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-   -   why residuals fluctuate? (http://www.cfd-online.com/Forums/cfx/22352-why-residuals-fluctuate.html)

 sagar March 25, 2006 04:29

why residuals fluctuate?

I am doing transient analysis of a pressure vessel.During solve the residuals fluctuate after 20 iterations.And they are not converging .time step is 0.15 sec. I have given 100 time steps.I have included blockage in this vessel.so is blockage is responsible for these fluctuations?or is these related to mesh problem.I have used k-e model.should i use another model.i do not think that model may be reason.can anybody have reason? Thanks in advance.

 Chebeba March 25, 2006 10:34

Re: why residuals fluctuate?

I have had the exact same effect when my (hexahedral) mesh had too may cells with bad aspect ratios, i e very long or flat cells. Improving mesh aspect ratios made the oscillations go away.

/C

 James Date March 26, 2006 18:05

Re: why residuals fluctuate?

I'd check to make sure your time step is something less than the Courant limit i.e. the Courant number for each time step is less than 1. The time step will obviously depend on the mesh size and velocity of the flow.

James

 sagar March 27, 2006 01:13

Re: why residuals fluctuate?

k.thaks sir.I have refined my mesh.mesh has been imporved.but the problem has not fully solved,yet it has been solved partially.I have given 100 timesteps.In run upto 20 timesteps the residual nature is ok.but from 20 to t30 timesteps it is suddenly fluctruaing.agin from 30 to 90 timesteps it is ok.but between 90 to 100 it is again fluctuating .why this has been happened.Boundary conditons are at inlet velocity varying expression (Velo varies sinusodially) and outlet pressure (constatn) wit hopening condition (with specified loss co effcient).can anybody tell me why this has been happened? Thaks all

 satyendra October 11, 2011 10:45

residuals fluctuating

hi all,

i am doing analysis of a furnace in fluent 6.3(geometry modeled in gambit 2.24). i have made structured mesh in most of the volume except at the corners of the mould.

initially when the hot gases' velocity were low it was converging, but i was not getting required temperature inside mould. hence i increased the flow rate without changing the model.

But now except energy, all other residuals i.e continuity, k, x, y, z velocity and epsilon are fluctuating. following are the specs:
2. turbulent k-epsilon and RNG model
3. time step = 1

satyendra

 Far October 11, 2011 14:02

please attach the pic of residual monitor

 ghorrocks October 11, 2011 17:33

This is the CFX forum. For Fluent questions go to that forum. Also, please start another thread for new questions. Do not hijack other threads.

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