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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?
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.


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