171 research outputs found
Linear global stability of a confined plume
No full textAbstractA linear stability analysis is performed for a plume flow inside a cylinder of aspect ratio 1. The configuration is identical to that used by Lopez and Marques (2013) for their direct numerical simulation study. It is found that the first bifurcation, which leads to a periodic axisymmetric flow state, is accurately predicted by linear analysis: both the critical Rayleigh number and the global frequency are consistent with the reported DNS results. It is further shown that pressure feedback drives the global mode, rather than absolute instability
Artificial eigenmodes in truncated flow domains
Get PDFInternational audienceWhenever linear eigenmodes of open flows are computed on a numerical domain that is truncated in the streamwise direction, artificial boundary conditions may give rise to spurious pressure signals that are capable of providing unwanted perturbation feedback to upstream locations. The manifestation of such feedback in the eigenmode spectrum is analysed here for two simple configurations. First, explicitly prescribed feedback in a Ginzburg-Landau model is shown to produce a spurious eigenmode branch, named the 'arc branch', that strongly resembles a characteristic family of eigenmodes typically present in open shear flow calculations. Second, corresponding mode branches in the global spectrum of an incompressible parallel jet in a truncated domain are examined. It is demonstrated that these eigenmodes of the numerical model depend on the presence of spurious forcing of a local k + instability wave at the inflow, caused by pressure signals that appear to be generated at the outflow. Multiple local k + branches result in multiple global eigenmode branches. For the particular boundary treatment chosen here, the strength of the pressure feedback from the outflow towards the inflow boundary is found to decay with the cube of the numerical domain length. It is concluded that arc-branch eigenmodes are artifacts of domain truncation, with limited value for physical analysis. It is demonstrated, for the example of a non-parallel jet, how spurious feedback may be reduced by an absorbing layer near the outflow boundary
Modes globaux non-lineaires et generation de son dans les jets chauds.
No full textThe objective of this thesis is twofold: firstly, to study the dynamics inherent in the near field of axisymmetric hot jets by direct numerical simulation, and describe the synchronized oscillations in terms of the theory of nonlinear global modes. On the other hand, characterize the aero-acoustic far field generated by the synchronized oscillations of the near field, also by direct numerical simulation, and identify the physical nature of the aero-acoustic sources in the jet. Thus it is established that the intrinsic synchronized oscillations are observed in the simulations have all the characteristics of a nonlinear global mode, governed by a stationary front located at the upstream edge of an absolutely unstable environment. A linear stability analysis reveals that a jet becomes absolutely unstable hot enough near the nozzle, through the destabilizing effect of the baroclinic torque. The emergence of self-sustained oscillations and their frequency, observed numerically, precisely follow the predictions derived from theoretical criteria. The numerical results are in good agreement with experiments from the literature. The global mode of a hot jet emits a dipole sound field and an analysis of the Lighthill equation shows that this radiation is due to fluctuations in entropy, in contrast to an isothermal jet forced, which sources linked quadrupole the Reynolds stresses are dominant.L'objectif de cette thèse est double : d'une part, étudier la dynamique intrinsèque dans le champ proche des jets chauds axisymétriques par simulation numérique directe, et décrire ces oscillations synchronisées sous l'angle de la théorie des modes globaux non-linéaires. D'autre part, caractériser le champ lointain aéro-acoustique généré par les oscillations synchronisées du champ proche, également par simulation numérique directe, et identifier la nature physique des sources aéro-acoustiques dans le jet. Ainsi il est établi que les oscillations synchronisées intrinsèques que l'on observe dans les simulations possèdent toutes les caractéristiques d'un mode global non-linéaire, régi par un front stationnaire situé au bord amont d'un milieu absolument instable. Une analyse d'instabilité linéaire révèle qu'un jet suffisament chaud devient absolument instable près de la buse, grâce à l'effet déstabilisant du couple barocline. L'apparition des oscillations auto-entretenues ainsi que leur fréquence, observées numériquement, suivent précisement les prédictions déduites des critères théoriques. Les résultats numériques sont en bon accord avec les expériences tirées de la littérature. Le mode global d'un jet chaud émet un champ sonore dipolaire; une analyse de l'équation de Lighthill révèle que ce rayonnement est dû aux fluctuations d'entropie, à la différence d'un jet isotherme forcé, pour lequel les sources quadripolaires liées au tenseur de Reynolds sont dominantes
Structures cohérentes dans des écoulements turbulents
