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November 20, 2015, 15:37 
Flow through ribbed pipePeriodic BC

#1 
New Member
CFDlover
Join Date: Nov 2015
Posts: 3
Rep Power: 3 
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: xmomentum 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 kw SST. Thank you for reading through! 

November 22, 2015, 12:30 

#2 
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,372
Rep Power: 20 
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 postprocessed 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 massflow rate. Because the pressure gradient doesn't change each iteration, your boundary conditions remain fixed whereas with the massflow rate approach the boundary conditions are iterated in. Plus you are doing a transient simulation so your pressure gradient is changing each timestep. 1e58 is basically infinity, if you are overpredicting the pressure gradient that much it looks like you have a serious problem. 

November 22, 2015, 13:37 

#3 
New Member
CFDlover
Join Date: Nov 2015
Posts: 3
Rep Power: 3 
Thank you for replying........Several thousands !?........wow!!!
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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