# Total pressure in rel frame and total pressure

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 March 12, 2012, 09:45 Total pressure in rel frame and total pressure #1 New Member   Join Date: Mar 2012 Posts: 3 Rep Power: 10 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: 16,460 Rep Power: 128 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. saha2122 likes this.

 March 13, 2012, 09:53 #3 New Member   Join Date: Mar 2012 Posts: 3 Rep Power: 10 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: 16,460 Rep Power: 128 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: 10 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=Ustn-omega*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: 16,460 Rep Power: 128 Eqn 3 is not total pressure. It is rothalpy. Look it up in the documentation (eqn 1-56 in the theory guide) or on google.

December 1, 2014, 06:11
#7
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saleh
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Quote:
 Originally Posted by ghorrocks 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.
Hi Dear ghorrocks;
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=Ustn-omega*R and Ptotal(rot)=pstatic+1/2*rho*Urel^2, as aresult: Ptotal(rot)=pstatic+1/2*rho*(Ustn-omega*R)^2
but in cfx theory guide(equation 1-53): 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: 16,460 Rep Power: 128 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
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saleh
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Yes, In CFD-Post, 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]
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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 October 6, 2015, 05:26 #10 New Member   Join Date: Sep 2010 Posts: 1 Rep Power: 0 Hi guys, I was reading the forum and have still a question regarding the total pressure in rotating frame. I could reproduce the values for "TotalPressure in relative and stationary frame" with the given formulas. But for the TotalPressure itself I didnt get the value in [Pa] as CFX gives me, I get to high values. Total Pressure in Stn Frame=p_stat+1/2*rho*Ustn^2 OK Total Pressure in Rel Frame=p_stat+1/2*rho*Urel^2 OK Total Pressure = p_stat+1/2*rho*(Urel^2-(omega*R)^2) Fail For R I use the maxValue at the plane I want to investigate. Would be nice if someone could give me some advice. Thanks

February 19, 2016, 21:38
Rothalpy shows the loss without output work
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Liu Meng
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Quote:
 Originally Posted by sfallah Yes, In CFD-Post, 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?
I believe another post had the similar discussion and offerred more detailed resources.

Difference between total pressure, total pressure in Stn and in Rel frames

Anyhow, if you understand what rothalpy is, these three definitions of "total pressure" should not be a problem. The definiton of Total Pressure here, which is used in CFD-Post, is deduced from rothalpy. It gets rid off the work which the fluid output against the rotors.

The work will be computed differently in the stationary frame and rotational frame. If the stationary work is added to rothalpy, the result is the total enthalpy in stationary frame. And the difference between rothalpy and the total enthalpy in relative frame showes the effect of Coriolis force. The equation of rothalpy includes a part that (Omega X Radius)^2 should be taken away. So if the calculation is about rotors, and the output work is very large, it is quite possible that the rothalpy is negative, or the total pressure is less than the static pressure.

I think this Total Pressure definition based on rothalpy can be very convenient for loss assessment, especially for the centrifugal pump or radial flow turbine. For these flowfields, the Omega X Radius may be quite different on the same streamline within a rotational domain.

Last edited by sodynamic; February 20, 2016 at 00:53.

 February 20, 2016, 10:09 #12 Senior Member   Join Date: Jun 2009 Posts: 174 Rep Power: 13 Read my post below : http://www.cfd-online.com/Forums/cfx...lications.html