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Contributor
Posts: 14
Registered: ‎03-13-2013

Fixed or Variable Environment Settings

61 Views, 4 Replies
05-30-2013 01:27 AM

Hello everybody,

 

I have a question to environment settings. I have created my own material (hydraulic oil) with partly linearly viscosity as a function. Now i simulated a hydraulic valve and got strange results with high Delta P. Then i noticed, that my settings on Materials (for the temperature i've set 50°C  like we have it on the test bench) were set as "variable".

Than i have switched to "fixed" and ran the simulation again. The Result was the 50% smaller Delta P and it is closer to our test bench results.

Now is the question when do i use for a static simulation "fixed" and "variable" settings. I have read in the FAQ that "variable" is recommended for natural circulation.. but this is not enough information for me, i want to describe this difference in my thesis- simply what are the consequences of using this two different settings. 

Appreciate youre answers!

 

best regards

 

CFAsmo

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apolo.vanderberg
Posts: 263
Registered: ‎08-31-2011

Re: Fixed or Variable Environment Settings

05-30-2013 05:37 AM in reply to: CFAsmo

For materials that have properties that vary with respect to Temperature:

 

Fixed: holds the properties constant (fixed) at the Scenario/Material Environment Temperature

 

Variable: properties will initialize based on the Scenario/Material Environment Temperature and then are allowed to change (vary) in the domain based on the thermal solution - which is why this is required for Natural Convection or Compressible models as the density change has a direct impact on the flow solution

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Contributor
Posts: 14
Registered: ‎03-13-2013

Re: Fixed or Variable Environment Settings

05-30-2013 06:13 AM in reply to: apolo.vanderberg

Thanks for reply, so covered on my case could be that "variable" settings allowing the oil to cool down during the run through the valve and so increasing the viscosity+Delta P ? Or what could be the reason that "fix" settings deliver less pressure loss.. I'm a little bit confused why this is having such an impact on my incompressible static flow?

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apolo.vanderberg
Posts: 263
Registered: ‎08-31-2011

Re: Fixed or Variable Environment Settings

05-30-2013 06:16 AM in reply to: CFAsmo

Yes,

Depending on your setup of the model Variable would allow the viscosity to change with the local temperatures in the model such that the viscosity can change in the domain.

 

As fixed it will be 1 viscosity for the entire domain regardless of the thermal solution. It would be held constant at the value dictated by the Environment Temperature

If you did the Variable run as a separate scenario, you should be able to look at the viscosity results through the domain and see how/where it changed and compare it to the Fixed run that is constant

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Contributor
Posts: 14
Registered: ‎03-13-2013

Re: Fixed or Variable Environment Settings

05-30-2013 06:43 AM in reply to: apolo.vanderberg

Okay so now i think what is more plausible.. the variable settings should be, because the temperature is increasing on recirculation and separations spots, so viscosity goes down and pressure loss too. But in my case the fix temperature 50°C on the whole valve way brings better results (less Delta P). 

I supose that has to do with my variable Temperature settings on my Material. I will take a look on viscosity changes to compare the cases tomorrow..

 

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