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The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sat, 18 Mar 2023 21:08:58 GMT2023-03-18T21:08:58ZThree-dimensional instability of a ow past a sphere: Mach evolution of the regular and Hopf bifurcations
http://hdl.handle.net/10985/14215
Three-dimensional instability of a ow past a sphere: Mach evolution of the regular and Hopf bifurcations
SANSICA, Andrea; ROBINET, Jean-Christophe; ALIZARD, Frédéric; GONCALVES, Eric
A fully three-dimensional linear stability analysis is carried out to investigate the unstable bifurcations of a compressible viscous fluid past a sphere. A time-stepper technique is used to compute both equilibrium states and leading eigenmodes. In agreement with previous studies, the numerical results reveal a regular bifurcation under the action of a steady mode and a supercritical Hopf bifurcation that causes the onset of unsteadiness but also illustrate the limitations of previous linear approaches, based on parallel and axisymmetric base flow assumptions, or weakly nonlinear theories. The evolution of the unstable bifurcations is investigated up to low-supersonic speeds. For increasing Mach numbers, the thresholds move towards higher Reynolds numbers. The unsteady fluctuations are weakened and an axisymmetrization of the base flow occurs. For a sufficiently high Reynolds number, the regular bifurcation disappears and the flow directly passes from an unsteady planar-symmetric solution to a stationary axisymmetric stable one when the Mach number is increased. A stability map is drawn by tracking the bifurcation boundaries for different Reynolds and Mach numbers. When supersonic conditions are reached, the flow becomes globally stable and switches to a noise-amplifier system. A continuous Gaussian white noise forcing is applied in front of the shock to examine the convective nature of the flow. A Fourier analysis and a dynamic mode decomposition show a modal response that recalls that of the incompressible unsteady cases. Although transition in the wake does not occur for the chosen Reynolds number and forcing amplitude, this suggests a link between subsonic and supersonic dynamics.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10985/142152018-01-01T00:00:00ZSANSICA, AndreaROBINET, Jean-ChristopheALIZARD, FrédéricGONCALVES, EricA fully three-dimensional linear stability analysis is carried out to investigate the unstable bifurcations of a compressible viscous fluid past a sphere. A time-stepper technique is used to compute both equilibrium states and leading eigenmodes. In agreement with previous studies, the numerical results reveal a regular bifurcation under the action of a steady mode and a supercritical Hopf bifurcation that causes the onset of unsteadiness but also illustrate the limitations of previous linear approaches, based on parallel and axisymmetric base flow assumptions, or weakly nonlinear theories. The evolution of the unstable bifurcations is investigated up to low-supersonic speeds. For increasing Mach numbers, the thresholds move towards higher Reynolds numbers. The unsteady fluctuations are weakened and an axisymmetrization of the base flow occurs. For a sufficiently high Reynolds number, the regular bifurcation disappears and the flow directly passes from an unsteady planar-symmetric solution to a stationary axisymmetric stable one when the Mach number is increased. A stability map is drawn by tracking the bifurcation boundaries for different Reynolds and Mach numbers. When supersonic conditions are reached, the flow becomes globally stable and switches to a noise-amplifier system. A continuous Gaussian white noise forcing is applied in front of the shock to examine the convective nature of the flow. A Fourier analysis and a dynamic mode decomposition show a modal response that recalls that of the incompressible unsteady cases. Although transition in the wake does not occur for the chosen Reynolds number and forcing amplitude, this suggests a link between subsonic and supersonic dynamics.System Identification of Two-Dimensional Transonic Buffet
http://hdl.handle.net/10985/23062
System Identification of Two-Dimensional Transonic Buffet
SANSICA, Andrea; KANAMORI, Masashi; HASHIMOTO, Atsushi; LOISEAU, Jean-Christophe; ROBINET, Jean-Christophe
When modeled within the unsteady Reynolds-Averaged Navier-Stokes framework, the
shock-wave dynamics on a two-dimensional aerofoil at transonic buffet conditions is char-
acterized by time-periodic oscillations. Given the time series of the lift coefficient at different
angles of attack for the OAT15A supercritical profile, the sparse identification of nonlinear dy-
namics (SINDy) technique is used to extract a parametrized, interpretable and minimal-order
description of this dynamics. For all of the operating conditions considered, SINDy infers
that the dynamics in the lift coefficient time series can be modeled by a simple parametrized
Stuart-Landau oscillator, reducing the computation time from hundreds of core hours to sec-
onds. The identified models are then supplemented with equally parametrized measurement
equations and low-rank DMD representation of the instantaneous state vector to reconstruct
the true lift signal and enable real-time estimation of the whole flow field. Simplicity, accuracy
and interpretability make the identified model a very attractive tool towards the construction
of real-time systems to be used during the design, certification and operational phases of the
aircraft life cycle.
Tue, 01 Feb 2022 00:00:00 GMThttp://hdl.handle.net/10985/230622022-02-01T00:00:00ZSANSICA, AndreaKANAMORI, MasashiHASHIMOTO, AtsushiLOISEAU, Jean-ChristopheROBINET, Jean-ChristopheWhen modeled within the unsteady Reynolds-Averaged Navier-Stokes framework, the
shock-wave dynamics on a two-dimensional aerofoil at transonic buffet conditions is char-
acterized by time-periodic oscillations. Given the time series of the lift coefficient at different
angles of attack for the OAT15A supercritical profile, the sparse identification of nonlinear dy-
namics (SINDy) technique is used to extract a parametrized, interpretable and minimal-order
description of this dynamics. For all of the operating conditions considered, SINDy infers
that the dynamics in the lift coefficient time series can be modeled by a simple parametrized
Stuart-Landau oscillator, reducing the computation time from hundreds of core hours to sec-
onds. The identified models are then supplemented with equally parametrized measurement
equations and low-rank DMD representation of the instantaneous state vector to reconstruct
the true lift signal and enable real-time estimation of the whole flow field. Simplicity, accuracy
and interpretability make the identified model a very attractive tool towards the construction
of real-time systems to be used during the design, certification and operational phases of the
aircraft life cycle.