# pressure evaluation and fully developed flow

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 July 4, 2016, 10:53 pressure evaluation and fully developed flow #1 New Member   Gioacchino La Rochelle Join Date: Jul 2016 Posts: 5 Rep Power: 2 Hello everyone, I'm a beginner on STAR CCM+, and I'm analysing a non-circular duct. I need to study only the fully developed flow motion, in order to evaluate a sort of Moody chart for this pipe. I've set the periodic boundary condition with interface, but now physical boundary conditions have overriden by "Pressure drop" or "mass flow rate" in interface. So my question is if this is normal, and to find Reynold number i've to do my calculations with and average velocity, and identify the corresponding flow rate. Once i've set the flow rate, to evaluate the pressure drop i've used two surface at the fluid boundaries with "total pressure", while in report for each surface the representative scalar value of pressure is computed with "surface average". Is this correct ? I don't get value 0 for the outlet as I fixed in Physics boundary conditions.

 July 5, 2016, 01:46 #2 Senior Member   kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 6 If you create a fully developed interface, periodic, than yes it is correct that you have to specify a pressure drop or flow rate. Yes take an average of velocity at a given cross section, (best if the pressure is somewhat uniform at this section) where you define the hydraulic diameter, to get the Re# likewise use a surface average for density and viscosity if they are not constant. I dont understand if you are using fully developed interface, periodic and setting the flow rate, which is the right way to get a pressure drop, how are you setting the pressure at the outlet, it is periodic. For pressure boundaries when you set 0, that is relative to your reference pressure.

 July 5, 2016, 16:23 #3 New Member   Gioacchino La Rochelle Join Date: Jul 2016 Posts: 5 Rep Power: 2 Thank for your quick reply! I've get the solution to my first doubt. But I don't understant the meaning of your last words. I repeat the question: which tool on star ccm should I use in order to evaluate the pressure gradient between two cross-plane ? Thank again for your coperation!

 July 5, 2016, 16:48 #4 Senior Member   kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 6 OK say you have crossPlane1 and crossPlane2, create two surfaceAverage reports, call them p1 and p2 for the report p1 use crossPlane1 for part and scalar is pressure and for p2 use crossPlane2 for part and pressure for scalar Create an expression report, call it dp, type for your expression the following \$p2Report-\$p1Report For surfaceAverage report you can choose totalpressure if you want, just try it and see if there is any different to pressure or static pressure, to learn about it,

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