Scrutiny of buffet mechanisms in transonic flow

Memmolo, Antonio; Bernardini, Matteo; Pirozzoli, Sergio

doi:10.1108/hff-08-2016-0300

Cited by 17 publications

(21 citation statements)

References 19 publications

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“…Performing a large-eddy simulation of a supercritical laminar airfoil (OALT25) at α = 4 • and M = 0.735, but at a higher Reynolds number of Re = 3 • 10 6 , Dandois et al [43] reported shock motion (a permanent back and forth moving shock wave) at significantly higher Strouhal numbers (St = 1.2) compared to typical buffet frequencies, and linked it to a breathing phenomenon of the separation bubble associated with downstream convecting vortices. Similar phenomena were reported by Memmolo et al [17] analysing the V2C airfoil at α = 7 • and high Reynolds numbers using large-eddy simulation. In a URANS parameter study exploring the buffet domain varying the angle of attack (3 ≤ α ≤ 9.5) and Mach number (0.55 < M < 0.75), an interesting coexistence between type A and type C shock motion was reported by Giannelis et al [44].…”

Section: Dynamic Mode Decompositionsupporting

confidence: 88%

“…While high-amplitude lift fluctuations associated with buffet are typically observed at the same frequency as the back-and forth-moving shock waves, there is no obvious correlation in the present case, as upstream-propagating shock waves are generated at significantly higher frequencies (St = 0.4 − 0.7) and leave the airfoil via the leading edge. While studies of the same airfoil at M = 0.7 and significantly higher Reynolds numbers close to experimental conditions (Re = 3 • 10 6 ) do not suggest transonic buffet at α = 4 • , low-frequency oscillations at St ≈ 0.1 are observed for α > 5 • [17,20]. The latter results, comparing delayed detached-eddy simulations (DDES), implicit large-eddy simulations (ILES), and unsteady Reynolds averaged Navier-Stokes (URANS) approaches, show high sensitivity of the low-frequency buffet phenomenon to the modelling methodology applied.…”

Section: Unsteady Flow Structures At α = 4 •mentioning

confidence: 53%

“…In the present contribution direct numerical simulations (DNS) are performed over wingsections at a moderate Reynolds numbers of Re = 500,000 (based on the chord length c) and a Mach number of M = 0.7 considering Dassault Aviation's V2C profile [16]. In the course of the TFAST project, experimental as well as numerical analysis has been carried out on that profile under buffet conditions [17][18][19][20]. There is great interest on global stability analysis of airfoils under buffet conditions (i.e.…”

Section: Introductionmentioning

confidence: 99%

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Modal Analysis of a Laminar-Flow Airfoil under Buffet Conditions at Re = 500,000

Zauner

Sandham

2019

Flow Turbulence Combust

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An airfoil undergoing transonic buffet exhibits a complex combination of unsteady shockwave and boundary-layer phenomena, for which prediction models are deficient. Recent approaches applying computational fluid mechanics methods using turbulence models seem promising, but are still unable to answer some fundamental questions on the detailed buffet mechanism. The present contribution is based on direct numerical simulations of a laminar flow airfoil undergoing transonic buffet at Mach number M = 0.7 and a moderate Reynolds number Re = 500,000. At an angle of attack α = 4 • , a significant change of the boundary layer stability depending on the aerodynamic load of the airfoil is observed. Besides Kelvin Helmholtz instabilities, a global mode, showing the coupled acoustic and flow-separation dynamics, can be identified, in agreement with literature. These modes are also present in a dynamic mode decomposition (DMD) of the unsteady direct numerical solution. Furthermore, DMD picks up the buffet mode at a Strouhal number of St = 0.12 that agrees with experiments. The reconstruction of the flow fluctuations was found to be more complete and robust with the DMD analysis, compared to the global stability analysis of the mean flow. Raising the angle of attack from α = 3 • to α = 4 • leads to an increase in strength of DMD modes corresponding to type C shock motion. An important observation is that, in the present example, transonic buffet is not directly coupled with the shock motion.

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Section: Dynamic Mode Decompositionsupporting

confidence: 88%

Section: Unsteady Flow Structures At α = 4 •mentioning

confidence: 53%

Section: Introductionmentioning

confidence: 99%

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Modal Analysis of a Laminar-Flow Airfoil under Buffet Conditions at Re = 500,000

Zauner

Sandham

2019

Flow Turbulence Combust

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“…On the numerical side, there are very few studies on the laminar transonic buffet phenomenon on airfoils. Memmolo, Bernardini & Pirozzoli (2016) have performed a large-eddy simulation of transonic buffet in laminar conditions. Two main peaks were identified: a low-frequency peak at St 0.1 concentrated around the mean shock position associated with transonic buffet; and a secondary peak at higher frequency (St 1) attributed to a Kelvin-Helmholtz (K-H) vortex shedding process in the separation.…”

Section: Introductionmentioning

confidence: 99%

Large-eddy simulation of laminar transonic buffet

2018

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A large-eddy simulation of laminar transonic buffet on an airfoil at a Mach number $M=0.735$, an angle of attack $\unicode[STIX]{x1D6FC}=4^{\circ }$, a Reynolds number $Re_{c}=3\times 10^{6}$ has been carried out. The boundary layer is laminar up to the shock foot and laminar/turbulent transition occurs in the separation bubble at the shock foot. Contrary to the turbulent case for which wall pressure spectra are characterised by well-marked peaks at low frequencies ($St=f\cdot c/U_{\infty }\simeq 0.06{-}0.07$, where $St$ is the Strouhal number, $f$ the shock oscillation frequency, $c$ the chord length and $U_{\infty }$ the free-stream velocity), in the laminar case, there are also well-marked peaks but at a much higher frequency ($St=1.2$). The shock oscillation amplitude is also lower: 6 % of chord and limited to the shock foot area in the laminar case instead of 20 % with a whole shock oscillation and intermittent boundary layer separation and reattachment in the turbulent case. The analysis of the phase-averaged fields allowed linking of the frequency of the laminar transonic buffet to a separation bubble breathing phenomenon associated with a vortex shedding mechanism. These vortices are convected at $U_{c}/U_{\infty }\simeq 0.4$ (where $U_{c}$ is the convection velocity). The main finding of the present paper is that the higher frequency of the shock oscillation in the laminar regime is due to a different mechanism than in the turbulent one: laminar transonic buffet is due to a separation bubble breathing phenomenon occurring at the shock foot.

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“…These waves are scattered at the TE, generating new waves that travel backward outside the boundary layer up to the shock. The numerical studies of Deck [2] and Memmolo et al [3] suggest that the waves propagating downstream are hydrodynamic waves while those propagating upstream are acoustic waves. Memmolo et al describe all the possible acoustic rays displaying the right frequency, which are emitted at the TE and hit the shock front halfway of the sonic line.…”

mentioning

confidence: 98%