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STUDY OF BUOYANCY EFFECTS ON A THERMAL MIXING LAYER USING AN X-WIRE PROBE OPERATED SEQUENTIALLY AT DIFFERENT OVERHEATS

Kodjovi Sodjavi
Irstea, University Europeenne de Bretagne, F-35044 Rennes, France

Johan Carlier
Irstea, University Europeenne de Bretagne, F-35044 Rennes, France

Résumé

The 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 x-wire probe. A particular attention was paid to the effect of buoyancy forces on two counter-gradient configurations so-called 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.