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-   -   Time scale factor and physical timescale (http://www.cfd-online.com/Forums/cfx/105766-time-scale-factor-physical-timescale.html)

saisanthoshm88 August 8, 2012 13:49

Time scale factor and physical timescale
 
Could some one please let me know when to use the time scale factor of the auto timescale and when to change to physical timescale when the intention is to accelerate convergence by changing the time scale in CFX.

ghorrocks August 8, 2012 18:48

Starting a simulation with auto time scale is usually OK. If you have convergence problems then make it smaller. Once the simulation is converging nicely then you can start increasing the time scale size to accelerate convergence.

bratzinger August 9, 2012 03:29

or if u want to calculate it by yourself and u know the massflow and the typical cross-section area:
v(_char)=m_dot/(rho*A) -> t_char=length/v(_char)
u should get roughly the same (depending of the complexity of your geometrie the same value or at least ~ the same power of tchar)

with heat flux i often use e.g. 100s for phys. timescale - a time where the casing in my project should easily got the right temperatures ... in my steady state simulation (hope i understood that right that u can avoid transient influence through initial conditions in that domain if u set it suficiantly high?)

for the energy equation i often use large timescale factors(100, 500 or more) to accelerate convergence

saisanthoshm88 August 22, 2012 04:00

Hi Glenn,

So if there is an auto time scale of 9 [s] and if I wanted it to be 0.9 [s] then does it make the same sense with the two approaches viz..,:

1. Setting the Time Scale Factor to 0.1 in the auto time scale.

2. Directly switching over to physical time scale and setting it to 0.9 [s]

ghorrocks August 22, 2012 06:46

Both approaches are equivalent, they both set the solver to 0.9s. Do which ever is easiest for you.


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