Pressure solved by CFX & Total pressure

MissCFD · September 29, 2017, 15:54

Hi,

In my CFX model (see the simplified picture attached), I impose a velocity at inlet (section 1) and a relative static pressure at outlet (section 2). For example: $V_{x,1}$ = 5 m/s & $P_{s,2}$ = 0 Pa

Physically, the net head is equal to: $H_{n}$ = $P_{1}$ + $\frac{1}{2}$ $\rho$ $V^{2}_{1}$ + $\rho$ g $z_{1}$ - ( $P_{2}$ + $\frac{1}{2}$ $\rho$ $V^{2}_{2}$ + $\rho$ g $z_{2}$ )

In CFX, when I do the difference of total pressure between the two sections: massFlowAve(Total Pressure)@inlet - massFlowAve(Total Pressure)@outlet
for me, CFX do this calculation: $P_{1}$ + $\frac{1}{2}$ $\rho$ $V^{2}_{1}$ - ( $P_{2}$ + $\frac{1}{2}$ $\rho$ $V^{2}_{2}$ ), so the term of the potential energy ( $\rho$ g $z_{i}$ ) is not taken into account, it that perception right ?

Or maybe, the pressure solved by CFD takes into account this term ?
Or should I use the absolute pressure to obtain the $H_{n}$ exact value ?

Can someone explain me this point ? What does correspond exactly the pressure solved by CFX in this case ?

Thank you for you help !

Opaque · October 2, 2017, 08:24

Not certain, but did you activate buoyancy? That is the only known mechanism to include gravity into the equations.

You should be able to verify if the elevation is included by comparing Absolute Pressure as well, correct?

Hope the above helps

ghorrocks · October 3, 2017, 19:52

Note that the pressure reported by the CFX solver in buoyant simulations is corrected for the hydrostatic head.

MissCFD · October 5, 2017, 10:01

What is the pressure you are talking about ? The absolute pressure ?

Quote:

Originally Posted by ghorrocks

Note that the pressure reported by the CFX solver in buoyant simulations is corrected for the hydrostatic head.

evcelica · October 5, 2017, 10:12

Absolute Pressure takes into account gravity head. You will see higher absolute pressure at the bottom of a tank.

Pressure and Total Pressure have this term taken out. The bottom and top of a tank will show the same pressure and total pressure.

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October 2, 2017, 08:24		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	Not certain, but did you activate buoyancy? That is the only known mechanism to include gravity into the equations. You should be able to verify if the elevation is included by comparing Absolute Pressure as well, correct? Hope the above helps

October 3, 2017, 19:52		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Note that the pressure reported by the CFX solver in buoyant simulations is corrected for the hydrostatic head.

October 5, 2017, 10:12		#5
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	Absolute Pressure takes into account gravity head. You will see higher absolute pressure at the bottom of a tank. Pressure and Total Pressure have this term taken out. The bottom and top of a tank will show the same pressure and total pressure.