# Moment convergence

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 January 5, 2006, 18:14 Moment convergence #1 Bunmi Guest   Posts: n/a how do i know that my moment has converged? Pls help me in this.I need to monitor moment as my performance indicator WAITING

 January 6, 2006, 01:44 Re: Moment convergence #2 sharuk Guest   Posts: n/a solve->monitor->forces and drop down the list to moment and specify the rotational axis.

 January 6, 2006, 03:41 Re: Moment convergence #3 Bunmi Guest   Posts: n/a yeah. that is what i did but how do i know when it has converged.Am i going to have a constant moment for a long time or what?

 January 6, 2006, 04:47 Re: Moment convergence #4 sharuk Guest   Posts: n/a exactly..

 January 6, 2006, 07:54 Re: Moment convergence #5 Bunmi Guest   Posts: n/a so, if the moment is not constant(i.e it is changing) that means my moment has not converged

 January 6, 2006, 09:31 Re: Moment convergence #6 Jason Guest   Posts: n/a It either means you haven't reached a converged solution, or that you have an unsteady problem (typically due to vortex production in a region of separated flow... like behind a bluff body or off the separation on a wing). Good luck, Jason

 January 6, 2006, 10:13 Re: Moment convergence #7 Bunmi Guest   Posts: n/a ok.i will try to leave d solution awhile and see if it'll d moment will constant

 January 6, 2006, 10:16 Re: Moment convergence #8 Bunmi Guest   Posts: n/a What could be responsible for this vortex production? thanks

 January 11, 2006, 04:24 Re: Moment convergence #10 Bunmi Guest   Posts: n/a Thanks.My model is a mixing tank.I'm carrying out a kind of parametric sensitivity of the turbulence model which is standard k-e model.There has been issues around the use of this model but the computing requirements & robustness still makes it best.But I am trying to optimise the model by changing the constant(C1,C2, Cm) so as to see the effect on the predictions. Using the standard k-e model, the moment converged but changing some of these constants, the moment refuse to converge.I was wondering what could be responsible for this. The surpising thing is that the moment is not converging but the variation of the constants has a great impact on the constant.For instance, some combinations of C1, C2 & Cm gave me good predictions with least error values between the experimental and predicted values. But the moment did not converge. I am really surprised about this. So u can see the issue now. Waiting yr response

 January 16, 2006, 10:09 Re: Moment convergence #11 Jason Guest   Posts: n/a Now you're getting into areas where I'm not too familiar. I pick my turbulence models based on a knowledge database we have where we compare different turbulence models to different situations. I have never gone in to customize a turbulence model. I hope you've researched the turbulence model and what the different coefficients do. Also, you should do some research to see what other people have used for custom coefficients. I have a feeling that some of the constants effect the stability of the model, which is why you're having trouble getting a converged value. Also, do you know for sure that your mesh is optimized? Have you run mesh sensitivities with the default coefficients? Hope this helps, and good luck, Jason

 January 18, 2006, 07:29 Re: Moment convergence #12 kharicha Guest   Posts: n/a I have played a little with the k-epsilon constants, I could make converging the solution on a very bad grid.... this effect was due the increase of the turbulent viscosity ratio which is known to stabilize the flow(just like normal viscosity). So it is always possible to modify the constants so that the magnitude and the distribution of the turbulent viscosity damp instabilities. So I totally agree with Jason, this is dangerous, we should first verify our grid, y+, symmetry of the grid...etc before playing with the parameters.

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