CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   Under relaxation factor for external coupling (http://www.cfd-online.com/Forums/cfx/67853-under-relaxation-factor-external-coupling.html)

dhxlxz August 28, 2009 06:41

Under relaxation factor for external coupling
 
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!

kingjewel1 August 29, 2009 11:01

Quote:

Originally Posted by dhxlxz (Post 227796)
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.:)

stumpy September 4, 2009 20:59

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.

smagmon September 7, 2009 07:37

Quote:

Originally Posted by stumpy (Post 228612)
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.

Pocket May 14, 2010 11:10

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

stumpy May 14, 2010 13:09

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.

Pocket May 14, 2010 23:36

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 :)

stumpy May 17, 2010 17:13

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.

smak.quadri August 29, 2012 13:06

How to tackle error in FSI convergence
 
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.

smak.quadri September 13, 2012 11:42

Please help:
 
Dear Stumpy,

I have been following many of the replies from your end with regard to applying source term @ interface to achieve convergence rather than using under-relaxation factor.


When I couldnt achieve convergence using underlaxation factor, so i tried using mass flux pressure coefficient of 1e-8. I used stagger iterations of 10 and mesh displacment coefficient of 10 and max coefficient of fluid flow as 5 and settling relaxation factor to 1.

I got the complete fsi transient run possible after running steady state. I used this condition to a concentric stenosis.

But when i use the same BC for eccentric type of stenosis, inspite of using several trial and error values of stagger iteration, mass flux cofficient etc. I am unable to get the convergance for steady state itself.

Please help me out, @ the earliest. also suggest is it advisable to first set the stagger iterations and then slowy vary the mass flux coefficient or how this funda works.

WANGFIRE August 11, 2015 20:52

hello,for one way and two way in fluid and solid interaction ,under relaxation factor is set different value?
thank you


All times are GMT -4. The time now is 10:20.