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http://hdl.handle.net/10985/17999
Various approaches to determine active regions in an unstable global mode: application to transonic buffet
PALADINI, Edoardo; MARQUET, Olivier; SIPP, Denis; DANDOIS, Julien; ROBINET, Jean-Christophe
The transonic flow field around a supercritical airfoil is investigated. The objective of the present paper is to enhance the understanding of the physical mechanics behind two-dimensional transonic buffet. The paper is composed of two parts. In the first part, a global stability analysis based on the linearized Reynolds-averaged Navier–Stokes equations is performed. A recently developed technique, based on the direct and adjoint unstable global modes, is used to compute the local contribution of the flow to the growth rate and angular frequency of the unstable global mode. The results allow us to identify which zones are directly responsible for the existence of the instability. The technique is firstly used for the vortex-shedding cylinder mode, as a validating case. In the second part, in order to confirm the results of the first part, a selective frequency damping method is locally used in some regions of the flow field. This method consists of applying a low-pass filter on selected zones of the computational domain in order to damp the fluctuations. It allows us to identify which zones are necessary for the persistence of the instability. The two different approaches give the same results: the shock foot is identified as the core of the instability; the shock and the boundary layer downstream of the shock are also necessary zones while damping the fluctuations on the lower surface of the airfoil; and outside the boundary layer between the shock and the trailing edge or above the supersonic zone does not suppress the shock oscillation. A discussion on the several physical models, proposed until now for the buffet phenomenon, and a new model are finally offered in the last section.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/179992019-01-01T00:00:00ZPALADINI, EdoardoMARQUET, OlivierSIPP, DenisDANDOIS, JulienROBINET, Jean-ChristopheThe transonic flow field around a supercritical airfoil is investigated. The objective of the present paper is to enhance the understanding of the physical mechanics behind two-dimensional transonic buffet. The paper is composed of two parts. In the first part, a global stability analysis based on the linearized Reynolds-averaged Navier–Stokes equations is performed. A recently developed technique, based on the direct and adjoint unstable global modes, is used to compute the local contribution of the flow to the growth rate and angular frequency of the unstable global mode. The results allow us to identify which zones are directly responsible for the existence of the instability. The technique is firstly used for the vortex-shedding cylinder mode, as a validating case. In the second part, in order to confirm the results of the first part, a selective frequency damping method is locally used in some regions of the flow field. This method consists of applying a low-pass filter on selected zones of the computational domain in order to damp the fluctuations. It allows us to identify which zones are necessary for the persistence of the instability. The two different approaches give the same results: the shock foot is identified as the core of the instability; the shock and the boundary layer downstream of the shock are also necessary zones while damping the fluctuations on the lower surface of the airfoil; and outside the boundary layer between the shock and the trailing edge or above the supersonic zone does not suppress the shock oscillation. A discussion on the several physical models, proposed until now for the buffet phenomenon, and a new model are finally offered in the last section.Transonic buffet instability: From two-dimensional airfoils to three-dimensional swept wings
http://hdl.handle.net/10985/17897
Transonic buffet instability: From two-dimensional airfoils to three-dimensional swept wings
PALADINI, Edoardo; BENEDDINE, Samir; DANDOIS, Julien; SIPP, Denis; ROBINET, Jean-Christophe
The objective of the present study is to explain the evolution of the transonic buffet phenomenon from two-dimensional airfoils to three-dimensional swept wings by a global stability analysis. With respect to two-dimensional buffet, shock oscillation frequency increases by a factor of 4 to 7 in the case of a swept 30◦ wing and three-dimensional patterns in the detached boundary layer are convected outboard. Crouch et al. [J. Comput. Phys. 224, 924 (2007)] explained the two-dimensional transonic buffet phenomenon by the appearance of a real positive complex eigenvalue of the linearized Jacobian matrix. In the case of an infinite unswept wing, the present study shows that two unstable modes actually exist: The two-dimensional transonic buffet mode already identified by Crouch et al. [J. Fluid Mech. 628, 357 (2009)] and a strongly amplified three-dimensional zerofrequency mode. The latter exhibits regular patterns in the separated boundary layer, which relates to the so-called buffet cells as named by Iovnovich et al. [AIAA J. 53, 449 (2015)]. The nonzero sweep angle generates a spanwise velocity component on the wing which convects the cells outboard. This impacts both modes identified in the unswept case: The two-dimensional mode is weakly damped by the sweep while the three-dimensional buffet cells mode, even if weakly damped, remains strongly unstable and now exhibits a nonzero frequency which increases with the sweep angle. The frequency and wavelength of the most unstable three-dimensional mode for a sweep angle of 30◦ agree well with numerical and experimental values of the three-dimensional transonic buffet on wings. The analysis of the wavemaker of the three-dimensional modes indicates that the core of the instability is nearly solely located in the separated region, with a maximum along the separation line. In contrast, the wavemaker of the two-dimensional buffet mode exhibits stronger values all along the shock wave.