Implementation of DO Radiation in Ansys Fluent
I have been trying to implement the DO Radiation Model to simulate a radiation beam in Ansys Fluent since the past 1 month without much success. The computational domain consists of a cylinder of 10 cm radius and 1 m length and the front section of the cylinder is specified as irradiated wherein incident radiation enters the domain. Air and dust particles are present inside the cylinder.
I have set the absorptivity of air to zero in the simulation, and the theta and phi extent corresponding to the beam width is set to 1e06. I have also set the theta and phi pixels and divisions equal to 15. I am expecting the incident radiation to be constant intensity over the entire domain, however, I what I am observing is that the intensity of incident radiation decreases with the axial length, and finally attains a constant value after a certain distance.
I would highly appreciate your support in solving the issue. Please let me know in case any additional information is required.
Best Answers

Kremella Admin
Hello,
It seems to me that you might not be directing the radiation beam correctly. Please note that the final direction of your beam is dependent on your theta and phi discretization.
The 'Beam Direction' is defined as the vector for the centroid of the solid angle (beam width). This is not the same as your catesian axis of the cylinder.
I'd strongly recommend running simple tests to see how changing the theta and phi discretizations as well as the beam direction changes the overall intensity of your radiation beam.
I hope this helps.
Thanks.
Karthik

Kremella Admin
That is what I was trying to highlight. It is not the same as the cartesian direction vector of your axis of your cylinder. The 'Beam Direction' is defined as the vector for the centroid of the solid angle (beam width), which is based on the theta  phi discretization of the DO model.
Thanks.
Karthik

subhamdas Member
Hi @Kremella Tried different combinations of the beam direction, but that also didn't help me to achieve a collimated radiation beam. What I have observed is that the beam remains aligned to the cylinder when I set the beam direction as (1 0 0) however, for other values of the beam direction it is some what tilted from the axis.
I had also suspected this because the problem was not with the direction but as I mentioned earlier, the intensity of incident radiation decreases with the axial length. The figure below somewhat represents the same. I want the entire portion to be red, but it is not the case here.
Answers
Hello,
It seems to me that you might not be directing the radiation beam correctly. Please note that the final direction of your beam is dependent on your theta and phi discretization.
The 'Beam Direction' is defined as the vector for the centroid of the solid angle (beam width). This is not the same as your catesian axis of the cylinder.
I'd strongly recommend running simple tests to see how changing the theta and phi discretizations as well as the beam direction changes the overall intensity of your radiation beam.
I hope this helps.
Thanks.
Karthik
@Kremella Thanks a lot for your response. The actual configuration is quite complex and hence I have taken the cylinder to study the effect of various parameters on the incident radiation. So far I've tried the following:
Keeping other parameters constant I've changed:
Also, in the fluent manual, it is mentioned that the parameters for beam width should be ~1e06 to acheive a collimated radiation beam, however, I am not getting the same even after increasing the discretisation parameters to 75(which in highly intensive in terms of computational cost).
You should be changing Beam Direction, not Beam width.
And, no need to go to 75. Keep 5 to 7 divisions should be sufficient.
Karthik
Thanks but is than not supposed to be the coinciding with the axis of the cylinder(1 0 0). And if its not, then how to estimate the proper beam direction?
That is what I was trying to highlight. It is not the same as the cartesian direction vector of your axis of your cylinder. The 'Beam Direction' is defined as the vector for the centroid of the solid angle (beam width), which is based on the theta  phi discretization of the DO model.
Thanks.
Karthik
Hi @Kremella Tried different combinations of the beam direction, but that also didn't help me to achieve a collimated radiation beam. What I have observed is that the beam remains aligned to the cylinder when I set the beam direction as (1 0 0) however, for other values of the beam direction it is some what tilted from the axis.
I had also suspected this because the problem was not with the direction but as I mentioned earlier, the intensity of incident radiation decreases with the axial length. The figure below somewhat represents the same. I want the entire portion to be red, but it is not the case here.
Hello,
What does the Incident Radiation contour look on the opposite face of your cylinder?
Thanks.
Karthik
I hope this is what you are asking for:
As you can see from the figure, the intensity decreases with increasing axial length.