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convergence and measurement problem

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Old   June 27, 2014, 08:23
Default convergence and measurement problem
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hashim chaudhry
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Hi, I am working on a simulation of axial fan with inlet and outlet duct in order to investigate the pressure drop across the fan. My fan Dia is 116 mm and i made the inlet of 300 mm, outlet of 500mm. I am using moving frame refernece for this case in which i am giving fixed rpm to a rotating region with velocity inlet and pressure outlet. The issue is whenever i run the simulation reversed flow occur and after few iterations it shows the error of floating point overflow/ kturbukence.
I am using surface remesher, segregated flow and polyhedral mesh as a mesh condition and steady, constant density with k-epsilon turbulence model. I also attached the pictures of fan geometry and pressure and velocity results.

Please help me out I would be very thankful to you

Regards
Hashim
Attached Images
File Type: jpg geometry.jpg (40.3 KB, 18 views)
File Type: jpg pressure_plot.jpg (89.4 KB, 18 views)
File Type: jpg velocity_plot.jpg (20.6 KB, 17 views)

Last edited by chaudhry_hashim; June 27, 2014 at 09:39. Reason: add pictures
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Old   June 29, 2014, 15:37
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Your mesh is probably not adequate, check your prism mesh and wall y+ values.

If you're going to post results, please do them using relative frames for MRF cases.
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Old   June 30, 2014, 02:47
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Thankyou so much for your guidance i am working on these issues and also could you help me out in finding the pressure drop across the fan. At which face or point i have to calculate total pressure at upstream and at which face or point i have to calculate static pressure in downstream. Which report should I consider surfaced average or mass averged flow and where at the interfaces or inlet and outlet.
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Old   July 1, 2014, 13:21
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Generally these measurements are conducted at interfaces up and downstream of the fan. Usually there are extruded meshes on either side of the inlet and outlet that provide these interfaces.
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Old   July 3, 2014, 03:18
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thanks for your reply if i use stagnation inlet as a inlet condition and and pressure outlet as outlet in order to get the fan curve is it correct ? and should I give the value of total pressure there or keep zero

I am trying to get the value of pressure at the interface upstream and downstream but the pressure drop is too high. I am calculating total pressure at upstream by using surface average report and static pressure at downstream interface. It may be because of the meshing problem ?

the interface I am talking about is the interface between the upstream/downstream and the rotating region i.e fan
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Old   July 3, 2014, 10:29
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No, you are measuring too close to the fan. Like I said, you should have extruded regions upstream and downstream of your BCs.
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Old   July 6, 2014, 10:23
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Thanks for reply I am now using upstream extruded duct which is 6 times of fan dia and downstream extruded duct which is 12 times of fan dia. I changed velocity inlet to stagnation inlet and I gave the pressure value at outlet which is basically the maximum value of pressure difference in the fan curve that I have and giving the rpm to the rotating region.
I am measuring the total pressure at 5 times of fan dia in the upstream by using the surface average report and static pressure at 10 times of fan dia in the downstream by the using the same report and also the mass flow at this plane.
Kindly guide me that the changes I have made is ok? or should I stick to the same velocity inlet method.

Thankyou so much for your precious time
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Old   July 6, 2014, 16:27
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Yeah that sounds okay.
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Old   July 6, 2014, 16:44
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thanks I also convert the downstream internal interface to fan interface and also give the pressure jump(fan) at outlet where I gave the value of pressure
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Old   July 11, 2014, 08:41
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I changed my complete system and the boundary conditions as well as I told you before. I made 10 times of fan dia extruded duct on both side of the fan as upstream and downstrream. I am using Mass flow inlet as a inlet boundary condition and pressure outlet as outlet. I attached the pictures of my cfd model, pressure velocity result and residual plot. I am measuring total pressure at 10 times of fan dia at upstream and static pressure at 10 times of fan dia in downstream. The problem is I am getting zero static pressure at outlet also for the pressure results you can see that maximum pressure occur at the bottom of duct(using no gravity affect) and should I give the pressure at outlet with respect to the mass flow rate I am giving from the fan curve. Right now I am giving zero pressure at outlet.
Attached Images
File Type: jpg cfd model.jpg (29.5 KB, 8 views)
File Type: jpg pressure.jpg (76.0 KB, 8 views)
File Type: jpg velocity.jpg (76.7 KB, 9 views)
File Type: jpg pres.jpg (64.1 KB, 10 views)
File Type: jpg residual.jpg (79.4 KB, 5 views)
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Old   July 14, 2014, 00:54
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Your results look suspicious. Why should their be any variation in pressure across the fan plane? Did the forces on the fan converge? I doubt it.
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