
[Sponsors] 
May 1, 2014, 22:05 
Unsteady pressure calculation

#1 
New Member
Fraser Callaghan
Join Date: Oct 2013
Posts: 4
Rep Power: 10 
I have a temporal dataset of velocity vectors in a 3D volume. I also have a reference pressure at t=0. My flow is incompressible.
I can calculate the pressure gradient and define a pressure field relative to my reference pressure at t=0, however how could I calculate pressure fields at t=1, t=2, t=i ... ? (I have the pressure gradient fields at all these time points, but no reference pressure). Any tips, resources would be helpful. I haven't been able to turn up anything in the literature. Thanks, Fraser 

May 3, 2014, 18:33 

#2  
Senior Member
Join Date: Aug 2011
Posts: 270
Rep Power: 14 
Did you solve the NavierStokes equations? if so ,necessary you have the pressure on each grid point of your computational domain.
If it is not the case,you need a spatial reference pressure for each time step (if you have the pressure gradient at each time step) Quote:


May 4, 2014, 03:34 

#3  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,146
Rep Power: 66 
Quote:
1) for any time, solve a Poisson equation with the source terme obtained taking the divergence of the momentum equation. 2) However, for incompressible flows be careful that you do not define a thermodicamic pressure 

May 4, 2014, 17:58 

#4 
New Member
Fraser Callaghan
Join Date: Oct 2013
Posts: 4
Rep Power: 10 
Thanks for the reply.
Velocities are measured velocities. Pressure gradient is solved for from the momentum equations. I have no reference pressure for each time step. Approach so far is an implicit discretisation based on SIMPLE 

May 4, 2014, 23:11 

#5 
Senior Member
Join Date: Jul 2009
Posts: 330
Rep Power: 17 
Why can you not use the same reference pressure as at t = 0? What is causing the reference pressure to change?


May 4, 2014, 23:51 

#6 
New Member
Fraser Callaghan
Join Date: Oct 2013
Posts: 4
Rep Power: 10 
A beating heart


May 5, 2014, 08:23 

#7 
Senior Member
Join Date: Jul 2009
Posts: 330
Rep Power: 17 
But ordinarily the reference pressure is a quantity that primarily affects the nondimensionalization of the equations of motion. The actual pressure in a heart chamber will vary in time, but the pressure levels will vary relative to the reference pressure. So I guess I still don't understand how or why the reference pressure is changing in time.


May 5, 2014, 09:54 

#8 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,146
Rep Power: 66 
the blood pressure is measured by the device with reference to a pressure value, you should use this one...


May 8, 2014, 20:34 

#9 
New Member
Fraser Callaghan
Join Date: Oct 2013
Posts: 4
Rep Power: 10 
Sorry  grant deadlines
The velocities are measured and thus should be nondivergent  solution of Poisson eqn not required. I don't want to know pressures relative to single point at each time step. I want to know pressures relative to single point at time step one. All pressures, everywhere are time dep. Reusing ref pressure from t=0 would give me the former case. Thus I need some sort of coupling of the pressure gradient to the unsteady component. 

May 9, 2014, 03:24 

#10  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,146
Rep Power: 66 
Quote:
You can always write Div (Grad p') = Div ( diffusion  convection  acceleration) so, if you have a non divergencefree acceleration you can compute it (you have the velocity ad several time steps, right?) and consider it in the source term. 

Tags 
incompressible, navierstokes, pressure, unsteady 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
[Help!] impeller torque calculation: pressure and viscous moments  ghost82  Main CFD Forum  1  March 27, 2014 02:37 
reference pressure and compressible flow  bingo10  CFX  0  September 11, 2013 07:32 
Unsteady pressure pulses  delaneyluke  FLUENT  0  February 19, 2013 15:17 
Pressure drag calculation  lc05  Main CFD Forum  2  November 1, 2010 07:50 
pressure drop calculation multiinletoutlet syst.  timmyy  FLUENT  1  April 24, 2007 06:03 