Under relaxation factor for external coupling

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 August 28, 2009, 06:41 Under relaxation factor for external coupling #1 New Member   Join Date: Jul 2009 Posts: 16 Rep Power: 8 Hi all, In the solver control of external coupling, there should be a number for under relaxation factor, its default value is 0.75. Also the variables can be selected from the drop down list. But, what is the meaning of this under relaxation factor? Does it mean that the variables from ansys multibled by this under relazation factor are transferred to CFX? What about with a value 1.0 for this under relazation factor? Thank you for your attention!

August 29, 2009, 11:01
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 Originally Posted by dhxlxz Hi all, In the solver control of external coupling, there should be a number for under relaxation factor, its default value is 0.75. Also the variables can be selected from the drop down list. But, what is the meaning of this under relaxation factor? Does it mean that the variables from ansys multibled by this under relazation factor are transferred to CFX? What about with a value 1.0 for this under relazation factor? Thank you for your attention!
As far as I recall these are for use in the Rhie-Chow algorithm which avoids the checkerboard pressure problem. Check the CFX theory guide pdf.

 September 4, 2009, 20:59 #3 Senior Member   Join Date: Apr 2009 Posts: 516 Rep Power: 12 This is a relaxation factor for the loads (force, displacement, heat flow or temperature) passed between ANSYS and CFX. The default means that 75% of the change in the load will be passed. A value of 1 means no under-relaxation - this is what I usually use.

September 7, 2009, 07:37
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 Originally Posted by stumpy This is a relaxation factor for the loads (force, displacement, heat flow or temperature) passed between ANSYS and CFX. The default means that 75% of the change in the load will be passed. A value of 1 means no under-relaxation - this is what I usually use.
Hi, I have also some doubts in this topic.I am around a FSI problems and I thought that this parameter settings could help me, that is the why I am also trying to understand it.
When we leave the relaxation set to 75%, only 75% of the load will be transfered from CFX to ANSYS and vice-versa, PER STAGGERED ITERATION? I mean, if I set lower relaxation factor, i have smaller load transfers, but at the end, after some staggered iterations, 100% of my load is transfered? Am I right? Is it ok to think that? Bigger under relaxation needs more staggered iteractions?
If I use the relaxation parameter set to 1 (100%) I make my simulation more instable. And sometimes it crashes easier then setting under-relaxation, because the full load is transfered in the same iteration. Setting under relaxation, the load transfer happens gradually. Otherwise, if the relaxation limits my load to a percentage of the real load, the result of a simulation will never fit to reality.

Thank you.

 May 14, 2010, 11:10 #5 New Member   Daniel Paukner Join Date: Apr 2010 Posts: 17 Rep Power: 7 Hi, i am trying to perform a steady state aerothermoelastic simulation in a hypersonic flowfield. So far so good, BUT: In order to keep the FSI from crashing when commencing the second stagger iteration, i need to use very low under relaxation factors (down to 0.1 or 0.05) Now my problem is, that the results obtained don't seem right. My deformation with two way fsi is significantly smaller than with one way fsi (cht -> structural) also, if i take the pressure and temperature distribution from a converged FSI and apply them as loads to a structural solution, i get more reasonable results, as the calculated deformation is bigger than with the cht-structural. So could someone confirm what the under relaxation factor does? Do i only transmit this percentage of the load itself or does it just affect the CHANGE in loads? I hope you can help me, Daniel

 May 14, 2010, 13:09 #6 Senior Member   Join Date: Apr 2009 Posts: 516 Rep Power: 12 smagmon's assumption was correct. With 0.75, only 75% of the CHANGE in load is included PER STAGGER. So as you iterate within a timestep you get the full load. If you assume the load is constant within a timestep, then after 3 staggers you get: 1 - (1 - 0.75)^3 = 98.4% of the change in the load. As you reduce the under relaxation factor you need to increase the number of staggers to get the correct solution. With a value of 0.1, even after 20 staggers you only get about 88% of the true change in load. Rather than using under relaxation, there's a much better way increase stability by using source term coefficients. It's a little involved, but Ansys support or their FSI training class would cover this.

 May 14, 2010, 23:36 #7 New Member   Daniel Paukner Join Date: Apr 2010 Posts: 17 Rep Power: 7 Thanks for the clarification stumpy. Since i am transmitting forces and heat fluxes, i guess i would have to add source terms to the energy equation as well, right? I guess i'll just run with the long convergence for now, as i only have 30 days left and the simulations is very likely converge within this timeframe. But if i end up with some time left, i'll have a look at the source terms. At least it will make for a good proposition for further investigations Last edited by Pocket; May 15, 2010 at 02:53.

 May 17, 2010, 17:13 #8 Senior Member   Join Date: Apr 2009 Posts: 516 Rep Power: 12 That's correct, you'd need source term *coefficients* (not actual source terms) for energy if the heat fluxes were giving convergence problems. Note that if heat fluxes were fairly stable and forces were not, then just increase the under relaxation factor for the heat fluxes only, then use a small relaxation factor (or source term coefficients) for forces.

 August 29, 2012, 13:06 How to tackle error in FSI convergence #9 New Member   abdul khader Join Date: Dec 2010 Posts: 19 Rep Power: 6 Dear stummpy, thanks a lot for making more clear clarifications in load transfer and relaxation factor. I have a new prob. even after using timestep of 0.005 for total time of 0.8sec, and under relaxation factor of 0.2 with max. stagger iterations of 4 (load transfer comes to around 0.9984). When solu reaches around 0.72 sec, i get an error saying "Preconditional conjugate gradient solver error level 1, possibly the model is unconstrained or additional iterations may be needed. try running setting the multipier MULT on the EQSLC command to greater than 1(but less than 3.0)". Please suggest how to tackle such problems.

 August 11, 2015, 20:52 #11 New Member   wanglingjun Join Date: Jan 2015 Posts: 3 Rep Power: 2 hello，for one way and two way in fluid and solid interaction ，under relaxation factor is set different value？ thank you

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