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November 17, 2011, 09:19 
head losses or static pressure

#1 
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Juan
Join Date: Dec 2010
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Hi
I am computing head losses i a 90º elbow. The difference height between inlet point and outlet point is 1m. When I compute it using fluent the difference between both points regarding to static pressure is close to 0,5 m. The head losses into the elbow is 0,5 or 1,5 m?. Does fluent considere the difference of height between two points in order to compute the static pressure or i should add the difference height to the static pressure obtained with fuent Thanks 

November 18, 2011, 07:19 
gravity option

#2 
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Juan
Join Date: Dec 2010
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If I have an elbow and I measure the static pressure in 2 points with different height. If I don´t consider gravity, the static pressure will be different?. Shoul I considered the gravity when you try to find out the head losses between 2 points with different height?
What does it happen if i do not consider gravity? Thanks 

November 18, 2011, 09:37 

#3 
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Andrew
Join Date: Mar 2009
Location: Washington, DC
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static pressure and head loss are two different things..head loss is due to the velocity and losses and includes gravity..static pressure has nothing to do with the velocity. You should calculate out both and compare to your Fluent answer..it is pretty straight forward and any fluids book will show you how.
good luck 

November 21, 2011, 05:55 
head losses

#4 
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Juan
Join Date: Dec 2010
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hi
thanks i have understand you said. I try to explain what i have done I have measured difference pressure static between 2 points in the elbow with Fluent. The different static pressure obtained by using Fluent was 0,04 m. I know it is not the head losses (i should consider the differente height between both points), so after that, as I know the different height between inlet point and outlet point (0,05 m), my head losses are 0,09 m (0,05m + 0,04m)(similar to my experimental results), considering similar velocity between the 2 points. I have done it without considering gravity, is it right? if I would consider gravity, could I say that the pressure static obtained in fluent is the same as the head losses? I have made simulation using an horizontal pipe and other one with a vertical pipe. Simulating both of them i have obtained the same values for static pressure, dynamic pressure and total pressure. How is it possible?. I know the head losses should be the same but I do not understand how the static pressure are similar. How can I explain fluent if it is a vertical pipe or horizontal pipe? Using gravity maybe? Thanks Last edited by Tres; November 21, 2011 at 07:14. 

November 21, 2011, 17:02 

#5 
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Andrew
Join Date: Mar 2009
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it sounds like you don't have your fluent model set up correctly, and Fluent is seeing both pipes as the same. If I remember correctly, you can define which axis gravity is following, and that will tell fluent if it is vertical or horizontal.


November 22, 2011, 04:03 

#6 
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Juan
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Thanks
OK, I have made a simulation in a vertical pipe including gravity for axis Y (9.81m/s2). Now I have obtained different static pressure (higher in the point which has less height and lower in the point which has more height). After measure static pressure with fluent when I consider the different height between the 2 points I obtain the same head losses than a horizontal pipe. In spite of this, I have a question about specified operating density option. When you choose gravity you can choose a specified operating density option. In several tutorial says that you can choose 0 kg/m3 as density but I do not understand why I can not use the density of water. Ihave used velocity inlet and outflow as boundary conditions Thanks again 

November 22, 2011, 09:28 

#7 
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Andrew
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I believe if you defined your 'fluid' section of the model as water you don't have to apply any density definitions. You can try changing that and seeing how it affects your answer.


November 22, 2011, 12:51 

#8 
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Juan
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Thanks
I try to understand my solutions. When I choose 0 kg/m3 as density I obtain a difference static pressure between 2 points which is not the same as my head losses, but when I consider the different height betweem both points the solution is right. On the other hand If I do not choose anything density the difference static pressure is the same as my head losses, but it must be wrong because the static pressure in fluent is not the same as head losses. Thanks 

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