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

#1 
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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 
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Glenn Horrocks
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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 

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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 
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Glenn Horrocks
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My previous post explains why. Because the velocity in the two frames of reference is different.


March 14, 2012, 04:35 

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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 
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Glenn Horrocks
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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  
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saleh
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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 
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Glenn Horrocks
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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 
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saleh
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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? 

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