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## Simulation Mechanical and Multiphysics

Active Member
Posts: 10
Registered: ‎01-03-2012

# Change the reflectivity of a material

122 Views, 4 Replies
03-12-2012 02:59 AM

Hi,

I want to change the reflectivity of a material? At the moment all I can change is the emissivity which isn't the same...

Any help?

Thank you,

Emily

Employee
Posts: 259
Registered: ‎06-14-2010

# Re: Change the reflectivity of a material

03-12-2012 02:28 PM in reply to: Emily.I

Hi, Emily,

In general, the interplay of energy exchange by thermal radiation is characterized by the following equation, and has relation as below,

Absorptivity + Reflectivity + Transmissibility=1                ----------------   (1)

For opaque surface (ASIM/MP thermal radiation has this assumption),   Transmissibility=0

so   Absorptivity + Reflectivity =1                              ----------------   (2)

Using Kirchhoff's law states that in thermal equilibrium,

Emissivity = Absorptivity                                         ----------------   (3)

Combing (2) and (3), gets

﻿﻿﻿Emissivity  + Reflectivity =1

So here you can see the dependency between surface Emissivity and Reflectivity.

Two extrame examples:

- Black body (  perfect black body, Emissivity=1,  Reflectivity=0)

- White body ( Snow is close white body,  Reflectivity=0.90  at visible light,  Emissivity is small)

Hope the this helps.

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Active Member
Posts: 10
Registered: ‎01-03-2012

# Re: Change the reflectivity of a material

03-13-2012 04:33 AM in reply to: Joey.X

Well, what I really want to change is the albedo...

Employee
Posts: 259
Registered: ‎06-14-2010

# Re: Change the reflectivity of a material

03-13-2012 06:25 AM in reply to: Emily.I

Albedo could be converted to Reflectivity depending on what kind of Albedo and what the material properties.

Beware the cases of water, glass or snow on the surface, the transmissibility is not trivial, that violates the ASIM/MP's assumption of opaque surface.

﻿

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Employee
Posts: 492
Registered: ‎03-25-2010

# Re: Change the reflectivity of a material

03-13-2012 06:26 AM in reply to: Emily.I

Hi Emily,

Perhaps you can explain what you want to do, and someone can suggest a way to do it. Other than mentioning emissivity, which implies radiation which implies heat transfer, we do not know anything about your simulation.

P.S. What's the difference between albedo and reflectivity? In my circle of friends (astronomy), albedo is the amount of reflected light. If you exaggerate light to mean all forms of radiation, then wouldn't albedo and reflectivity be the same?

Sincerely,
John Holtz, P.E.
Senior User Experience Designer, Simulation
Autodesk, Inc.

Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)