5 research outputs found
Ătude de la turbulence et du scalaire passif dans un jet rond libre lĂ©gĂšrement chauffĂ©
Tableau d'honneur de la FacultĂ© des Ă©tudes supĂ©rieures et postdorales, 2014-2015Cette thĂšse prĂ©sente une Ă©tude de la turbulence et du scalaire passif dans la zone autosimilaire dâun jet rond libre faiblement chauffĂ©, caractĂ©risĂ© par un haut nombre de Reynolds ([symbol]). Dâabord, une analyse des limitations des sondes Ă fils chauds croisĂ©s stationnaires basĂ©e sur des mesures LDV est prĂ©sentĂ©e. Celle-ci met en Ă©vidence lâimportance des Ă©vĂ©nements de vitesse Ă grands angles dâincidence dans les valeurs mesurĂ©es. Ensuite, la turbulence et la chaleur employĂ©e comme scalaire passif dans le jet sont Ă©tudiĂ©es Ă lâaide du LDV, de la thermomĂ©trie Ă fil froid et dâune combinaison de ces deux mĂ©thodes. On mesure ainsi les distributions radiales des valeurs moyennes et des moments de vitesse, de tempĂ©rature et de leurs corrĂ©lations croisĂ©es. Ces statistiques incluent notamment [symbol]. Ces mesures sont caractĂ©risĂ©es par un contrĂŽle prĂ©cis des conditions aux limites et une grande convergence statistique. On remarque un bon accord entres les valeurs mesurĂ©es de [symbol] , [symbol] ainsi que [symbol] et leur expressions analytiques, dĂ©rivĂ©es des Ă©quations de continuitĂ©, du mouvement moyen et dâenthalpie, respectivement. Les bilans de transports de lâĂ©nergie cinĂ©tique turbulente (k), du scalaire passif ([symbol] ) ainsi que des flux de chaleur turbulents [symbol] sont ensuite Ă©tablis, dans le mĂȘme Ă©coulement de jet rond faiblement chauffĂ©. Pour le bilan de k, le modĂšle proposĂ© par Lumley (1978) est employĂ© pour reprĂ©senter les termes de diffusion de pression. Pour les deux bilans, lâintĂ©gration Ă zĂ©ro sur un volume de contrĂŽle des termes de diffusion est vĂ©rifiĂ©e et la portĂ©e de ce critĂšre de validitĂ© est analysĂ©e. Aussi, les restes des deux bilans constituent leurs termes de dissipation respectifs, ek et eq . Dans le cas de ek, le modĂšle prĂ©dictif mis de lâavant par Thiesset et al (2014) ainsi quâune analyse originale des bases de donnĂ©es de Panchapakesan & Lumley (1993a) et de Hussein et al (1994) laisse entrevoir une possibilitĂ© de rĂ©conciliation en ce qui a trait aux rĂ©sultats produits par ces deux expĂ©riences rĂ©putĂ©es. Les budgets des composantes du tenseur de Reynolds ([symbol] ) sont aussi Ă©tablis et analysĂ©s. La valeur de eq sur lâaxe du jet est employĂ©e comme rĂ©fĂ©rence dans une Ă©tude exhaustive des techniques de mesures directes de ce taux de dissipation. Cette Ă©tude expose clairement les difficultĂ©s de ces mesures, notamment par des analyses spectrales. Les donnĂ©es amassĂ©es permettent dâĂ©tablir les distributions radiales de paramĂštres dâintĂ©rĂȘt pour la modĂ©lisation, tels que la viscositĂ© tourbillonnaire ([symbol] ), la diffusivitĂ© tourbillonnaire ([symbol] ) et le nombre de Prandtl turbulent ([symbol] ). Lâassomption de [symbol] constant est examinĂ©e. Le ratio entre les Ă©chelles de temps de dissipation de tempĂ©rature et dâĂ©nergie cinĂ©tique turbulente (R) est aussi calculĂ© et analysĂ©.This thesis presents a study of the turbulence generated by a slightly heated turbulent round jet, evolving in still air. The study focuses on the self-preserving region of the high Reynolds number jet ([symbol] ). The jet is heated to only 20°C above ambient, so that temperature is treated as a passive scalar. In the first chapter of the thesis, a new and novel analysis of the stationary crossed hot wire probes based on unbiased LDV data is exposed, thus revealing the importance of the wide incidence angle events on the measured values. LDV, cold wire thermometry and a combination of both methods is used to study the slightly heated jet. Radial distributions of mean velocities and heating level, second and third order moments of fluctuating velocity and temperature ([symbol]) along with cross correlations, such as [symbol] are made available. During data acquisition, great attention was devoted to the close control of boundary conditions and statistical convergence. The measured values of V, uv as well as vq are in good agreement with their analytical expressions, provided by continuity, mean momentum and enthalpy equations, respectively. The transport budgets for the turbulent kinetic energy, ([symbol] ), passive scalar ([symbol] ), and temperature turbulent fluxes ([symbol] ) are then established using the data acquired in the very same, slightly heated, jet flow. The model introduced by Lumley (1978) for the pressure diffusion, is used as a surrogate for this term, in the case of the k budget. The integration to zero of the diffusion terms of both budgets over a control volume is verified. The significance of this validity criterion is analysed. The remainders of both budgets, are their respective dissipation terms, ek and eq . In the case of ek, the predictive model recently put forward by Thiesset et al (2014) along with a novel treatment of the databases of Panchapakesan & Lumley (1993a) and Hussein et al (1994), may help bring the outcome of the two famous experiments to a reconciliation. The Reynolds stress tensor component budgets were also established, leading to an analysis of the importance of the redistribution terms. The [symbol] value provided by the [symbol] budget is used as a reference in an exhaustive study of the means of dissipation measurement, on the centerline of the jet. This study reveals the shortcoming of the direct measurements of eq , namely thought spectral analysis. Radial distributions of turbulence modeling parameters, such as turbulent momentum viscosity ([symbol] ), turbulent thermal diffusivity ([symbol] ) and turbulent Prandtl number ([symbol] ) were established. The assumption of a constant PrT is scrutinized. The radial distribution of the thermal-to-mechanical timescale ratio (R) is calculated and analysed
Self-similarity in Slightly Heated Annular Jet with Large Diameter Ratios
International audienceThe study aims at furthering our understanding and quantifying the influence of coherent structures on small-scale turbulence and passive scalar mixing, in an annular jet configuration with large diameter ratios. This âbluff-bodyâ geometry is close to that widely used in combustion for flame stabilization [1]. A passive contaminant is introduced in the flow, through a slight heating. We report the evolution along the jet axis of the following quantities: mean values of the longitudinal velocity and passive scalar ( UÂŻ and ÎÂŻ ), as well as the energy and scalar dissipation rates ( Δ¯ and ÏÂŻ ). It is shown that these statistics: decay as xâ1 and xâ4 , where x is the streamwise direction, similarly to the decay in the far-field of classical jets (CJ);unlike the CJ, they reach self-similarity faster, a behaviour that may be attributed to the presence of coherent structures