25 ; 9:29 ; 9:4 , and 9:6 ). The authors found that self-sustained natural LFO is observed for all the angles.…”

Section: Introductionmentioning

confidence: 99%

Numerical investigation of the low-frequency flow oscillation over a NACA-0012 aerofoil at the inception of stall

ElAwad

ElJack

2019

International Journal of Micro Air Vehicles

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High-fidelity large eddy simulation is carried out for the flow field around a NACA-0012 aerofoil at Reynolds number of 9 Â 10 4 , Mach number of 0.4, and various angles of attack around the onset of stall. The laminar separation bubble is formed on the suction surface of the aerofoil and is constituted by the reattached shear layer. At these conditions, the laminar separation bubble is unstable and switches between a short bubble and an open bubble. The instability of the laminar separation bubble triggers a low-frequency flow oscillation. The aerodynamic coefficients oscillate accordingly at a low frequency. The lift and the drag coefficients compare very well to recent high-accuracy experimental data, and the lift leads the drag by a phase shift of p=2. The mean lift coefficient peaks at the angle of attack of 10:25 , in total agreement with the experimental data. The spectra of the lift coefficient does not show a significant low-frequency peak at angles of attack lower than or equal the stall angle of attack (10:25 ). At higher angles of attack, the spectra show two low-frequency peaks and the low-frequency flow oscillation is fully developed at the angle of attack of 11:0 . The behaviour of the flow-field and changes in the turbulent kinetic energy over one low-frequency flow oscillation cycle are described qualitatively.

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“…The second circulation region is slower than the main circulation region similar to Almutairi and AlQadi. 39 Also, note that the notion of reattachment and separation is relative to the effective free stream formed by the main circulation/separation region above the second circulation/separation region as can be clearly seen in the time averaged velocity profiles and streamlines in Figure 7c.…”

Section: A Convergence Studymentioning

confidence: 93%

Numerical Study of Intermittent Laminar Bubble Bursting and Vortex Shedding on an NACA 64_3-618 Airfoil

Jost

Zhang

2015

53rd AIAA Aerospace Sciences Meeting

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2D flows over an NACA 643-618 airfoil at Re = 64,200 at α = 8.64 o , 11.24 o and 13.84 o are simulated numerically using FLUENT. Spatial and temporal resolution studies indicate that sufficiently fine spatial and temporal resolutions are necessary to properly capture the dominant flow features. At α = 8.64 o , a long laminar separation bubble (LSB) and periodic vortex shedding are present. Fourier power spectra of probe data and aerodynamic coefficients identify a unique vortex shedding frequency. At α = 11.24 o , vortex shedding is no longer periodic, and intermittent bubble bursting from a short to a long bubble and intermittent vortex shedding are present. In addition, bubble bursting frequency, vortex shedding frequency and power distribution in Fourier power spectra of probe data are found to be location dependent. The intermittency in the flow makes time-averaging ill-posed as demonstrated. At α = 13.84 o , even though bubble bursting and vortex shedding are still intermittent, bubble bursting and vortex shedding frequencies appear to be more distinct in the Fourier power spectra indicating increased periodicity.

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Large Eddy Simulation of Natural Low-Frequency Oscillations of Separating-Reattaching Flow Near Stall Conditions

Cited by 5 publications

References 16 publications

Bifurcation scenario for a two-dimensional static airfoil exhibiting trailing edge stall

Bifurcation scenario for a two-dimensional static airfoil exhibiting trailing edge stall

Numerical investigation of the low-frequency flow oscillation over a NACA-0012 aerofoil at the inception of stall

Numerical Study of Intermittent Laminar Bubble Bursting and Vortex Shedding on an NACA 64_3-618 Airfoil

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