## Fluids

#### Absorptivity and Emissivity in Fluent

• Jiezougt
Subscriber

Hello All,

In fluent, I am trying to model asphalt on top of concrete under radiation, conduction, and convection. Below are the questions:

1. There is a Absorption Coefficient [m-1] in the material definition.
2. There is a Absorptivity under Boundary Conditions/Wall/Any wall window. There is a section call "Absorptivity" for both Direct Visible and Direct IR.

Which one is the place to set absorptivity of concrete (absorption coefficient of concrete = 0.65)? Why there are two different locations for the potentially same parameter?

Emissivity of concrete (0.88) should be set under Boundary Conditions/Wall/Any wall window, under "Internal Emissivity". Is this correct?

Thanks!

Brandon

• DrAmine
Ansys Employee
Absorption Coefficient is related to the medium participating in radiation and refers to the amount of radiation being absorbed over certain length (second term of the RTE equation on the LHS). Absorptivity is surface related phenomenon. If radiation hits an opaque wall a portion is is absorbed and part is reflected (1-emissivity). When a ray travel through a medium it gets absorbed according to the absorption coefficient so the intensity is reduced.

I think what you are referring to it to be provided under Radiation Tab for the walls.
The internal emissivity is used to compute the radiative exchange in the domain whereas external emissivity refers to radiation exchange with the exterior and it is there even if you do not activate any radiation model. -> Internal property.

• Jiezougt
Subscriber
I would love to explain the whole model information.
A concrete box girder bridge with concrete deck.
Top surface of the concrete deck to be modeled as asphalt (through changing the parameters under Radiation Tab for the Walls)
All walls are opaque. No transparent walls.
Please see attachments: Overall model, Absorption Coefficient, Absorptivity, and Internal Emissivity.