Statistical analysis of high-order moments in a turbulent boundary layer with large density gradients

An experimental study of the effects of large density variations on the turbulent transfers in a boundary layer is carried out in this paper. Density differences were generated by tangential injection of air or helium in a mixture of these two components. In order to study the fine structure of wall turbulence in the presence of significant density variations, a statistical analysis of the experimental data, obtained in a wind tunnel, is carried out. The results show that the relaxation of the skewness factor of u′(Su′) is carried out more quickly in the external layer than close to the wall, for the air injection as well as for the helium injection. Su′ grows close to the injection slot in an appreciable way, and this increase definitely is stronger for the air injection than for the helium injection. This growth of the skewness factor close to the injection slot can be explained by the increase in the longitudinal convective flux of turbulent energy in this zone. For the distributions of the flatness factor Fu′ the results show that there is no significant effect of the density gradient on the intermittent structure of the instantaneous longitudinal velocity in the developed zone, x/δ≥5, where δ is the boundary-layer thickness. Differences remain, of course, between the profiles of Fu′ for density gradients Open image in new window in the relaxation zone, where the longitudinal gradients are significant. It should, however, be noted that Open image in new window has a smaller effect on the flatness factor compared to the skewness factor. The statistical analysis carried out in this study shows that the helium injection in the boundary layer generates more violent ejections than the air injection. This result is confirmed by the significant contribution of the ejections to turbulent mass flux.