# why residuals fluctuate?

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 March 25, 2006, 04:29 why residuals fluctuate? #1 sagar Guest   Posts: n/a 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.

 March 25, 2006, 10:34 Re: why residuals fluctuate? #2 Chebeba Guest   Posts: n/a 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

 March 26, 2006, 18:05 Re: why residuals fluctuate? #3 James Date Guest   Posts: n/a 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 satyendra likes this.

 March 27, 2006, 01:13 Re: why residuals fluctuate? #4 sagar Guest   Posts: n/a 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

 October 11, 2011, 10:45 residuals fluctuating #5 New Member   satyendra Join Date: Jun 2010 Posts: 15 Rep Power: 9 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: 1. unsteady flow. 2. turbulent k-epsilon and RNG model 3. time step = 1 satyendra

 October 11, 2011, 14:02 #6 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,400 Blog Entries: 6 Rep Power: 46 please attach the pic of residual monitor

 October 11, 2011, 17:33 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,313 Rep Power: 110 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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