Hey!!
I need to multiply viscous dissipation/heating term with a factor of 0.4( or say is as efficiency factor) and the resulting term should be further used as effective viscous dissipation term in further numerical calculations in fluent.
I searched a lot but could not find a way to implement this in fluent whereas several research papers that I am referring have implemented this in fluent.
Kindly help me in implementing this in fluent. Help will be highly appreciated.
Thanks
To change the viscous dissipation term, all you would need to do is to change the viscosity value; either by typing in a constant, or modifying it via a UDF.
I am not sure whether you can simply save the value of the dissipation term and use it later on. However, you can surely define it again via a UDF and use it where it is needed.
Dear Kalal
Thanx for the reply.
Surely I can write a UDF for viscous dissipation term and add it to energy. But I don't know, where to hook that UDF. Because for activating viscous heating, I just click that option and it I do not see any option to attach UDF.
If you want to add a term to the energy equation, write a UDF for the energy source and hook it there.
You may consider another approach - the viscous heating is proportional to the effective viscosity, i.e., dynamic + turbulent viscosity. Just modify those. This approach you may use only if it does not mess up the viscosities in the other equations of your model.
Thanx for the prompt reply and suggestion
I am using viscous laminar flow. Changing effective viscosity will affect momentum equation. So I can't use this option
I have defined viscosity as a function of strain rate and temperature (I am simulating FSW process)
Do you wish to say, I should write a UDF and hook up in source term of cell zone conditions and disable the viscous heating option in FLUENT and effectively code the required viscous heating as the volumetric user defined heat source.
Any theory notes in viscous dissipation UDF will be appreciated.
You have laminar flow, the effective viscosity is then equal to the dynamic viscosity (since the turbulent viscosity is zero).
Anyway, if you want to introduce the viscous heating term with an efficiency factor of 0.4, I do not have a better idea right now what to do than to define the whole term as a source term (without turning on the Viscous heating option).
@zbynek
Thanx you very much for the insightful suggestions.... I will approach in that direction
Dear kalal
I have written the udf of source code to calculate the viscous dissipation term. But that udf also require to mention the dS[eqn] (derivative of the equation). Since the formula depends on more than one variable, I am unable to understand how to calculate it.
Screen shot of the equation is shown in the below image.
I never did this so I cannot pass on any personal experience. However, as you may have noticed in the UDF Guide, you can set the dS[eqn] to zero and so the whole term will be treated explicitly. This may or may not have an adverse effect on convergence.
@kalal....
Ya, I noticed about that explicit part, but just wondering how much it will affect the results. As I think FLUENT analysis is an implicit one.
If the terms of the governing equations are discretized in the implicit way, it does not mean that no terms that are expressed explicitly cannot be added. To understand it a bit more, you should read a book about FVM.
As for your case, you will not know whether the explicit form causes any troubles till you try it.
Thanx for the valuable suggestions........
I really appreciate it. Thanx a lot again
Hi,
Did you find the solution to this problem. Could you suggest me something please.
Because, as soon as I include viscous heating under viscous laminar model, the temperature is ridiculously increasing, which is not true and change should not be more than 5%.
Thanks in advance
Buchibabu
You need to multiply that viscous heating equation with a small constant like 0.05 (as reported in the literature) by using UDF.
Thank you so much for your swift response.
With best regards
Buchibabu
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