No full textCette thèse s'intéresse à un formalisme récent, le formalisme résolvent, pour proposer une modélisation à ordre faible d'écoulements turbulents. Cette approche linéaire a un intérêt académique en permettant une meilleure compréhension des mécanismes concernés, mais aussi pour l'industrie en permettant des cycles de recherche et développement moins coûteux en calcul. Les travaux réalisés incluent une approche résolvante enrichie du tenseur de Reynolds appliquée dans le cas de l'écoulement canal, et une étude en profondeur du comportement d'un jet turbulent en rotation. Purement numérique, cette contribution s'appuie sur la méthode éléments finis et le formalisme Reynolds-Averaged-Navier-Stokes. Parmi les résultats obtenus, on notera la découverte de nouveaux comportements des jets à basse fréquence ainsi que leur interprétation. Certains de ces effets ne sont pas à portée des méthodes précédemment employées.This thesis is focused on a recent formalism called resolvent formalism, in order to put forward a low rank model of turbulent flows. This linear approach is of interest from an academical perspective as a way to better understand the mechanisms at play, as well as from an industry perspective by allowing for cheaper development cycles. Works detailed in this thesis include a resolvent approach enriched by the Reynolds stress tensor applied in the channel flow case, and an in depth study of a swirling turbulent jet. Based on purely numerical endeavours, this contribution is making ample use of finite element methods and Reynolds-Averaged--Navier-Stokes formalism. Amongst the obtained results, new jet behaviours at low frequency were brought to light as well as interpreted. Some of these effects are simply out of reach more traditional linear analysis methods
Linear impulse response in hot round jets
No full textInternational audienceThe linear impulse response is retrieved from a numerical solution of the spatial eigenvalue problem, which is derived from the fully compressible equations of motion. Changes in the spatiotemporal stability of heated versus isothermal jets are shown to arise solely from the effect of the baroclinic torque. By considering the full linear impulse response, the competition between jet column modes and shear layer modes is characterized. Jet column modes are only found to occur for axisymmetric disturbances. In thin shear layer jets, the jet column mode is shown to prevail at low group velocities, whereas axisymmetric and helical shear layer modes dominate at high group velocities. The absolute mode of zero group velocity is found to always be of the jet column type. Although only convectively unstable, the maximum growth rates of the shear layer modes greatly exceed those of the jet column modes in thin shear layer jets. In thick shear layer jets, axisymmetric modes of mixed jet column/shear layer type arise. The weakened maximum growth rate of mixed modes accounts for the dominance of helical modes in temporal stability studies of thick shear layer jets. © 2007 American Institute of Physics
Optimal velocity and density profiles for the onset of absolute instability in jets
No full textInternational audienceThe absolute/convective character of the linear instability of axisymmetric jets is investigated for a wide range of parallel velocity and density profiles. An adjoint-based sensitivity analysis is carried out in order to maximize the absolute growth rate of jet profiles with and without density variations. It is demonstrated that jets without counterflow may display absolute instability at density ratios well above the previously assumed threshold rho(jet)/rho(infinity) = 0.72, and even in homogeneous settings. Absolute instability is promoted by a strong velocity gradient in the low-velocity region of the shear layer, as well as by a step-like density variation near the location of maximum shear. A new efficient algorithm for the computation of the absolute instability mode is presented
Sensitivity-based optimization of jet profiles for the onset of absolute instability
No full textInternational audienc
Sensitivity-based optimization of jet profiles for the onset of absolute instability
No full textInternational audienc
Confinement effects in laminar swirling jets
No full textInternational audienceThis paper explores the effect of axial and radial confinement on the flow topology of laminar swirling jets. Its objective is to provide a unifying perspective toward swirling jet mechanics that connects earlier reports across a variety of confined and unconfined flow situations and over a range of swirl ratio S values. The analysis focuses separately on the influence of the jet's injection depth L in a radially-unconfined flow and of the chamber diameter C in radially-confined jets. In the former case, it shows that axial confinement strongly influences the jet's behaviour when L is small, allowing bistable steady-states: a central jet (CJ) solution with or without a small central recirculation zone (CRZ), and a wall jet (WJ) solution with a wide-open CRZ spreading along the reservoir's edge. Similar behaviour is identified for radially-confined jets, where bistable CJ and WJ states appear over a range of moderate C values, and the WJ state adopts a conical CRZ. In either case, the WJ solution appears or disappears via saddle-node bifurcations when the confinement is made sufficiently strong or weak, respectively. This dynamics is attributed to an exchange of dominance between central and outer low pressure regions as the flow transitions from CJ to WJ or vice-versa. The findings demonstrate that the hysteresis widely associated with swirling jets is controlled not just by vortex breakdown, but also by confinement through the Coandă effect. Such confinement is found to significantly alter the state-space structure even when the walls are far from the nozzle
- …
