# What number do i have to type about boussinesq

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 October 10, 2018, 12:53 What number do i have to type about boussinesq #1 New Member   HakMin Lee Join Date: Oct 2018 Posts: 3 Rep Power: 7 Hi i'm student in south korea. i'm sorry i can't enlglish very well. First question is that i want to use a boussinesq in density. but i don't know what number do i have to type in material density boussnisq. and what is operating density in operating condition? is it the density in boussnisq apporximation? So, I type the number density under the operating tempeature condition, is that right? where is typing the enumber in material boussinsq menu or operating density in operating condition? then what number do i have to type operating temperature? And I know about that thermal expansion coefficient is the reciprocal of temperature. what number do i have to type temperature in condition for reciptrocal?

 October 10, 2018, 13:55 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,683 Rep Power: 66 Boussinesq approximation is still a constant density assumption. You need to specify the constant density and the thermal expansion coefficient. The thermal expansion coefficient is a general material property. In general, it's not always 1/T. That's why you have to also specify it. You say you know it is 1/T, but you are actually using a model in the same way that the ideal gas equations of state is simply a model for how a gas behaves (real materials don't need to follow any human-made rules). The buoyancy term is relative to a specified reference temperature. This reference temperature is the one that you specify as the operating temperature in operating conditions. In most cases, this is the same temperature that you used to calculate the density and thermal expansion coefficient at in the material properties. If you use the Boussinesq approximation, you normally do not want to specify the operating density. Do not check this box. Last edited by LuckyTran; October 10, 2018 at 16:16.

October 10, 2018, 23:51
#3
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HakMin Lee
Join Date: Oct 2018
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Quote:
 Originally Posted by LuckyTran Boussinesq approximation is still a constant density assumption. You need to specify the constant density and the thermal expansion coefficient. The thermal expansion coefficient is a general material property. In general, it's not always 1/T. That's why you have to also specify it. You say you know it is 1/T, but you are actually using a model in the same way that the ideal gas equations of state is simply a model for how a gas behaves (real materials don't need to follow any human-made rules). The buoyancy term is relative to a specified reference temperature. This reference temperature is the one that you specify as the operating temperature in operating conditions. In most cases, this is the same temperature that you used to calculate the density and thermal expansion coefficient at in the material properties. If you use the Boussinesq approximation, you normally do not want to specify the operating density. Do not check this box.

Thanks your reply.

I know about that how can i select the number in density due to your reply.

But I don't know exactly about thermal-expansion-coefficient what number i have to type.

If) The operating temperature 300K -> density(boussinesq) value to be include under the same conditions(300K).. Is it right?

ex) air density is 1.177m/s^2 in 300K

Then what is thermal-expansion-coefficient value? Is not 1/300K?

October 11, 2018, 13:03
#4
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Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
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If you say you have air nominally at 300K w/ a density of 1.177 then by all means input 1.177 in material properties and 300K for the operating temperature.

Quote:
 Originally Posted by mhl0706 Then what is thermal-expansion-coefficient value? Is not 1/300K?
Thermal expansion coefficient is a material property. It's 1/T for ideal gases. Imagine you have a block of solid plutonium. The thermal coefficient of solids is very different than gases. Do you want to use the real thermal expansion coefficient for air at 300K? Then you need to look up what this is from a property database. You can use 1/300K if you like. But the reason you have to input a number is because, you need to input a number. Also note that by default in Fluent you input a constant value, even though you know it should at least be temperature dependent for ideal gases (it's 1/T).

When you set up any model you make a million assumptions. I hope you notice and are aware that you are choosing to model your problem a certain way.

October 11, 2018, 23:15
#5
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HakMin Lee
Join Date: Oct 2018
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Quote:
 Originally Posted by LuckyTran If you say you have air nominally at 300K w/ a density of 1.177 then by all means input 1.177 in material properties and 300K for the operating temperature. Thermal expansion coefficient is a material property. It's 1/T for ideal gases. Imagine you have a block of solid plutonium. The thermal coefficient of solids is very different than gases. Do you want to use the real thermal expansion coefficient for air at 300K? Then you need to look up what this is from a property database. You can use 1/300K if you like. But the reason you have to input a number is because, you need to input a number. Also note that by default in Fluent you input a constant value, even though you know it should at least be temperature dependent for ideal gases (it's 1/T). When you set up any model you make a million assumptions. I hope you notice and are aware that you are choosing to model your problem a certain way.

Thank you for your reply again.

The experiment condition is natural convection of air and ideal gas.

So I just adopt 1/300K for thermal expansion coefficient because operating temperature is 300K.

Thanks a lot!

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