Linear modal instabilities around post-stall swept finite wings at low Reynolds numbers

Burtsev, Anton; He, Wei; Zhang, Kai; Theofilis, Vassilios; Taira, Kunihiko; Amitay, Michael

doi:10.1017/jfm.2022.420

Cited by 11 publications

(6 citation statements)

References 73 publications

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“…Furthermore, linear instabilities around swept wings were examined for a variety of swept and unswept wings, showing that the sweep angle suppresses the emergence of wake modes (Burtsev et al. 2022; Ribeiro et al. 2022 a ).…”

Section: Introductionmentioning

confidence: 99%

“…Similar attenuation of flow unsteadiness was further studied for forward-swept wings revealing that wing sweep has a strong effect on attenuating wake oscillations in laminar flows. Furthermore, linear instabilities around swept wings were examined for a variety of swept and unswept wings, showing that the sweep angle suppresses the emergence of wake modes (Burtsev et al 2022;Ribeiro et al 2022a).…”

Section: Introductionmentioning

confidence: 99%

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Triglobal resolvent analysis of swept-wing wakes

2023

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Through triglobal resolvent analysis, we reveal the effects of wing tip and sweep angle on laminar separated wakes over swept wings. For the present study, we consider wings with semi-aspect ratios from $1$ to $4$ , sweep angles from $0^\circ$ to $45^\circ$ and angles of attack of $20^\circ$ and $30^\circ$ at a chord-based Reynolds number of $400$ and a Mach number of $0.1$ . Using direct numerical simulations, we observe that unswept wings develop vortex shedding near the wing root with a quasi-steady tip vortex. For swept wings, vortex shedding is seen near the wing tip for low sweep angles, while the wakes are steady for wings with high sweep angles. To gain further insights into the mechanisms of flow unsteadiness, triglobal resolvent analysis is used to identify the optimal spatial input–output mode pairs and the associated gains over a range of frequencies. The three-dimensional forcing and response modes reveal that harmonic fluctuations are directed towards the root for unswept wings and towards the wing tip for swept wings. The overlapping region of the forcing–response mode pairs uncovers triglobal resolvent wavemakers associated with self-sustained unsteady wakes of swept wings. Furthermore, we show that for low-aspect-ratio wings optimal perturbations develop globally over the entire wingspan. The present study uncovers physical insights on the effects of tip and sweep on the growth of optimal harmonic perturbations and the wake dynamics of separated flows over swept wings.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Triglobal resolvent analysis of swept-wing wakes

2023

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“…2020 a ) and identify global modes (Burtsev et al. 2022; Ribeiro, Yeh & Taira 2023 b ) that give rise to fundamental global coherent structures of flow separation around swept wings. It is noteworthy that some of the wing-sweep effects on laminar post-stall flows are topologically analogous over a wide range of Reynolds numbers.…”

Section: Introductionmentioning

confidence: 99%

“…Wing sweep also has a strong influence on post-stall wake dynamics. For laminar flow regimes, a number of experimental and numerical efforts were made to examine the effects of backward and forward wing sweep (Yen & Hsu 2007;Yen & Huang 2009;Zhang et al 2020a) and identify global modes (Burtsev et al 2022;Ribeiro, Yeh & Taira 2023b) that give rise to fundamental global coherent structures of flow separation around swept wings. It is noteworthy that some of the wing-sweep effects on laminar post-stall flows are topologically analogous over a wide range of Reynolds numbers.…”

Section: Introductionmentioning

confidence: 99%

Laminar post-stall wakes of tapered swept wings

Ribeiro,

Neal,

Burtsev

et al. 2023

J. Fluid Mech.

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While tapered swept wings are widely used, the influence of taper on their post-stall wake characteristics remains largely unexplored. To address this issue, we conduct an extensive study using direct numerical simulations to characterize the wing taper and sweep effects on laminar separated wakes. We analyse flows behind NACA 0015 cross-sectional profile wings at post-stall angles of attack $\alpha =14^\circ$ – $22^\circ$ with taper ratios $\lambda =0.27$ – $1$ , leading-edge sweep angles $0^\circ$ – $50^\circ$ and semi aspect ratios $sAR =1$ and $2$ at a mean-chord-based Reynolds number of $600$ . Tapered wings have smaller tip chord length, which generates a weaker tip vortex, and attenuates inboard downwash. This results in the development of unsteadiness over a large portion of the wingspan at high angles of attack. For tapered wings with backward-swept leading edges, unsteadiness emerges near the wing tip. On the other hand, wings with forward-swept trailing edges are shown to concentrate wake-shedding structures near the wing root. For highly swept untapered wings, the wake is steady, while unsteady shedding vortices appear near the tip for tapered wings with high leading-edge sweep angles. For such wings, larger wake oscillations emerge near the root as the taper ratio decreases. While the combination of taper and sweep increases flow unsteadiness, we find that tapered swept wings have more enhanced aerodynamic performance than untapered and unswept wings, exhibiting higher time-averaged lift and lift-to-drag ratio. The current findings shed light on the fundamental aspects of flow separation over tapered wings in the absence of turbulent flow effects.

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“…2020 b ; Burtsev et al. 2022) finite-aspect-ratio wings in the incompressible and compressible (Paladini et al. 2019; Timme 2020; He & Timme 2021; Plante et al.…”

Section: Introductionmentioning

confidence: 99%

Global stability, sensitivity and passive control of low-Reynolds-number flows around NACA 4412 swept wings

et al. 2023

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The stability and sensitivity of two- and three-dimensional global modes developing on steady spanwise-homogeneous laminar separated flows around NACA 4412 swept wings are numerically investigated for different Reynolds numbers ${\textit {Re}}$ and angles of attack $\alpha$ . The wake dynamics is driven by the two-dimensional von Kármán mode whose emergence threshold in the $\alpha \unicode{x2013}{\textit {Re}}$ plane is computed with that of the three-dimensional centrifugal mode. At the critical Reynolds number, the Strouhal number, the streamwise wavenumber of the von Kármán mode and the spanwise wavenumber of the leading three-dimensional centrifugal mode scale as a power law of $\alpha$ . The introduction of a sweep angle attenuates the growth of all unstable modes and entails a Doppler effect in the leading modes’ dynamics and a shift towards non-zero frequencies of the three-dimensional centrifugal modes. These are found to be non-dispersive as opposed to the von Kármán modes. The sensitivity of the leading global modes is investigated in the vicinity of the critical conditions through adjoint-based methods. The growth-rate sensitivity map displays a region on the suction side of the wing, wherein a streamwise-oriented force has a net stabilising effect, comparable to what could have been obtained inside the recirculation bubble. In agreement with the predictions of the sensitivity analysis, a spanwise-homogeneous force suppresses the Hopf bifurcation and stabilises the entire branch of von Kármán modes. In the limit of small amplitudes, passive control via spanwise-wavy forcing produces a stabilising effect similar to that of a spanwise-homogeneous control and is more effective than localised spherical forces.

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Linear modal instabilities around post-stall swept finite wings at low Reynolds numbers

Cited by 11 publications

References 73 publications

Triglobal resolvent analysis of swept-wing wakes

Triglobal resolvent analysis of swept-wing wakes

Laminar post-stall wakes of tapered swept wings

Global stability, sensitivity and passive control of low-Reynolds-number flows around NACA 4412 swept wings

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