these simulation numerique et modelisation de l'ecoulement autour ...

A. Analysis of the small-scale reference LES results
Small-scale simulations were performed in order to learn more about the fine structure of the
flow around and inside a perforated plate in an isothermal configuration corresponding to the
LARA experiment. A complete description of the methodology can be found in Mendez et
al. 30 In FCFC experiments, the flow is known to be different depending on the number of
rows of perforations considered. This is very difficult to handle from a modeling point of
view. Furthermore, as already said, this dependance on the row number is not obvious to
transpose in full burners. It has been decided to deal with this difficulty by considering the
case where the perforated plate is infinite. The computational domain can then be reduced
to a small box containing only one perforation, using periodic boundary conditions in order
to reproduce the periodicity of the staggered pattern (see Fig. 3). This is also consistent
with the construction of a local model.
CALCULATION
DOMAIN
a
b
PRIMARY FLOW
SECONDARY FLOW
INFINITE
PERFORATED
WALL
24 d
2 d
10 d
y
x
z
11.68 d 6.74 d
Figure 3. From the infinite plate to the ‘bi-periodic’ calculation domain. (a): Geometry
of the infinite perforated wall. (b): Calculation domain centered on a perforation; the bold
arrows correspond to the periodic directions. The dimensions of the computational domain
are provided.
The calculation grid contains 1,500,000 tetrahedral cells: fifteen points describe the diameter
of the hole and on the average the first off-wall point is located 5 wall units apart
from the wall. Typically the cells along the wall to be cooled and in the hole are sized to a
height of 0.3mm (recall that the aperture diameter is 5 mm).
As natural mechanisms that normally drive the flow in a non-periodic domain are absent
in such a configuration, an appropriate method has to be set up to generate and sustain the
flow: the main tangential flow at both sides of the plate is enforced by a constant source
term added to the momentum equation, as is usually done in channel flow simulations. The
effusion flow through the hole is sustained by imposing a uniform vertical mass flow rate
at the bottom boundary of the domain. The source terms and the uniform vertical mass
flow rate were tuned in order to reproduce the operating conditions given in section IIB. A
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