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Flow through ribbed pipe-Periodic BC

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Old   November 20, 2015, 15:37
Default Flow through ribbed pipe-Periodic BC
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Hi !!!
This is my first post and I am a new user to FLUENT. I am working on a ribbed pipe with air as the fluid and am using a 2D geometry with axisymmetry & periodic BC. This BC offers only 2 input options & both give the following errors:
1.Pressure gradient: My experiments show I have a pressure loss of about 1500Pa in a 1.75m long pipe. My geometry is 19mm radius and only 10mm in the streamwise direction. I apply a pressure gradient of (1500/1.75) Pa/m (which is the input option in Fluent). I however get a simulation which keeps showing an increasing velocity with time steps. I do not know why..... I think it could be because
a) CFD would not be able to simulate the actual pressure losses and hence I would have to reduce the pressure gradient.
b) I am not changing the dimensions of length and area in the reference values section.......... The problem is I am not sure if I have to change them to the actual geometry values or whether I can proceed without making any changes to them.
However, I must mention that the simulation predicts flow phenomena similar to other papers on the same.

2. Mass Flow rate: I calculate Mass flow rate as: 30[lit/sec]*1.225(air density)*10^-3(lit/sec - kg/sec)=3.675*10^-3 [Kg/sec]
Now, the problem is when I give this as an input without any pressure gradient, Fluent seems to be overpredicting the pressure ( order of e58) and exits citing divergence in AMG solver: x-momentum
Why do you think this is happening?
I have also plotted the contours of the cell courant number and get values as high as 300 in some regions when I use time steps of 0.001. Could this be responsible. If so, why is the solution not giving problems when the pressure gradient is applied?
All suggestions are welcome as I am not able to get past these steps! Btw,The turbulence model is k-w SST.
Thank you for reading through!
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Old   November 22, 2015, 12:30
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First: 30 L/s @ 1.225 kg/m^3 is a massflow rate of 0.037 kg/s

I have run into some numerical issues using the axissymmetric boundary condition that I didn't have on a 2D planar simulation. It was prone to divergence which was eventually solved by lowering the urf's.

Reference values don't affect your solution, they only affect post-processed quantities.

I have performed several thousand simulations using 2D axissymmetric + periodic boundary conditions on circular and rectangular ribbed channel flow, so I know it works.

The pressure gradient approach is much more robust than specified mass-flow rate. Because the pressure gradient doesn't change each iteration, your boundary conditions remain fixed whereas with the mass-flow rate approach the boundary conditions are iterated in. Plus you are doing a transient simulation so your pressure gradient is changing each time-step.

1e58 is basically infinity, if you are over-predicting the pressure gradient that much it looks like you have a serious problem.
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Old   November 22, 2015, 13:37
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Thank you for replying........Several thousands !?!!!
Thank you for pointing out the mistake, I replaced the 10^-2 with 10^-3.
I have kind of given up on the mass flow rate because of the crazy pressure gradients issue.
Just by prescribing a pressure gradient, lets say 300Pa/m,the solution never seems to stop, i.e. with time, the velocity keeps increasing. Does this mean that:
1. The pressure gradient I imposed is very high ?
2. Something else I must have done wrong?
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