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Shljuki June 10, 2013 04:35

Total pressure loss
 
Hi, i am simulating flow through a duct, two ends of the duct (inlets) are open to external space (ambient condition with relative pressure of 0 pa) and third end is outlet with 2.2 kg/s of air. I am interested in velocity distribution inside the duct and total pressure loss.
I am bit confused with total pressure distribution isnide the duct, it drops from starting 0 to -900 and starts increasing once the flow approach outlet boundary. I expect constant drop (loss) of total pressure. I have run simulation with different options for outlet: constant flux, shift presure, scale mass flows.
Can someoene advise why do i have total pressure increase at the end of the duct, close to boundary outlet?

oj.bulmer June 10, 2013 05:56

Can you post an image

OJ

Shljuki June 10, 2013 07:49

1 Attachment(s)
I dont have my CFD files here, attached is a schematic of the problem - hope it helps, if not i will be able to upload better image tomorrow.
Thanks

singer1812 June 10, 2013 11:34

Are you measuring Total Pressure along a stream line?

oj.bulmer June 10, 2013 11:44

Are you sure the solution is converged? Also, are you seeing area averaged values?

OJ

Shljuki June 10, 2013 19:05

The solution seem to be converged to below 10e-4, i am monitoring 4 different parameters and no changes.

I have been checking area average total pressure at different sections of the duct.

singer1812 June 11, 2013 09:05

Use massflow average. Or measure the total pressure at different points along a streamline.

oj.bulmer June 11, 2013 10:07

Contrary to what NS equations predict, it has been documented that in viscous flows, there is a possibility that total pressure may rise locally and this is a result of the redistribution of energy by the viscous stresses.

http://adsabs.harvard.edu/abs/1995AIAAJ..33..772I

The phenomenon is aggravated if models like k-eps are used since they sometimes over-predict the viscosity further aggravating this problem. And thus the modelling assumptions make this situation worse and this is the likelier reason for inconsistencies.

What model/discretization schemes are you using? Is the solution mesh independent?

OJ

Shljuki June 12, 2013 22:45

2 Attachment(s)
Thanks everyone for your support, mass flow average helped understanding results.
Total pressure increase along a streamline has ben recorded at the location where two air streams at about 25 m/s clash and enter third vertical duct - refer to attached images.

oj.bulmer June 13, 2013 07:41

Exactly, it has been documented that the rise in total pressure can happen due to reasons mentioned in above post, at stagnation point:
http://fluidsengineering.asmedigital...icleid=1429388

Now, in your case, where the two fluids meet and suddenly come to halt, it seems to be equivalent to what happens at a stagnation point and this may explain the increase.

However, you need to do a bit of literature search to understand if the extent of increase you see there is adequate or too much!

OJ


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