19 research outputs found
"Pensées des morts. Paroles de de Lamartine. Musique de Félicien David" (manuscrit autographe)
No full textAu titre : "manuscrit F. O. Alizard. Pensée des morts dédiée à son ami Alizard par Félicien David. Octobre 1839". - Foliotation ajoutée. - Au fol. 6 v° "Mr Guillet. Rue Croix des Petits champs N°31.". - On a joint une partie de 2d hautbois (342 x 263 mm). - A la suite de cette partie, esquisse pré-instrumentale "Lever de soleil
"Pensées des morts. Paroles de de Lamartine. Musique de Félicien David" (manuscrit autographe)
No full textAu titre : "manuscrit F. O. Alizard. Pensée des morts dédiée à son ami Alizard par Félicien David. Octobre 1839". - Foliotation ajoutée. - Au fol. 6 v° "Mr Guillet. Rue Croix des Petits champs N°31.". - On a joint une partie de 2d hautbois (342 x 263 mm). - A la suite de cette partie, esquisse pré-instrumentale "Lever de soleil
Optimal transient growth in compressible turbulent boundary layers
No full textThe structure of zero-pressure-gradient compressible turbulent boundary layers is
analysed using the tools of optimal transient growth theory. The approach relies
on the extension to compressible flows of the theoretical framework originally
developed by Reynolds & Hussain (J. Fluid Mech., vol. 52, 1972, pp. 263–288) for
incompressible flows. The model is based on a density-weighted triple decomposition
of the instantaneous field into the contributions of the mean flow, the organized
(coherent) motions and the disorganized background turbulent fluctuations. The
mean field and the eddy viscosity characterizing the incoherent fluctuations are here
obtained from a direct numerical simulation database. Most temporally amplified
modes (optimal modes) are found to be consistent with scaling laws of turbulent
boundary layers for both inner and outer layers, as well as in the logarithmic
region, where they exhibit a self-similar spreading. Four free-stream Mach numbers
are considered: Ma ∞ = 0.2, 2, 3 and 4. Weak effects of compressibility on the
characteristics length and the orientation angles are observed for both the inner- and
the outer-layer modes. Furthermore, taking into account the effects of mean density
variations, a universal behaviour is suggested for the optimal modes that populate the
log layer, regardless of the Mach number. The relevance of the optimal modes in
describing the near-wall layer dynamics and the eddies that populate the outer region
is discussed
Sensitivity and forcing response in separated boundary-layer flow
No full textThe stability of separating boundary-layer flow on a flat plate is numerically investigated by means of three-dimensional
eigenmodes of the linearized Navier-Stokes equations obtained by linearization about the steady state. The disturbance variables
are approximated using a Chebyshev-Chebyshev collocation technique in inhomogeneous directions. Due to the large size of the
generalized eigenvalue problem, an Arnoldi iterations method using ARPACK routines is employed. By expanding the flow disturbance
variables in the basis of eigenmodes the growth potential is revealed by the computation of the optimal initial condition. This yields a
low-dimensional model of the flow and a unified view on its stability characteristics. Furthermore, a general formalism is developed to
assess how harmonic forcing may alter the stability properties of flows studied by a global approach of the linear stability theory. This
formalism is based on the sensitivity analysis performed by the pseudo-spectrum calculation and the resolution of the forced problem.
These results are compared and extended with direct numerical simulation
Global and Koopman modes analysis of sound generation in mixing layers
No full textIt is now well established that linear and nonlinear instability waves play a significant role in the noise generation process for a wide variety of shear flows such as jets or mixing layers. In that context, the problem of acoustic radiation generated by spatially growing instability waves of two-dimensional subsonic and supersonic mixing layers are revisited in a global point of view, i.e., without any assumption about the base flow, in both a linear and a nonlinear framework by using global and Koopman mode decompositions. In that respect, a timestepping technique based on disturbance equations is employed to extract the most dynamically relevant coherent structures for both linear and nonlinear regimes. The present analysis proposes thus a general strategy for analysing the near-field coherent structures which are responsible for the acoustic noise in these configurations. In particular, we illustrate the failure of linear global modes to describe the noise generation mechanism associated with the vortex pairing for the subsonic regime whereas they appropriately explain the Mach wave radiation of instability waves in the supersonic regime. By contrast, the Dynamic Mode Decomposition (DMD) analysis captures both the near-field dynamics and the far-field acoustics with a few number of modes for both configurations. In addition, the combination of DMD and linear global modes analyses provides new insight about the influence on the radiated noise of nonlinear interactions and saturation of instability waves as well as their interaction with the mean flow
The onset of three-dimensional centrifugal global modes and their nonlinear development in a recirculating flow over a flat surface
No full textPublisher version : http://pof.aip.org/resource/1/phfle6/v22/i11/p114102_s1?isAuthorized=noThe three-dimensional stability dynamics of a separation bubble over a flat plate has been studied in both linear and nonlinear conditions. Using a global eigenvalue analysis, two centrifugal global modes are identified: an asymptotically unstable three-dimensional weakly growing mode which appears to be originated by a Rayleigh instability; a marginally stable three-dimensional steady mode which is originated by a convective Gortler instability. Direct numerical simulations show that both modes play a role in the route to transition toward the turbulent flow. A structural sensitivity analysis is used to investigate the mechanism of selection of the path toward transition when small perturbations are considered. Finally, a scenario of transition via Gortler modes breakdown is studied in detail, revealing the formation of trains of hairpin vortices in streamwise succession
