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.
