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

18 S. Mendez and F. Nicoud
Figure 11. Time-averaged quantities from Run C on the centreline plane (zoom over the hole
region) (a): Contours and isolines of the time-averaged pressure. (b): Contours and isolines
of the time-averaged velocity magnitude |V |. White arrows show the flow direction in both
channels.
is observed just downstream of the jet, near the wall (3). This separation is known to
appear for relatively high blowing ratios and is responsible for a key feature of this type
of flow, the entrainment phenomenon, visualised in § 4.4 and 4.5.
Further in the primary main stream, the jet loses its strength by mixing with the main
flow (4). Note that due to the periodic configuration, the jet that goes out of the domain
reenters by the other side (5). Figure 11 strongly suggests that the shape of the micro-jet
(1–4) is influenced by the aspiration side and that computing only the injection side
would be questionable.
4.2. Flow on the suction side
Figure 12 presents the structure of the flow on the suction side by displaying, in a horizontal
plane located 0.5 d under the suction wall, contours and isolines of the three components
of the time-averaged velocity in figures 12(a–c) and contours of the Q criterion
(Hunt, Wray & Moin 1988) calculated from the time-averaged velocity in figure 12(d).
The acceleration of fluid entering the hole can be seen in figure 12(b), on the timeaveraged
vertical velocity field, which is very inhomogeneous. The spatial-averaged vertical
velocity over the horizontal plane is 0.02 V j but locally in the cutting plane, it reaches
0.3 V j . Note also that the maximum of the vertical velocity is not centred under the
hole inlet but is located downstream of the centre. This can be related to the pressure
gradients observed in figure 11(a): the maximum pressure variations are observed at the
downstream edge of the hole inlet.
The suction through the hole influences the three components of the velocity: figure
12(a) shows its effect on the streamwise velocity: under the upstream edge of the
hole, the aspiration induces a small acceleration and under the downstream edge of the
hole, a deceleration. Near the plate, negative values of the streamwise velocity are even
observed, showing that the fluid turns back to enter the hole. The aspiration makes the
fluid come from all sides of the hole, as observed on the time-averaged spanwise velocity
field (figure 12c), which shows how the fluid comes from lateral sides. The streamwise
velocity field also shows the presence of two bands of low velocity on each side of the
hole. The lateral aspiration visualised in figure 12(c) creates a velocity deficit on both
sides of the hole. Figure 12(d) presents isocontours of Q criterion (Hunt et al. 1988).