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Average Nusset number and Standard Deviation Calculation
Posted 7 set 2012, 15:10 GMT-4 3 Replies
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Please help me How to calculate Average Nusselt number over the heated wall and standard deviation of the temperature over the whole surface at different Ra values in COMSOL?
(I am cosidering a square cavity and trying to validate benchmark buoyancy flow posed by de Vahl Davis).
Thanks
(I am cosidering a square cavity and trying to validate benchmark buoyancy flow posed by de Vahl Davis).
Thanks
3 Replies Last Post 10 set 2012, 18:06 GMT-4
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Posted:
1 decade ago
Hi
well I would say write out the equation (but I believe COMSOl has already defined a Nusselt number, no ? and integrate over the desired area, then divide by the area to get the local average.
If you know the area of interest in advance, you can split up your domain and then select the desired domain and average it over this domain.
Ifyou do not split your domain, you must use bolean operatrs to limit the integratin area to the desired sub-part of the domain. But then DO NOTuse the build in average operator, as it will normalise over the FULL selected domain area, you need to write explicitely out the integration average:
average = int_overMyDomains(my equation*(X>X0)*(X<X1)*(Y>Y0)*(Y<Y1)) / int_over...(1*(X>X0)*(X<X1)*(Y>Y0)*(Y<Y1))
Hope you understand my explnations. try it out on a simple case (rectangle in 2D with initial conditions) you can estimate by hand calculations, when you are convinced, apply it to your model.
--
Good luck
Ivar
well I would say write out the equation (but I believe COMSOl has already defined a Nusselt number, no ? and integrate over the desired area, then divide by the area to get the local average.
If you know the area of interest in advance, you can split up your domain and then select the desired domain and average it over this domain.
Ifyou do not split your domain, you must use bolean operatrs to limit the integratin area to the desired sub-part of the domain. But then DO NOTuse the build in average operator, as it will normalise over the FULL selected domain area, you need to write explicitely out the integration average:
average = int_overMyDomains(my equation*(X>X0)*(X<X1)*(Y>Y0)*(Y<Y1)) / int_over...(1*(X>X0)*(X<X1)*(Y>Y0)*(Y<Y1))
Hope you understand my explnations. try it out on a simple case (rectangle in 2D with initial conditions) you can estimate by hand calculations, when you are convinced, apply it to your model.
--
Good luck
Ivar
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Posted:
1 decade ago
Hi
well I would say write out the equation (but I believe COMSOl has already defined a Nusselt number, no?) and integrate over the desired area, then divide by the area to get the local average.
If you know the area of interest in advance, you can split up your domain and then select the desired domain and average it over this domain.
--
Good luck
Ivar
Dear Ivar,
I did not find where they defined Nusselt number in COMSOL. Could you please elaborate how to do it and which equation do I need to use? My geometry is simple 2D square cavity of area 1m^2.
Thanks!
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Posted:
1 decade ago
Hi
you are right, I got confused about the "numbers" you have the Cell Rynolds number and Cell Pecklet number but not the Nusselt number predefined (I could not find it back) but as normally Nusselt is "ht*L/k" with
ht is the heat transfer coefficient (SI unit: W/(m2·K)).
L is the characteristic length (SI unit: m).
k is the thermal conductivity of the fluid (SI unit: W/(m·K)).
as L is a "characterisitic length", COMSOL uses "h" the average element length for the characterisitc length, hence it has the "cell" Reynolds and Prandtl number defined, in this way you can define the "cell" (mesh dependent) Nusselt numbe toor.
Or from you model you can define a "L" as a constant and then define your own Nusselt number variable depening on the other model values
--
Good luck
Ivar
you are right, I got confused about the "numbers" you have the Cell Rynolds number and Cell Pecklet number but not the Nusselt number predefined (I could not find it back) but as normally Nusselt is "ht*L/k" with
ht is the heat transfer coefficient (SI unit: W/(m2·K)).
L is the characteristic length (SI unit: m).
k is the thermal conductivity of the fluid (SI unit: W/(m·K)).
as L is a "characterisitic length", COMSOL uses "h" the average element length for the characterisitc length, hence it has the "cell" Reynolds and Prandtl number defined, in this way you can define the "cell" (mesh dependent) Nusselt numbe toor.
Or from you model you can define a "L" as a constant and then define your own Nusselt number variable depening on the other model values
--
Good luck
Ivar