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November 16, 2011, 09:43 

#21 
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Sijal Ahmed Memon (turboenginner@gmail.com)
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While looking at the static pressure at different planes from inlet to outlet, it is clearly visible that the average static pressure condition permits the solution to be developed naturally than the enforced uniform static pressure at outlet.
From physical point of view enforced uniform static pressure condition assumes that the flow is completely mixed out which may be true at the outlet boundary placed at far far downstream. Another physical interpretation is : it is basic design philosophy of turbo machine that there should be minimum radial velocity gradient, this can only be minimised if the static pressure increases from hub to shroud. 

November 16, 2011, 10:03 

#22  
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But you have to take care some design allow a pressure gradient to induce some radial velocity gradient. i.e designs not sticking with raidal equilibrium design/theory
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November 16, 2011, 12:17 

#23 
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Now question comes, when to use static pressure or average static pressure condition. Can you name three three cases for each boundary condition as per your understanding? e.g. what should be boundary condition for fully developed pipe flow with uniform cross section?


November 16, 2011, 13:38 

#24  
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November 16, 2011, 13:44 

#25  
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November 16, 2011, 22:08 

#26 
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First i would like to point out that the radial equilibrium theory is only valid in the blade passages where there actually is a constant centrifugal force acting on the flow. When the flow leaves the passage the flow does not have such kind of force acting on it, but still the pressure difference is there, so the flow 'diffuses' in the radial direction after it leaves the blade passage and hence after some axial length static pressure condition would be correct and it need not be far far away, it all depends on the blade geometry and loading.
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November 19, 2011, 13:30 

#28 
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Finally got the definite answer. The answer is
" For axial compressor rotor, atleast, the uniform static pressure does not represent the true physics and is therefore technically wrong. However, surprisingly, the performance map with both conditions is overlapping with some shift (on same curve) on map for the same back pressure and for both conditions". The reason is unknown and your comments shall be very much helpful to me. For confirmation the following procedure was adopted : 1. Another domain of constant area ratio is constructed at the outlet of rotor extending approximately 8.18 chords from the interface plane. And approximately 10.12 chords from leading edge. The walls of the extended domain are modelled as free slip as frictional effects are not important and to save the computational resources as well. 2. Rotor has Y+ = 1 and around 1.3 million nodes. SST turbulence model and automatic wall treatment. 3. Interface is modelled as mixing plane and frozen rotor and at the exit of extended domain two boundary condition are specified a) uniform static pressure b) Average static pressure (For results shown here it is 115000 Pa) Conclusions are: 1. For four cases mass flow rate is same and convergence is good and residuals converged to 1e05 1e06 2. Results are shown at three planes a) plane upstream of interface at 1.45 chord from leading edge (Plane 1) b) plane of interface at 1.94 chords from leading edge (Interface) c) plane downstream of interface at 5.1 chords from leading edge (Plane 3) 3. Values at the interface (original outlet boundary for the rotor simulation) shows the radial variation of static pressure from hub to shroud. It is natural to have this pressure gradient to account for the radial flow and other effects. 4. Results at interface are interesting to note where for both boundary conditions, we get the similar pressure profile at this boundary, which confirm the validity of average static pressure condition. 5. Even at far downstream plane static pressure variation from hub to shroud is visible and gradually decreases to nearly uniform value at far downstream boundary. Although some variation is still there. 

November 21, 2011, 23:02 

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So what do you conclude ?? From your analysis, it seems that at far far away boundary both B.C's are good enough ( the point I was trying to make earlier ). Now ,at the interface ( original outlet ) I am unable to see much difference in pressure distribution in both cases. but a pressure variance occurs throughout the domain, hence at no point it is logical to apply static pressure unless it is far far away.
It would be interesting to see what is the pressure distribution for both the cases at 8.18 chords downstream of interface ( the current outlet ). Could you post that too ?
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November 30, 2011, 22:40 

#30  
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However if you define the average static pressure = 0 or 101325 or any other value with blend factor = 0 does specify the neumann BC on pressure, velocity and scalars (By default blending factor = 0.05 or 5%). @DB I just ran few more cases to understand it thoroughly and now I have made my conclusions and shall post the comments with pics within few days. @Glenn Can I post my results along with pics so that it can be of help to everyone. Can you please guide me how to do this? 

December 1, 2011, 05:11 

#31 
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You have previously posted images in this thread, so I trust you know how to do that. For things like CCL files put them in as attachments.


December 2, 2011, 10:31 

#33  
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Two important conclusions are: 1. More important is their effect at interface (original outlet), which is very much equal for both conditions. Hence the mass flow rate, pressure ratio and efficiency values are also same. 2. However in original case (without extended domain), they produce very different results for same outlet static pressure condition. 

December 6, 2011, 04:34 

#34 
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Hi,
Could you compare the interface results for far far outlet boundary domain with the small domain for both the b.c's ( avg S.P and S.P) and tell which B.C is giving interface values much closer to case of far far outlet boundary case interface value. ANd what is the % for both B.C's as compared to far away boundary. This would let us know which B.C can be used with much greater accuracy for a small domain size ( Again I think Avg Static Pressure would be better )
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December 6, 2011, 14:26 

#35 
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yes you are right, Av Static pressure BC is more realistic. I shall share full analysis here as you have suggested within couple of days.
Thanks for giving me new ideas for data interpretation. 

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boundary condition, cfxpre, outlet 
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