Conditional analysis in a turbulent boundary layer with strong density differences

A study of the fine structure of wall turbulence in the presence of strong density variations through a conditional analysis of experimental data obtained in a wind tunnel is presented in this paper. Density variations within the boundary layer are produced by injection of air or helium into a mixture of these two components. The conditional analysis carried out in this study shows that the injection of helium into the boundary layer generates more violent ejections than in the case of an injection of air. This result is confirmed by the significant contribution of the ejections to the turbulent mass flux. The results obtained show that in the fully turbulent zone the ejections are associated with strong velocity fluctuations for the two injected gases. This is even more so for the fluctuations of density when an injection of helium is considered. At two locations of the flow (x/e = 8.3 and x/e = 33.3, where e = 3 mm is the slot width) we show that the contributions of a large excursion in the two quadrants of the u' – ρ' plane, corresponding to the two events of the fluid movement, the ejection and sweeping, are very intermittent owing to the fact that they are associated only to a very low number of measurement points. The ejections at y+ > 50 are more significant and much more energetic in the case of helium.