CFX Boundary conditions

vitulaaak · October 23, 2012, 06:32

Hello,

I am modeling compressor in CFX.To prevent from switching boundary conditions, i tried to use opening on the outlet with pressure loss ( i would use outlet, but there is no such possibility to add pressure loss to this type of boundary).

when I use this approach, it sometime fall on overflow. To prevent from doing this I would like to do either:

1. specify different initialization file than current in workbench - I dont know how
2. specify a linear function of pressure loss vs. number of iterations

Could you please let me know, how it can be done.

V.

vitulaaak · October 23, 2012, 08:23

I would add one point.

I know, how to initialize from initial conditions and not from current solution. Is there a way to initialize from last design point? This would probably be the preffered option.

cdegroot · October 23, 2012, 15:50

You can initialize from any .res file, so it shouldn't be a problem to use the "last design point" as you put it. Do this in Solver Manager when you set up the case.

With regards to having a pressure loss that is a function of the iteration number, use a CEL expression.

vitulaaak · October 24, 2012, 08:53

Hi,

that was exactly what I was looking for, but when you check the options that ansys offers you in the expressions, iteration number is not there. Anyway I found it in the help, so now it seems to work fine.

brunoc · October 25, 2012, 12:13

Why do you need to specify a pressure loss at the outlet? Could you explain us what is it you're modelling at the outlet?

vitulaaak · October 26, 2012, 06:20

Hello,the reason is following:
You can either specify mass flow or static pressure at the outlet. These conditions are not always met and you can get simulation problem...by specifying a pressure loss further downstream the compressor you basicaly try to simulate the reality. When you measure compressor performance you throttle the exit to get to different operating conditions. The other possibility would be to specify a variable porosity at the outlet to create again a pressure loss. This solution should be self sustainable.you should not need to swit h between different boundary conditions.

October 23, 2012, 06:32	CFX Boundary conditions	#1
vitulaaak Member Vit Houst Join Date: Apr 2012 Posts: 35 Rep Power: 14	Hello, I am modeling compressor in CFX.To prevent from switching boundary conditions, i tried to use opening on the outlet with pressure loss ( i would use outlet, but there is no such possibility to add pressure loss to this type of boundary). when I use this approach, it sometime fall on overflow. To prevent from doing this I would like to do either: 1. specify different initialization file than current in workbench - I dont know how 2. specify a linear function of pressure loss vs. number of iterations Could you please let me know, how it can be done. V.

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October 23, 2012, 08:23		#2
vitulaaak Member Vit Houst Join Date: Apr 2012 Posts: 35 Rep Power: 14	I would add one point. I know, how to initialize from initial conditions and not from current solution. Is there a way to initialize from last design point? This would probably be the preffered option.

October 23, 2012, 15:50		#3
cdegroot Senior Member Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 414 Rep Power: 17	You can initialize from any .res file, so it shouldn't be a problem to use the "last design point" as you put it. Do this in Solver Manager when you set up the case. With regards to having a pressure loss that is a function of the iteration number, use a CEL expression.

October 24, 2012, 08:53		#4
vitulaaak Member Vit Houst Join Date: Apr 2012 Posts: 35 Rep Power: 14	Hi, that was exactly what I was looking for, but when you check the options that ansys offers you in the expressions, iteration number is not there. Anyway I found it in the help, so now it seems to work fine.

October 25, 2012, 12:13		#5
brunoc Senior Member Bruno Join Date: Mar 2009 Location: Brazil Posts: 277 Rep Power: 21	Why do you need to specify a pressure loss at the outlet? Could you explain us what is it you're modelling at the outlet?

October 26, 2012, 06:20		#6
vitulaaak Member Vit Houst Join Date: Apr 2012 Posts: 35 Rep Power: 14	Hello,the reason is following: You can either specify mass flow or static pressure at the outlet. These conditions are not always met and you can get simulation problem...by specifying a pressure loss further downstream the compressor you basicaly try to simulate the reality. When you measure compressor performance you throttle the exit to get to different operating conditions. The other possibility would be to specify a variable porosity at the outlet to create again a pressure loss. This solution should be self sustainable.you should not need to swit h between different boundary conditions.