Effect of mesh grids on the turbulent mixing layer of an axisymmetric jet

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.This paper focuses on the effect that two different mesh grids have on the structure of the mixing layer of an axisymmetric jet. Detailed measurements of mean velocity and turbulent velocity fluctuations are made with an X hot-wire probe in the range 0.5 ≤ x/d ≤ 10, where x is the longitudinal distance from the nozzle exit plane and d is the nozzle diameter. The grids are introduced just downstream of the nozzle exit plane: one completely covers the nozzle (full mesh or FM), the other covers the central, high speed zone (disk mesh or DM). With reference to the undisturbed jet, FM yields a significant reduction in the turbulence intensity and width of the shear layer whereas DM enhances the turbulence intensity and increases the width of the shear layer. Both grids suppress the formation of the Kelvin-Helmholtz instability in the mixing layer. Results are presented, mainly at x/d = 5, both in the spectral domain and in physical space. In the latter context, second and third-order structure functions associated with u (the longitudinal velocity fluctuation) and v (the lateral or radial velocity fluctuation) are presented. All mesh geometries have a more significant effect on the second-order structure function of u than on that of v. The third-order energy transfer term is affected in such a way that, relative to the undisturbed jet, its peak location is shifted to a smaller scale with FM is used and to a larger scale with DM. This is consistent with our observations that FM reduces the turbulence in the shear layer whilst DM enhances it. It is suggested that the large scale vortices that are formed at the edge of the grids play a significant role in the transfer of energy.