Experimental study of thermal mixing layer using variable temperature hot-wire anemometry

International audienceThe buoyancy effects on the development of the thermal mixing layer downstream from a horizontal separating plate were studied by comparing stable and unstable counter-gradient configurations. In this study, the novel experimental technique called parameterizable constant temperature anemometer, proposed by Ndoye et al. (Meas Sci Technol 21(7):075401, 2010), was improved to make possible the simultaneous measurement of temperature and two velocity components with an x-wire probe. The buoyancy effects on the flow are discussed through the transport equations of turbulent kinetic energy and temperature variance. In view of the low Richardson numbers at stake (Ri f < 0.03), the buoyancy forces appeared logically to be quantitatively negligible compared to the main driving forces, but such a low-energy forcing mechanism was in fact sufficient in unstable configurations to increase the shear stress and the expansion rate of the mixing layer significantly, both phenomena being associated with enhanced production of turbulence

E&amp;#769;tude expe&amp;#769;rimentale des flux de quantite&amp;#769; de mouvement et de chaleur dans une couche de me&amp;#769;lange anisotherme

21e Congre&amp;#768;s Franc&amp;#807;ais de Me&amp;#769;canique, Bordeaux, FRA, 26-/08/2013 - 30/08/2013National audienceL'étude porte sur une couche de mélange plane horizontale généree par la rencontre de deux écoulements parallèles à vitesse et température différentes. La thermo-anémométrie la température de fil variable, étendue à l'utilisation de sonde fils croisés, a permis la mesure instantanée des fluctuations de vitesse (composantes longitudinale et transversale) et de température dans l'écoulement. Les densités de probabilité jointes ont pu ainsi être obtenues, et leur analyse a mis en évidence les mécanismes et évènements qui contribuent signicativement aux flux transversaux de quantifiées de mouvement et de chaleur. Ces différentes contributions ont été différenciées et quantifiées par une analyse en quadrant qui a fait ressortir la prépondérance des mouvements d'entraînement et d'éjection, et leur dissymétrie

Turbulent mixing and entrainment in a stratied horizontal plane shear layer: joint velocitytemperature analysis of experimental data

International audienceBuoyancy effects on the turbulent mixing and entrainment processes were analysed in the case of a stratified plane shear layer between two horizontal air flows in conditions leading to relatively low values of the flux Richardson number (|Rif|max≃0.02|Rif|max≃0.02). Velocity and temperature measurements were made with a single ××-wire probe thermo-anemometry technique, using multi-overheat sequences to deliver simultaneous velocity–temperature data at high frequency. The spatial resolution was found to be fine enough, in relation to the dissipative scale and the thermal diffusive scale, to avoid false mixing enhancement in the analysis of the physical mechanisms through velocity–temperature coupling in statistical turbulence quantities. Probability density functions (PDFs) and joint probability density functions (JPDFs) were used to distinguish between the different mechanisms involved in turbulent mixing, namely entrainment, engulfing, nibbling and mixing, and point to the contribution of entrainment in the mixing process. When comparing an unstably stratified configuration to its stably stratified equivalent, no significant difference could be seen in the PDF and JPDF quantities, but a conditional analysis based on temperature thresholding enabled a separation between mixed fluid and two distinct sets of events associated with unmixed fluid entrained from the hot and cold sides of the mixing layer into the mixing layer. This separation allowed a direct calculation of the entrainment velocities on both sides of the mixing layer. A significant increase of the total entrainment could be seen in the case of unstably stratified configuration. The entrainment ratios were compared to their prediction by the Dimotakis model and both a rather good relevance of the model and some need for improvement were found from the comparison. It was hypothesised that the improvement should come from better taking into account the distinct contributions of nibbling and engulfing inside the process of entrainment and mixing

Study of buoyancy effects on a thermal mixing layer using an x-wire probe operated sequentially at different overheats

