Effect of gravity in verticle pipe on static pressure

Astik Kulkarni · January 3, 2019, 23:01

Hello,
I am trying to simulate the flow of water in verticle pipe. and the effect of its velocity on total pressure (static + dynamic)

I used a simple pipe without any flow of water (static water column). but the static pressure obtained in the results is different than the theoretical calculation of pressure by rho*g*h.

I can't seem to find the results of theoretical pressure in any of the fluent software.

For example. I simulated the verticle water column of 1000mm height and 100mm diameter. The calculated theoretical pressure should be 1000*10*1 = 10000 pascal.

The obtained pressure is varying between -90k to 100k pascal.

Please advise on what should I do.

I am using k- epsilon method with all other default options.

Thank You

LuckyTran · January 4, 2019, 00:04

When you plot the static pressure in Fluent, it does not contain the hydrostatic part.

Astik Kulkarni · January 6, 2019, 21:18

What does it actually content? Just Dynamic pressure? Rho*v2/2

LuckyTran · January 7, 2019, 11:26

The static pressure is a modified pressure which does not contain the hydrostatic part (the rho*g*z part). It contains the part of the pressure associated with flow (i.e. the part of the pressure needed to satisfy the momentum & continuity equation). The static pressure does not contain the dynamic part. The reason it is done this way is because you do not ever need to solve for the hydrostatic part (which is simply adding a rho*g*z factor). Fluent reports this pressure that is actually calculated by the solver (it gives the user access to this low level information). Unfortunately it labels this in the GUI as static pressure which confuses a lot of people (it's just poor GUI design, the variable is simply called pressure without any meaning if you look under-the-hood).

You can visualize the pressure with the hydrostatic part by generating a custom field and adding the rho*g*z to the pressure.

Btw, this is actually well documented in the Fluent manual and even mentioned in the tutorial.

Astik Kulkarni · January 9, 2019, 02:32

If I want to obtain total pressure acting on the base area (inlet) of the pipe? including the hydrostatic part and the dynamic part, is there any way to do it?

January 3, 2019, 23:01	Effect of gravity in verticle pipe on static pressure	#1
Astik Kulkarni New Member Astik Kulkarni Join Date: Sep 2018 Posts: 4 Rep Power: 7	Hello, I am trying to simulate the flow of water in verticle pipe. and the effect of its velocity on total pressure (static + dynamic) I used a simple pipe without any flow of water (static water column). but the static pressure obtained in the results is different than the theoretical calculation of pressure by rhogh. I can't seem to find the results of theoretical pressure in any of the fluent software. For example. I simulated the verticle water column of 1000mm height and 100mm diameter. The calculated theoretical pressure should be 1000101 = 10000 pascal. The obtained pressure is varying between -90k to 100k pascal. Please advise on what should I do. I am using k- epsilon method with all other default options. Thank You

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
defining a term for a domain using DEFINE_ADJUST	MASOUD	Fluent UDF and Scheme Programming	1	September 24, 2010 05:08
Gas pressure question	Dan Moskal	Main CFD Forum	0	October 24, 2002 22:02
Terrible Mistake In Fluid Dynamics History	Abhi	Main CFD Forum	12	July 8, 2002 09:11
fluid flow fundas	ram	Main CFD Forum	5	June 17, 2000 21:31
Hydrostatic pressure in 2-phase flow modeling (long)	DS & HB	Main CFD Forum	0	January 8, 2000 15:00

January 4, 2019, 00:04		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	When you plot the static pressure in Fluent, it does not contain the hydrostatic part.

January 6, 2019, 21:18		#3
Astik Kulkarni New Member Astik Kulkarni Join Date: Sep 2018 Posts: 4 Rep Power: 7	What does it actually content? Just Dynamic pressure? Rho*v2/2

January 7, 2019, 11:26		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	The static pressure is a modified pressure which does not contain the hydrostatic part (the rhogz part). It contains the part of the pressure associated with flow (i.e. the part of the pressure needed to satisfy the momentum & continuity equation). The static pressure does not contain the dynamic part. The reason it is done this way is because you do not ever need to solve for the hydrostatic part (which is simply adding a rhogz factor). Fluent reports this pressure that is actually calculated by the solver (it gives the user access to this low level information). Unfortunately it labels this in the GUI as static pressure which confuses a lot of people (it's just poor GUI design, the variable is simply called pressure without any meaning if you look under-the-hood). You can visualize the pressure with the hydrostatic part by generating a custom field and adding the rhogz to the pressure. Btw, this is actually well documented in the Fluent manual and even mentioned in the tutorial.

January 9, 2019, 02:32		#5
Astik Kulkarni New Member Astik Kulkarni Join Date: Sep 2018 Posts: 4 Rep Power: 7	If I want to obtain total pressure acting on the base area (inlet) of the pipe? including the hydrostatic part and the dynamic part, is there any way to do it?