CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

question on the physical meaning of p_rgh

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By Phicau

Reply
 
LinkBack Thread Tools Display Modes
Old   September 26, 2014, 03:46
Default question on the physical meaning of p_rgh
  #1
lin
Senior Member
 
Hua Zen
Join Date: Mar 2009
Posts: 116
Rep Power: 9
lin is on a distinguished road
Hello, I have found that p_rgh is often used in OpenFOAM.

According to Henrik thesis (4.21)

p_rgh=p-rho*g*h

"the modified pressure p_rgh is obtained by removing the hydrostatic pressure from the pressure."

I think rho*g*h is not the static pressure, as rho is not a constant, thus it should be int(rho*g)dz. So the question is what's the physical meaning of p_rgh.
lin is offline   Reply With Quote

Old   September 26, 2014, 04:55
Default
  #2
Senior Member
 
Pablo Higuera
Join Date: Jan 2011
Location: Singapore
Posts: 340
Rep Power: 8
Phicau is on a distinguished road
Hi Lin,

p_rgh does not have any physical meaning, it is just a convenient numerical technique.

The expression is not with h, but with z:

p_rgh = p - rho*g*z

This means that if a cell is at z = 0, p_rgh = p.

Also, as you pointed out, if a cell is in water at one instant p_rgh = p - rhoWater*g*z, and if at the next time step it is in air: p_rgh = p - rhoAir*g*z, it may be induce quite a huge variation in p_rgh.

In short, p_rgh is not hydrostatic pressure because it takes into account what happens at a point, and not at all the points above. I hope this clarifies things a little.

Best,

Pablo
jherb likes this.
Phicau is offline   Reply With Quote

Old   September 26, 2014, 21:18
Default
  #3
lin
Senior Member
 
Hua Zen
Join Date: Mar 2009
Posts: 116
Rep Power: 9
lin is on a distinguished road
Thanks Pablo for your reply. It helps me a lot.
lin is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Physical meaning of TKE jibreel Main CFD Forum 2 January 6, 2014 05:31
Partition does not start on physical sector boundary!!?? immortality Lounge 1 December 8, 2013 16:04
viscosity ratio in wake of ellipse - physical meaning? MachZero Main CFD Forum 8 July 22, 2013 16:32
Some question about channeloodles and average value spwater OpenFOAM 0 August 21, 2009 15:28
Dual-time stepping question lost.identity Main CFD Forum 0 June 4, 2009 03:43


All times are GMT -4. The time now is 23:30.