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-   -   variant of Tutorial15 (https://www.cfd-online.com/Forums/cfx/21993-variant-tutorial15.html)

jemteo January 3, 2006 03:37

variant of Tutorial15
 
Hi,

i will like to run tutorial 15 using a non-newtonian fluid instead of water. i would like to ask a few questions.

(1) my fluid changes viscosity with time, do i use a transient or steadystate setup? i notice that under steadystate, i can specificy physical timescale. does this timescale affect my viscosity? (viscosity is keyed in as an expression by myself)

(2) i tried using transient, but when running errors surfaced, stating that in transient simulations initialisation value cannot be left blank. i.e. i cannot use AUTOMATIC but have to use AUTOMATIC WITH VALUE. what are the initial values i should key in pertaining to tutorial 15?

txs in advance


test January 4, 2006 03:46

Re: variant of Tutorial15
 
Hi,

CFX uses the approach of false time step approach for steady state cases. It is similar to using an under-relaxation factor. So this value of time step will not be of any use for you if your case is transient in nature.

For a transient case, you need to specify what is the state of the system at when you start the simulations. You can get away from specifying the "initial guess" for steady state runs where the solver uses the boundary condition to apply intial guess. However for transient cases you have to specify the initial state of the system. You can assume that the system is stationary initially and thus specify velocity components to be zero. You can keep the k-e values to be automatic. The static pressure can be that specified in the domain form.

Regards, test

jemteo January 4, 2006 04:19

Re: variant of Tutorial15
 
I did run the modified tut15 in transient mode. however i have problems showing the progression of bubble dispersion within my fluid. Simulation time span =10sec, for every 1sec i requested for a output. in CFXpost, i used isosurface (geometry=volume fraction of bubbles) to view the progression. Only time step 10sec had any results, time step 1-9 showed nothing.

anyone knows why?


TB January 4, 2006 09:09

Re: variant of Tutorial15
 
Have you specified the time intervals to store transient results PRE (under output control)? If not, it will only store results at the final step.

jemteo January 4, 2006 19:27

Re: variant of Tutorial15
 
TB: yes i did specify a time interval of 1sec to store the transient results. hmmm.

TB January 5, 2006 04:38

Re: variant of Tutorial15
 
It's weird. Intermediate results are stored in *.trn file. If you don't delete these files and set things right in Pre, you should get those transient results. Do you see any other timestep when you click on "show timestep selector" in Post?


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