# Average static Pressure

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November 16, 2011, 09:43
#21
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Sijal
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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.
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 average_sp.JPG (66.6 KB, 115 views) sp.JPG (66.8 KB, 87 views)

November 16, 2011, 10:03
#22
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DB
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Quote:
 Originally Posted by Far 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.
Yes this is to be expected

Quote:
 Originally Posted by Far 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.
Precisely the point I made earlier.

Quote:
 Originally Posted by Far 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.
I don't get it how do you interpret this from the fact of average or fixed static pressure ???
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 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54 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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Quote:
 Originally Posted by Turbomachinery flow physics and dynamic performance By Meinhard Schobeiri: Springer The rotating fluid is subjected to centrifugal forces that must be balanced by the pressure gradient in order to maintain the radial equilibrium
In this context static pressure condition is totally wrong for turbo machinery flows ???? right?

November 16, 2011, 13:44
#25
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Quote:
 Originally Posted by MS thesis If the radial equilibrium condition is employed at the outlet, flow separation does not occur for any of the investigated hub-to-tip ratios. It is evident that the boundary layer separation changes dramatically the flow downstream of the cascade. The question arises what would be the differences between the flow with and without radial equilibrium when separation does not appear for both boundary condition types. To investigate this, a plane at constant y=0 was constructed downstream of the cascade. Figures 18 and 19 show the comparison of the axial velocity for the cascade KA with two different boundary conditions at the outlet. A velocity drop due to the excessive pressure value is visible near the hub of the cascade KA when uniform pressure distribution is applied. The same feature is Figure 22 depicts exemplary behaviour of the flow when uniform pressure distribution is applied at the outlet and a backflow appears at the outlet.The same feature compared in Figure 20 and Figure 21 for the cascade KB. A visible decrease in the axial velocity can be noticed near the hub if uniform pressure distribution is applied for this cascade. A small backflow appears in the vicinity of the hub wall near the outlet. On the one hand, the flow in the investigation area, i.e. on the measuring plane, and the flow upstream do not seem to be remarkably influenced by the uniformity of the pressure distribution. On the other hand, it is obvious that it is substantial to provide realistic boundary conditions if it is deserved to simulate the reality in detail. The further CFD results are presented only for the simulations when radial equilibrium condition was applied at the outlet.
So again average static pressure makes more sense right?

November 16, 2011, 22:08
#26
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Quote:
 Originally Posted by Far In this context static pressure condition is totally wrong for turbo machinery flows ???? right?
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 17, 2011, 14:55
#27
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please look at this
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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, at-least, 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 1e-05 -1e-06

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.
Attached Images
 planes.jpg (47.2 KB, 57 views) pressure_plane1.jpg (69.8 KB, 59 views) pressure_interface.jpg (69.4 KB, 53 views) pressure_plane3.jpg (70.3 KB, 46 views)

 November 21, 2011, 23:02 #29 Member   DB Join Date: Apr 2011 Posts: 87 Rep Power: 15 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 ? __________________ -D.B

November 30, 2011, 22:40
#30
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Quote:
 Does average static pressure=0 at (outlet) signify Neumann boundary conditions. If I require Streamwise gradient=0 at the outlet, will the above condition suffice it? Thanks Santhosh
Quote:
 This is all discussed in the documentation. But from memory it defines dirichlet BCs on pressure and neumann BCs on velocity and scalars. So yes, a static pressure BC will give you zero streamwise gradient in other variables providing the boundary is perpendicular to the flow.
Specifying average static pressure = 0 or 101325 or any other value has the same effect and as mentioned by glenn it defines dirichlet BCs on pressure and neumann BCs on velocity and scalars.

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 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,731 Rep Power: 143 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 1, 2011, 07:41 #32 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54 Well, I am not talking about this particular thread.

December 2, 2011, 10:31
#33
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Quote:
 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 ?
Well at the 8.18 chords downstream the variation in static pressure distribution for average static pressure condition is very less (127000 to 132000 pa) as compared to interface zone. So at the far far downstream both condition have equal effect.

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.
Attached Images
 outlet_staticpressure.PNG (61.0 KB, 46 views)

 December 6, 2011, 04:34 #34 Member   DB Join Date: Apr 2011 Posts: 87 Rep Power: 15 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 ) __________________ -D.B

 December 6, 2011, 14:26 #35 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54 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.

 September 27, 2020, 11:47 #36 Member   Abdullah Arslan Join Date: Apr 2019 Posts: 94 Rep Power: 7 1.If I set Static pressure 1 bar at inlet, and mass flow rate at outlet, my simulation is not converging. Neither vice versa. 2.But if I put mass flow rate as inlet and average static pressure 0Pa. Solution converges. My results are qualitatively perfect, but are 40% of paper. I have thoroughly checked the CEL expressions and others input. (I put 1bar in reaction rate equation, besides Pref as 1bar) Using 2nd option the pressure and total pressure in the domain is like 0-0.2 Pa. So the discrepancy can be regarded to that? Note: Changing Pref changes inlet density though.

 Tags boundary condition, cfx-pre, outlet

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