# Influence of initial conditions in SteadyState

 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 29, 2012, 09:14 Influence of initial conditions in SteadyState #1 Senior Member   Join Date: Dec 2011 Posts: 121 Rep Power: 6 If i am right,the initial condition should not have any influence if our solution is converged. In my Conjugate Heat Transfer simulation,After about 10 iteration i get the residual about 10^-5 and this residual isn't changed anymore... . Therefor i expect that the results aren't changed by continuing iteration.But by each iteration(in spite of no changes in residual) my results are changed. Also the initial condition has huge influence on my results(in spite of having the same residual about 10^-5) Do you know why? Thank you in advance.

 March 29, 2012, 12:40 #2 Senior Member   Join Date: Mar 2009 Location: Austin, TX Posts: 137 Rep Power: 10 You are correct that in a valid steady state simulation that you should end up with the same solution regardless of initial conditions. Your problem is your interpretation of what is being presented to you as "Residuals". In commercial CFD software, residuals are of absolutely no use in determining convergence. They may help in determining reasons for divergence, but that is about it. The residual values provided by Star-CCM+ are normalized to 1 in the first iteration*. So, if you start your simulation with initial conditions wildly different than the steady state solution, the residuals will rapidly fall and appear to level off long before the simulation has actually converged. This is likely the behavior you are seeing. At the other end of the spectrum, if you use initial conditions that are very near the steady state solution, the residuals will never fall very far, and you may get a valid converged solution with residuals above 1e-1. This is why it is a very bad idea to have a convergence rule based on residual values. To properly gauge convergence, you need to monitor other values. For a CHT simulation such as yours, I would monitor the heat transfer rates at each of your solid-fluid and solid-solid interfaces, the temperature of the flow at the outlet (assuming you have an outlet), and possibly the temperatures of a few individual solid cells that you expect to converge last. Once all of those values have stopped changing, or at least begin osculating about a constant value, then your simulation is converged. * This is a terrible design. It is only done so that the residual plots look pretty for the manager that CD-adapco is selling the software to. For all the fantastic meshing and solving algorithms in Star-CCM+, their design decisions do a fantastic job of obstructing natural workflow, inhibiting flexibility, and somehow simultaneously confusing users and treating them as if they are idiots. Pauli, siara817, fshak92 and 1 others like this.

 April 1, 2012, 16:09 #3 Senior Member   Join Date: Oct 2009 Location: Germany Posts: 637 Rep Power: 13 kyle, the residuals are normalised to the highest residual within the first 5 iterations. And I assume this is not done to better sell the software but for better judgement about a simulation. When a simulation converges very slowly, residuals will show a completely wrong picture without normalisation. fshak92 likes this.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post cheng1988sjtu OpenFOAM Bugs 15 May 1, 2016 16:12 vaina74 OpenFOAM Running, Solving & CFD 3 June 8, 2010 12:30 vw.cfd OpenFOAM 6 August 7, 2009 05:44 velan OpenFOAM Running, Solving & CFD 1 October 17, 2008 05:36 liugx212 OpenFOAM Running, Solving & CFD 3 January 4, 2006 19:07

All times are GMT -4. The time now is 01:41.