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-   -   how to get the right pressure value in the backflow domain? (http://www.cfd-online.com/Forums/cfx/96692-how-get-right-pressure-value-backflow-domain.html)

 rockzh January 30, 2012 00:50

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

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?
http://forum.simwe.com/data/attachme....jpg.thumb.jpg

 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?