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

24 S. Mendez and F. Nicoud
Figure 16. Isosurface of Q criterion (5 V j 2 /d 2 ) from Run C showing the instantaneous flow
structure. Top: top view. Bottom: side view.
fields. Note that they appear beneath the location where the vortices composing the CVP
get closer from each other, at approximately x = 3.5 d.
Instantaneous fields of Q-criterion offer a very different picture of the vortical flow
structure. Instantaneous structures are much intense, as shown in figure 16, where an
instantaneous iso-surface of Q-criterion at 5 V j 2 /d 2 is shown. Again, the solid wall is
partly transparent to allow observation of the in-hole behaviour. The instantaneous field
is dominated by shear layer vortices formed at the upstream face of the jet and also
inside the aperture, at the high shear zone that separates the low-momentum and the
jetting regions. These in-hole vortices have an hairpin form with two legs located in the
low-momentum region, in the direction of the hole. The counter-rotating vortex pair
observed within the hole in the average field actually results from the averaging of series
of hairpin vortices: they form a unique structure in an instantaneous visualisation. The
hairpin vortices seem to result from the roll-up of the vorticity located at the sharp edge
of the hole entrance. Outside the hole, in addition to the shear layer vortices, another
type of vortices are detected. They are located close to the wall and have a streamwise