Radiation in Flow Simulation - Part 2 : Refraction

Radiation in Flow Simulation – Part 2: Refraction

In my earlier blog on Radiation in Flow Simulation, I showed how Flow accurately ‘ray traces’ solar radiation and how surfaces can be set as ‘Symmetry’ Radiative Surfaces that perfectly reflect the radiation. I used a parabolic solar trough to illustrate this.

That work led me to wonder whether Flow could refract solar radiation through a transparent medium. Could it bend the radiation waves? If so, Flow can be used for all manner of radiation / heating / lighting applications.

I decided to test a magnifying glass and see if a convex lens would concentrate the sun’s solar radiation at a spot – like I used to do when a boy, setting fire to newspaper. If so, it would prove that the ray tracing bends the light as it passes through the glass. Here is my magnifying glass.

I have positioned it above a wooden surface and applied a transparent appearance to the lens body. The lens has a radius of 150 mm and I have positioned it such that the wooden floor is exactly 150 beneath the centre of the lens – i.e. at the focal point.


The set up in Flow is an External study with Solar Radiation and Gravity enabled. I used the ‘Direction and Intensity’ option for defining the solar values and, for convenience, set the sun to shine straight down the Y axis. The ‘Gravity’ option enables natural convection which will cool the surfaces by drawing in cold air as warm air rises from an expected hot spot.



There are some further Radiation options you can enable in the ‘Calculation Control Options’. Below you can see that I have set the ‘Solar ray tracing’ to ‘Forward’ meaning the rays start at the sun and are traced through the lens to the floor surface. You can also define the ‘Number of rays’ and the ‘View factor resolution level’. These will give more refined rays but take longer to run. I used the default values.



The results show a hot spot at the focal point. The surface of the wood has reached 368 degrees C.


The animation below shows how the hot spot creates a vertical current of warm air which in turn draws in cool air.

However, for thermal studies where radiation reflects or refracts solar or other radiation, Flow is an exceptional, easy-to-use tool that gives accurate real-life results.  

By Andy Fulcher
Technical Manager
Solid Solutions Management Ltd