how to get the right pressure value in the backflow domain?

rockzh · January 30, 2012, 00:50

The 3D model i finished is as following figure.The water flows into a pipe with different diameters.Inlet boundary type:velocity inlet(2m/s), outlet boundary type: 0pening(0Pa).

In CFX-Post,i got the presssure values of several faces as follows[areaAve(pressure)@plane]:
---------INLET -----PLANE1 -----PLANE2 -----PLANE3 ------OUTLET
static ---(-79 ) -----(-170 )------(-153 )-------(1.56) -----(-0.00019)
pressure

total ----(1343 )-------(-79 )-------(-70)-------( 7.36)------( 5.6)
pressure

the question:
1)why is the total pressure on plane1 and plane2 negative?
2)How to get the right pressure value on the plane located in the backflow domain in CFX-Post?

rockzh · January 31, 2012, 01:59

anyone can give me an answer?Thanks very much.

ghorrocks · January 31, 2012, 06:45

The recirculations at plane 1 and 2 are low pressure, low velocity regions where a lot of energy is lost, so the pressure is low.

rockzh · January 31, 2012, 20:03

why is the total pressure on plane1 and plane2 negative?(the reference pressure is setted as 0Pa or 1atm,and the value is the same.)
The plane 1 and 2 is in the backflow domain,and I get the pressure value using areaAve(total pressure)@plane1,is it right?
The pressure drop between inlet and plane1 is equal to areaAve(total pressure)@inlet-areaAve(total pressure)@plane1?

rockzh · January 31, 2012, 20:56

The value is caculated using areaAve()@.
this following table gives the values using ave()@
---------INLET -----PLANE1 -----PLANE2 -----PLANE3 ------OUTLET
static ---(-80 ) -----(-164 )------(-168 )-------(1.46) -----(-0.0008)
pressure

total ----(1342)-------(393)-------(338)-------( 8.95)------( 5.07)
pressure

This following tabe gives the values using massFlowAve()@
　 ---------INLET -----PLANE1 -----PLANE2 -----PLANE3 ------OUTLET
static ---(-79 ) -----(-156 )------(-165)-------(1.52) -----(-0.00022)
pressure

total ----(1343 )-------(1269)-------(1171)-------( 7.79)------( 5.7)
pressure

There are big different for total pressure value between the three tables in backfow domain,but the static pressure is approximately same.which is right for the case?
Thanks in advance for your answer.

ghorrocks · January 31, 2012, 22:54

Before wondering about what a result means, have you checked it it accurate? See this FAQ for tips:
http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

ave(), areaave() and massFlowAve() all integrate different quantities, so the mesh size and mass flow rates are skewing the results.

January 30, 2012, 00:50	how to get the right pressure value in the backflow domain?	#1
rockzh New Member Darren Zhang Join Date: Nov 2011 Location: China Posts: 20 Rep Power: 15	The 3D model i finished is as following figure.The water flows into a pipe with different diameters.Inlet boundary type:velocity inlet(2m/s), outlet boundary type: 0pening(0Pa). In CFX-Post,i got the presssure values of several faces as follows[areaAve(pressure)@plane]: ---------INLET -----PLANE1 -----PLANE2 -----PLANE3 ------OUTLET static ---(-79 ) -----(-170 )------(-153 )-------(1.56) -----(-0.00019) pressure total ----(1343 )-------(-79 )-------(-70)-------( 7.36)------( 5.6) pressure the question: 1)why is the total pressure on plane1 and plane2 negative? 2)How to get the right pressure value on the plane located in the backflow domain in CFX-Post? Last edited by rockzh; January 30, 2012 at 01:19.

January 31, 2012, 20:03		#4
rockzh New Member Darren Zhang Join Date: Nov 2011 Location: China Posts: 20 Rep Power: 15	why is the total pressure on plane1 and plane2 negative?(the reference pressure is setted as 0Pa or 1atm,and the value is the same.) The plane 1 and 2 is in the backflow domain,and I get the pressure value using areaAve(total pressure)@plane1,is it right? The pressure drop between inlet and plane1 is equal to areaAve(total pressure)@inlet-areaAve(total pressure)@plane1? Last edited by rockzh; January 31, 2012 at 20:50.

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January 31, 2012, 01:59		#2
rockzh New Member Darren Zhang Join Date: Nov 2011 Location: China Posts: 20 Rep Power: 15	anyone can give me an answer?Thanks very much.

January 31, 2012, 06:45		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,850 Rep Power: 144	The recirculations at plane 1 and 2 are low pressure, low velocity regions where a lot of energy is lost, so the pressure is low.

January 31, 2012, 20:56		#5
rockzh New Member Darren Zhang Join Date: Nov 2011 Location: China Posts: 20 Rep Power: 15	The value is caculated using areaAve()@. this following table gives the values using ave()@ ---------INLET -----PLANE1 -----PLANE2 -----PLANE3 ------OUTLET static ---(-80 ) -----(-164 )------(-168 )-------(1.46) -----(-0.0008) pressure total ----(1342)-------(393)-------(338)-------( 8.95)------( 5.07) pressure This following tabe gives the values using massFlowAve()@ 　 ---------INLET -----PLANE1 -----PLANE2 -----PLANE3 ------OUTLET static ---(-79 ) -----(-156 )------(-165)-------(1.52) -----(-0.00022) pressure total ----(1343 )-------(1269)-------(1171)-------( 7.79)------( 5.7) pressure There are big different for total pressure value between the three tables in backfow domain,but the static pressure is approximately same.which is right for the case? Thanks in advance for your answer.

January 31, 2012, 22:54		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,850 Rep Power: 144	Before wondering about what a result means, have you checked it it accurate? See this FAQ for tips: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F ave(), areaave() and massFlowAve() all integrate different quantities, so the mesh size and mass flow rates are skewing the results.