International audienceThe turbulent mixing was studied in a thermal plane turbulent mixing layer induced by two parallel and horizontal incident streams with velocity and temperature differences. The instantaneous velocity and temperature fluctuations in the mixing layer were measured using variable temperature hot wire thermo-anemometry with an ×-wire probe. A particular attention was paid to the effect of buoyancy forces on two counter-gradient configurations socalled stable and unstable, in relation to the sign of the vertical temperature gradient applied. The buoyancy effects were discussed in terms of transport equations of turbulent kinetic energy and temperature variance. In view of the low Richardson values at stake (Rif < 0.03) the buoyancy forces appeared logically to be quantitatively negligible compared to the main driving forces, but such a low energy forcing mechanism was in fact sufficient in the unstable configuration to increase significantly the shear stress and the expansion rate of the mixing layer, both phenomena being associated with enhanced production of turbulence. In addition, joint probability density function analysis highlighted the mechanisms and events that significantly contribute to the transverse momentum and heat fluxes. These contributions were differentiated and quantified through quadrant analysis which emphasized the dominance of the local movements of entrainment and ejection, their dissymmetries and the effects of buoyancy

Passive control strategy for mixing ventilation in heating mode using lobed inserts

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Airflow characteristics and thermal comfort generated by a multi-cone ceiling diffuser with and without inserted lobes

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Experimental investigation of nozzle shape effect on wall shear stress beneath impinging round jet

This paper reports on measurements of velocities and wall shear rates for an impinging round jet. The test parameter that we consider is the nozzle shape. The Particle Image Velocimetry (PIV) was used to measure the axial and radial velocity components. The limitations of the PIV technique in the vicinity of the target wall are addressed by using the electrodiffusion technique to achieve wall shear rate distribution. A round orifice perforated either on a flat plate (RO/P) or on a hemispherical surface (RO/H), is compared to a reference convergent nozzle (CONV). All the nozzles have the same exit diameter D. The exit volumetric flow rate was also conserved and led to the same Reynolds number based on the exit bulk-velocity, Re0=5290. The nozzle-to-wall distance was constant and equal to 2 D. The whole velocity field and wall shear rates in the three impinging round jets having different features are compared. This has improved the understanding of the jet/wall interaction. The wall shear rate is in a close relationship with the near field flow features, themselves affected by nozzle geometry. The orifice nozzles generate narrower exit profiles compared to the convergent nozzle. The vena contracta effect in orifice jets, more intense with RO/P than with RO/H, generates an increase of the exit centerline velocity. The hemispherical surface of RO/H nozzle leads to a venna-stretching attenuating somewhat the venna contracta effect at the jet exit. The instantaneous PIV fields indicated the formation of secondary vortices in the region where a secondary peak in the mean and rms distributions of wall shear rate emerged

Impinging jet passive control fo wall shear stress enhancement

Heat, mass and momentum transfer induced on a wall by an impinging jet are linked to vortices organization at the jet exit, themselves influenced by jet nozzle geometry. Particle Image Velocimetry (PIV) and electrodiffusion techniques were used to investigate the characteristics of three impinging jets. Two cruciform jets, one issuing from a plane orifice nozzle (CO/P) and the second from a hemispherical orifice nozzle (CO/H), were compared to a reference round jet issuing from a convergent nozzle. The distance between the jet exit and the target wall is equal to 2 nozzle equivalent diameters (De). The Reynolds number based on De and on the exit bulk-velocity was equal to 5290 in each flow. The analysis of mean and fluctuating flow fields from PIV enables the identification of flows characteristics. The jet spreading rate and the turbulence intensities before impingement were found to be higher in the jet issuing from CO/H nozzle. The switching-over phenomena observed in CO/P nozzle jet does not occur in CO/H nozzle jet. Electrodiffusion measurements reveal some differences in the shape and the level of radial distributions of wall shear rates. Of the most important observations is the large difference between the three jets in the wall shear stress levels. For the same exit bulk-velocity, the maximum wall shear rate in CO/P and CO/H nozzle jets are almost two and three times respectively higher than the one of the reference convergent jet