
[Sponsors] 
March 12, 2012, 09:45 
Total pressure in rel frame and total pressure

#1 
New Member
Join Date: Mar 2012
Posts: 3
Rep Power: 5 
Hi! Please let me know what is a principal difference between Total pressure in rel frame and Total Pressure functions in a case of axial turbine stage problem. CFX Post returns different fields of theese variables on turbo surface with constant span =0.5, but i think they must be identical. Thanks.


March 12, 2012, 17:25 

#2 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,658
Rep Power: 84 
Total pressure = p + 1/2 rho v^2
The velocity term is different depending on what frame of reference you measure it in. This means total pressure is different in different frames of reference. 

March 13, 2012, 09:53 

#3 
New Member
Join Date: Mar 2012
Posts: 3
Rep Power: 5 
Thanks, ghorrocks. I was made a mistake in my question, in fact I told about only rotating domain. The velocity term on the rotating frame of reference is the same for total pressure and total pressure in rel frame, isn't it? So why do the total pressure and total pressure in rel frame fields for look different in a same rotating domain?


March 13, 2012, 17:31 

#4 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,658
Rep Power: 84 
My previous post explains why. Because the velocity in the two frames of reference is different.


March 14, 2012, 04:35 

#5 
New Member
Join Date: Mar 2012
Posts: 3
Rep Power: 5 
Ok, probably it's my english problems.
I ask you directly: What is the differnce between: Total Pressure in Stn Frame=p+1/2*rho*Ustn^2 (1) Total Pressure in Rel Frame=p+1/2*rho*Urel^2 (2) Total Pressure = p+1/2*rho*(Urel^2(omega*R)^2) (3) I know, that Urel=Ustnomega*R in rotating systems, Physical meaning of 1 term  absolute total pressure Physical meaning of 2 term  relative total pressure But what's the physical meaning of term 3 ????!!!! Best wishes! 

March 14, 2012, 06:09 

#6 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,658
Rep Power: 84 
Eqn 3 is not total pressure. It is rothalpy. Look it up in the documentation (eqn 156 in the theory guide) or on google.


December 1, 2014, 06:11 

#7  
New Member
saleh
Join Date: Nov 2014
Posts: 16
Rep Power: 2 
Quote:
Total pressure in rotating frame is less than static pressure (In the outlet ), whereas according to equation: Total Pressure in Rel Frame=p+1/2*rho*Urel^2, it should be greater than static pressure because of positive values of "1/2*rho*Urel^2" in any condition(rotating or stationary frame) . My test case is NASA rotor37. What is the reason for this contradiction? Another question: Urel=Ustnomega*R and Ptotal(rot)=pstatic+1/2*rho*Urel^2, as aresult: Ptotal(rot)=pstatic+1/2*rho*(Ustnomega*R)^2 but in cfx theory guide(equation 153): Ptot(rot)=Pstatic+1/2*rho*(Urel*Urel(omega*R.omega*R)). What is the reason for this contradiction? 

December 1, 2014, 17:07 

#8 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,658
Rep Power: 84 
Are you saying you are getting total pressure in a rotating frame less than inlet static pressure in your case? Can you post an image of what you are modelling? And the boundary conditions?


December 2, 2014, 04:38 

#9 
New Member
saleh
Join Date: Nov 2014
Posts: 16
Rep Power: 2 
Yes, In CFDPost, 3 Total pressure is exist: Total Pressure, Total pressure in Rel Frame and Total Pressure in stn Frame. Total pressure is less than static pressure but total pressure in rel frame and total pressure in stn frame is greater than static pressure. Image of Mach contour in 80% span is attached. Boundary condition:
Test case: NASA ROTOR 37 Reference pressure:0 [pa] Inlet: Total pressure:1[atm] Outlet: Static pressure:106.5 [kpa] Angular velocity:1800[rad/s] High resolution discretization is used for all equations. High speed numeric, clip pressure and high resolution rhio chow algorithm are activated via advance parameter tab in solution tab. I can not reach to nominal mass flow rate (20.19 kg/s) and total pressure ratio (2.1) by variation in static pressure. I reached to pressure ratio 1.88 and mass flow rate 20.38 at best condition. What is its solution? 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Total pressure in CFX  famarcfd  CFX  0  June 17, 2011 10:33 
rotating frame of reference and total pressure  viking  ANSYS  1  November 24, 2010 00:25 
RPM in Wind Turbine  Pankaj  CFX  9  November 23, 2009 05:05 
Definition of Total Pressure in rel. Frame of Ref.  Thorsten  CFX  0  November 30, 2006 04:33 
what the result is negatif pressure at inlet  chong chee nan  FLUENT  0  December 29, 2001 06:13 