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

16 S. Mendez and F. Nicoud
ROWS
1 3 5 7 9 11
a
12
10
b
8
y/d
6
4
2
P b
P a
P c
0.4 0.6 0.8 1.0
< U > /V j
Figure 10. Experimental measurements: (a): perforated zone of the LARA plate with the
measurements locations (+ ,⋄ , • , ), (b): time-averaged streamwise velocity profiles evolution
in the injection region: + : fifth row, ⋄ : seventh row, • : ninth row, : eleventh row. Profiles are
measured 3 diameters downstream of each row, on the centreline plane.
located as in the experiment (figure 4c) and the RMS peak is well reproduced (figure 5c)
in both shape and level. At the same time, significant differences can be found in the film
core region (y > 2 d) where the numerical streamwise mean velocity is systematically
larger than the experimental values. A closer investigation of the experimental data
base supports the idea that this difference is mainly due to the difference between the
configurations that are studied: recall that simulations characterise the flow around an
infinite perforated plate while measurements correspond to the ninth row of a spatially
evolving flow. From figure 10, which displays the mean streamwise velocity profiles at
several locations along the perforated plate, the velocity of the film core (above the jet)
tends to increase with the number of upstream rows. Measurements are performed 3
diameters downstream of the centre of the hole located in the middle of each row, far
from the lateral walls (see figure 10a). Velocity profiles show the formation of a film
created by effusion through the plate. The jets interact together to form a film that
develops in the ‘hot’ side, modifying the primary flow in the neighborhood of the plate.
Time-averaged streamwise velocity profiles (figure 10b) are characterised by three peaks:
the first one (P a ), next to the wall, represents the jet core (y/d ≈ 1). The second peak
(y/d ≈ 3) represents the film core (P b ), which results from the interaction of all the
upstream jets with the main flow. The presence of a secondary velocity peak, located
below the jet core and due to the entrainment process, can also be observed (P c ) at
y/d ≈ 0.5.
An important feature visible from figure 10 is that the peaks behave differently: the
peak related to the jet just upstream does not change a lot from one row to the other,
whereas the peaks corresponding to the film core and the entrainment process are highly
influenced by the number of rows upstream of the measurement location. Since P a is
related to the jet just upstream and the flow rate is roughly uniform along the plate, this
peak does not depend on the position over the plate. On the contrary, P b and P c are
directly related to the velocity of the main flow just upstream the hole and thus their
amplitude depend on the number of row upstream, viz. on the position over the plate. It
is also obvious from figure 10 that the experimental results are not established at row 9:
the jet core is rapidly established, but peaks P b and P c continue to evolve. Now, since in