As you can see, we have merely erased
the cells that stand for the wall, since it is a lot easier than to define
them as "wall".
So, the walls in the domain do appears like holes.
Now, we can show the boundaries we have set. This is a difficult step with StarCD and we have managed to get the correct boundaries after a large number of tries.
Here they are
You can see that the entire upper surface is defined as
an inlet (in red), whereas the outlet (green colored) is defined only on
a small area.
Once again, we precise that all views are in 3D, but the study is a 2D one. The third dimension is only added by StarCD, which requires a volume.
Another thing interesting point is that we have finally set very simple boundaries, whereas we began with complicated ones. As a matter of fact, StarCD is very sensitive to the definition of the mesh, and it is then very easy to make an error. So, simplicity is the key word!
Typically, the velocity we put at
the inlet is equal to 1m/s.
The phase that will transport the particles is air, and the particles properties will be defined afterwards.
We also use a k\Epsilon model for the turbulence. There would be a lot of things to say about this particular point, but this is not the question here, so we just click to enable this model and then we launch the calculations.
This profiles are quite satisfying. We can observe the
recirculating zone where we were waiting for them. The flow is well materialized
and follows the shape of our filter.
Furthermore, the option "smooth" enables to shade the drawing thus giving this nice pictures!
Concerning the values, considering the conservation of the flow, it seems to be correct. The accelerating fluid in the smallest section is due to this conservation.
Now, we can draw the velocity vectors. We draw a general overview and we zoom in the interesting zones.
- general overview -
- what happens near the second and the third wall -
- velocity vectors at the outlet of the domain -
With those picture, we can see the recirculating phenomenon
very well. This is mainly in the dark zones (the velocity is very low),
that the particles will probably aggregate.
The following section is the interesting one: we will put particles in our flow and we will try to follow them.
- the initial positions of the particles -
- no initial velocity and a small velocity -
We set two different initial velocities for the particles.
On the left, the particles are injected with no initial velocity, whereas ,on the right, the velocity is taken equal to 10m/s for both the U and the V components.
As you can see, the difference is very slight at the beginning. But when looking the first wall, you can see that the trajectories are closer from the second obstacle.
Now, let us see if a bigger particle will be more sensitive to a change in the initial velocity.
Obviously, the particles are now heavy enough to be self
piloted. I mean that they are no more transported by the flow, they follow
their own way. Their inertia is strong enough.
If the first picture seems to be useless, on the other hand, the second one shows that, this time, the initial velocity conditions have a real effect on the particles. They behave as if the air flow merely did not exist!
The next try is to find a diameter, where the particles will be affected by the flow, even if the initial velocity as a visible effect.
- effect of initial velocity -
You can see that the initial tendency of the particles
is to follow the original motion that we gave them, and then, given that
they are not heavy enough, the air flow take the advantage, and they end
up following the air motion.
- stream lines -