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/178972019-01-01T00:00:00ZPALADINI, EdoardoBENEDDINE, SamirDANDOIS, JulienSIPP, DenisROBINET, Jean-ChristopheThe objective of the present study is to explain the evolution of the transonic buffet phenomenon from two-dimensional airfoils to three-dimensional swept wings by a global stability analysis. With respect to two-dimensional buffet, shock oscillation frequency increases by a factor of 4 to 7 in the case of a swept 30◦ wing and three-dimensional patterns in the detached boundary layer are convected outboard. Crouch et al. [J. Comput. Phys. 224, 924 (2007)] explained the two-dimensional transonic buffet phenomenon by the appearance of a real positive complex eigenvalue of the linearized Jacobian matrix. In the case of an infinite unswept wing, the present study shows that two unstable modes actually exist: The two-dimensional transonic buffet mode already identified by Crouch et al. [J. Fluid Mech. 628, 357 (2009)] and a strongly amplified three-dimensional zerofrequency mode. The latter exhibits regular patterns in the separated boundary layer, which relates to the so-called buffet cells as named by Iovnovich et al. [AIAA J. 53, 449 (2015)]. The nonzero sweep angle generates a spanwise velocity component on the wing which convects the cells outboard. This impacts both modes identified in the unswept case: The two-dimensional mode is weakly damped by the sweep while the three-dimensional buffet cells mode, even if weakly damped, remains strongly unstable and now exhibits a nonzero frequency which increases with the sweep angle. The frequency and wavelength of the most unstable three-dimensional mode for a sweep angle of 30◦ agree well with numerical and experimental values of the three-dimensional transonic buffet on wings. The analysis of the wavemaker of the three-dimensional modes indicates that the core of the instability is nearly solely located in the separated region, with a maximum along the separation line. In contrast, the wavemaker of the two-dimensional buffet mode exhibits stronger values all along the shock wave.Analysis and Comparison of Transonic Buffet Phenomenon over Several Three-Dimensional Wings
http://hdl.handle.net/10985/18001
Analysis and Comparison of Transonic Buffet Phenomenon over Several Three-Dimensional Wings
PALADINI, Edoardo; DANDOIS, Julien; SIPP, Denis; ROBINET, Jean-Christophe
The transonic buffet is a complex aerodynamic instability that appears on wings and airfoils at a high subsonic Mach number and/or angle of attack. It consists of a shock oscillation that induces pressure and notably lift fluctuations, thus limiting the flight envelope of civil aircraft. The aim of the present Paper is to improve the understanding of the flow physics of the three-dimensional transonic buffet over swept wings through the analysis and comparison of four different experimental databases. In particular, the objective is to identify characteristic values of the phenomenon such as Strouhal numbers, convection velocities, buffet onset, etc. It is shown that some dimensionless numbers are kept constant among the different databases and consequently can be considered as characteristics, whereas others change. The key factors in the understanding of the three-dimensional transonic buffet phenomenon lie in explaining common features but also the variability of transonic buffet characteristics in different configurations. In particular, it is shown that three-dimensional buffet is characterized by a Strouhal number in the range 0.2–0.3 and a spanwise convection velocity of 0.245 0.015 U∞, where U∞ denotes the freestream velocity. These characteristic ranges of frequencies are larger than those of the two-dimensional buffet phenomenon, which suggests different physical mechanisms.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/180012019-01-01T00:00:00ZPALADINI, EdoardoDANDOIS, JulienSIPP, DenisROBINET, Jean-ChristopheThe transonic buffet is a complex aerodynamic instability that appears on wings and airfoils at a high subsonic Mach number and/or angle of attack. It consists of a shock oscillation that induces pressure and notably lift fluctuations, thus limiting the flight envelope of civil aircraft. The aim of the present Paper is to improve the understanding of the flow physics of the three-dimensional transonic buffet over swept wings through the analysis and comparison of four different experimental databases. In particular, the objective is to identify characteristic values of the phenomenon such as Strouhal numbers, convection velocities, buffet onset, etc. It is shown that some dimensionless numbers are kept constant among the different databases and consequently can be considered as characteristics, whereas others change. The key factors in the understanding of the three-dimensional transonic buffet phenomenon lie in explaining common features but also the variability of transonic buffet characteristics in different configurations. In particular, it is shown that three-dimensional buffet is characterized by a Strouhal number in the range 0.2–0.3 and a spanwise convection velocity of 0.245 0.015 U∞, where U∞ denotes the freestream velocity. These characteristic ranges of frequencies are larger than those of the two-dimensional buffet phenomenon, which suggests different physical mechanisms.