Abstracts - KTH Mechanics

Simulation and evaluation of mixing in a plane compressible
wall jet
Daniel Ahlman ∗ , Geert Brethouwer ∗ , and Arne V. Johansson ∗
A plane wall jet is obtained by injecting fluid along a solid wall in such a way that the
velocity of the jet supersedes that of the ambient flow. The structure of a developed
turbulent wall jet can formally be described as two adjacent shear layers of different
character. The inner layer, reaching from the wall up to the maximum mean streamwise
velocity, resembles a thin boundary layer, while the outer part, positioned above
the first layer and reaching out to the ambient fluid, can be characterized as a free
shear flow. A consequence of this twofold nature is that properties such as mixing and
momentum transfer exhibit distinctively different character in the two shear layers.
An increased understanding of the mixing properties close to a wall is of prime
importance in connection to combustion. In all combustion applications some part of
the mixing and reaction will take place close to a wall, with properties distinctively
different from those further out in the mixture. How the mixing processes are affected
by the proximity to the wall, and to what extent these are captured by present
combustion models, is currently not fully understood, and it is of interest to add to
this knowledge.
In the present study, we analyze the mixing processes in a plane compressible
and turbulent wall jet, by means of three-dimensional direct numerical simulations.
The wall jet mixing is characterized by the evolution of a conserved passive scalar,
introduced in the jet in a non-premixed manner. The simulations are aimed at both
investigating the dynamics of the plane wall jet and to provide mixing statistics useful
for evaluation and model development. The developing wall jet provides an interesting
test case since it contains inhomogeneous mixing.
The simulations are performed, using a compressible code based on a compact
finite difference scheme of sixth order for the spatial discretization and a third order,
low-storage, Runge-Kutta scheme for the temporal integration. The domain used
in the simulations is rectangular with a no-slip wall positioned at the bottom. The
physical domain size, in terms of the jet inlet height h, isLx/h = 47, Ly/h =18and
Lz/h =9.6 in the streamwise, wall normal and spanwise directions respectively. The
domain is discretized using 384 × 192 × 128 nodes. The inlet of the jet is positioned
directly at the wall with the flow directed along it. The Reynolds number based on
the inlet height and velocity is 2000. Above the jet inlet a coflow of 10% of the jet
inlet velocity is applied. The flow field is compressible with a moderate inlet Mach
number of 0.5. The dynamic structure of the wall jet is investigated by averaged
profiles, velocity fluctuations and Reynolds stresses. Comparisons are made with the
experimental data of Eriksson et al. 1 and the LES data of Dejoan and Leschziner 2 .
The mixing processes are investigated through scalar fluxes, turbulent Schmidt numbers
and probability density functions of the passive scalar. Further statistics relating
to combustion modeling, such as the scalar dissipation rate, are also evaluated.
∗ KTH Mechanics OB 18, SE-100 44 Stockholm, Sweden.
1 Eriksson et al., Experiments in Fluids 25, 50 (1998).
2 Dejoan and Leschziner, Phys. Fluids 17, 025102 (2005).
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