Understanding Radiation

ChaChaLaLa · June 7, 2013, 09:59

Hi All! I am working on a CFD simulation with radiation implementation. The model I used mostly is the DO model with high angular discretization (say, 25 titah and 25 phi) and fine pixels too (say, 9 x 9). (3D problem with semi-transparent walls)

It took awhile to figure out how to properly set up a radiation problem but I believe I have understood and mastered that

.

Now, my question is to do with the results. In the simulation result, one of the result I have access to is: Incident Radiation with units (W/m2).

http://www.sharcnet.ca/Software/Flue...ug/node984.htm
(31.4-21)

My question is:
Why are the units for Incident Radiation (W/m2) when the equation in the link is clearly just W.

Can I hand calculate the Incident Radiation so I can compare theorectical background with Fluent results? (I have made quite a few tries but none successful

)

Thx for reading.

Rami · June 12, 2013, 05:21

Although your query is specific to Fluent (and better be posted in the Fluent forum), the Eq. (31.4-21) you cite is the explanation: The intensity I integrated over the whole solid angle w results in W/m^2 as the units of the incident radiation, G.

ChaChaLaLa · June 12, 2013, 09:46

Thanks for your reply. I will try to move this discussion over to the appropriate forum as soon as I figure that out. Thanks. Back to the query.

I am unsure of what Fluent considers intensity, I as. From my understanding of intensity; it is defined as power per steradian (solid angle) and doesn't change with distance from source/antenna. Therefore, it should have units of (W/sr) and after integrating with a range of solid angles, should result in simply (W).

I am aware however, there is fragmentation in definitions of intensity in the world of "optics" so I looked up Wikipedia to gain a better understanding.

http://en.wikipedia.org/wiki/Radiation_intensity
http://en.wikipedia.org/wiki/Power_density
http://en.wikipedia.org/wiki/Solid_angle

According to Wikipedia, radiation intensity is the power density per solid angle. This confuses me even more because if so, the units will be (W/m^3) * (m^2) = (W/m??)

I know I am not an expert in this topic so please be patient with me

Thanks for any help and feedback.

Rami · June 16, 2013, 04:51

The intensity, I, is defined (see e.g., Modest or Siegel and Howell or Sparrow books) as the power per area normal to rays per steradian, and its units are therefore W / (m^2 * sr).

June 7, 2013, 09:59	Understanding Radiation	#1
ChaChaLaLa New Member Join Date: Apr 2013 Posts: 8 Rep Power: 12	Hi All! I am working on a CFD simulation with radiation implementation. The model I used mostly is the DO model with high angular discretization (say, 25 titah and 25 phi) and fine pixels too (say, 9 x 9). (3D problem with semi-transparent walls) It took awhile to figure out how to properly set up a radiation problem but I believe I have understood and mastered that . Now, my question is to do with the results. In the simulation result, one of the result I have access to is: Incident Radiation with units (W/m2). http://www.sharcnet.ca/Software/Flue...ug/node984.htm (31.4-21) My question is: Why are the units for Incident Radiation (W/m2) when the equation in the link is clearly just W. Can I hand calculate the Incident Radiation so I can compare theorectical background with Fluent results? (I have made quite a few tries but none successful ) Thx for reading.

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June 12, 2013, 05:21		#2
Rami Senior Member Rami Ben-Zvi Join Date: Mar 2009 Posts: 155 Rep Power: 17	Although your query is specific to Fluent (and better be posted in the Fluent forum), the Eq. (31.4-21) you cite is the explanation: The intensity I integrated over the whole solid angle w results in W/m^2 as the units of the incident radiation, G.

June 12, 2013, 09:46		#3
ChaChaLaLa New Member Join Date: Apr 2013 Posts: 8 Rep Power: 12	Thanks for your reply. I will try to move this discussion over to the appropriate forum as soon as I figure that out. Thanks. Back to the query. I am unsure of what Fluent considers intensity, I as. From my understanding of intensity; it is defined as power per steradian (solid angle) and doesn't change with distance from source/antenna. Therefore, it should have units of (W/sr) and after integrating with a range of solid angles, should result in simply (W). I am aware however, there is fragmentation in definitions of intensity in the world of "optics" so I looked up Wikipedia to gain a better understanding. http://en.wikipedia.org/wiki/Radiation_intensity http://en.wikipedia.org/wiki/Power_density http://en.wikipedia.org/wiki/Solid_angle According to Wikipedia, radiation intensity is the power density per solid angle. This confuses me even more because if so, the units will be (W/m^3) * (m^2) = (W/m??) I know I am not an expert in this topic so please be patient with me Thanks for any help and feedback.

June 16, 2013, 04:51		#4
Rami Senior Member Rami Ben-Zvi Join Date: Mar 2009 Posts: 155 Rep Power: 17	The intensity, I, is defined (see e.g., Modest or Siegel and Howell or Sparrow books) as the power per area normal to rays per steradian, and its units are therefore W / (m^2 * sr).