How to reach steady state solution
 

Hello I am solving my case with unsteady state but I do not know in which time my solution reach a steady state is there anybody can guide me in this?

Sonia

Re: How to reach steady state solution
 

You should monitor several key variables (pressure, velocity) at some points and surfaces and once they reach a steady value the model is complete. For models which do not reach ss (multiphase, ..) you need to turn time averaging feature on and use the time average values. For those you need to time average it long enough to make sure the fluctuations are not effecting the time average results.

Re: How to reach steady state solution
 

Hi bashu thanks to the reply can you tell me to how can i turn on time averaging feature because I was actually use multiphase flow in my simulations?

Thank you Sally

Re: How to reach steady state solution
 

text command (for fluent 6.3, similar for prev. versions)

solve set d-s y 1 y y

or if running with gui check 'data sampling for time statistics' in the iterate window

Make sure you save the case file to be able to get the time average variables in the contour options.

Re: How to reach steady state solution
 

Hi bashu I did what you told me about activating data sampling from the command. I was made a monitor to a point near the outlet of the pipe to look at the velocity, for this monitor I chose under every list flow time and for unsteady statistics I chose mean X velocity but for report type what you think the better choice as you think ? please let me know.

Thanks Sally

Re: How to reach steady state solution
 

if you chose a point it really doesn't matter. I would choose are-weighted-average. if I understood right, you are monitoring the mean-x-velocity. if that's the case chances are that you will get an error. you need to do some iterations with the time-average on before turning the monitor on, since the mean values do not exist from beginning.

Re: How to reach steady state solution
 

Hi bashu my monitor for x mean velocity now fluctuates between (1.1607-1.1608 m/sec) and this happened when the time reach (25 sec) and still fluctuate between these values even when I let the time reach (30 sec) is this mean I reach the steady state solution? knowing that I tried with both of vertex-average and area- weighted average it give the same thing?

Thank you Sally

Re: How to reach steady state solution
 

if you plot the time average contours and you don't see sharp gradients of velocity, volume fraction and ...; your results should be steady now.

one other check is mass balance. you can look at the mass flow out for the time average period and the time average mass flow "in" should be equal to "out" for the period you are time averaging. if the mass "out" does not fluctuate, much then you should be OK

Re: How to reach steady state solution
 

Hi bashu I failed to find the time average contours on the menu display >contours so where can I find it please? Furthermore how can I estimate the value of mass flow at inlet and outlet ? did you mean by the second way of check if the mass flow rate at outlet equal to mass flow rate at inlet I reach the steady state case?

Sally

Re: How to reach steady state solution
 

you should see a new menu item 'unsteady statistics' and under that you will have the time average properties.

the mass flow in and out at any given time will not be equal in most multiphase transient cases. what you should do is to monitor the mass flow rate at the outlet and inlet and then plot them vs. time. if you time average these values over a range (outside fluent , in excel) the average values should be equal and the sum should be zero.

this means there is no accumulation of mass inside your system and what has entered eventually exited the ssytem. an alternative is to monitor totla volume fraction (which is mass if density is constant) and that should remain constant as well.

Re: How to reach steady state solution
 

Well bashu I plot the contours for time average (from unsteady statistics menu) for mean x velocity,mean pressure it appear nearly the same for the last time steps with the same limits so what this mean ?

Thank you Sally

Monitor the mass fluxes at inlet and outlet. At steady state, The sum should be zero.
If you monitor the flow variables at a point with respect to time, the conclusions/results may be misleading. For example, if you select small time step, the the difference in flow variables will not be much higher and you may conclude that the steady state has been achieved..Which is not the truth.
So, better you monitor the Fluxes (mASS FLOW RATE, kg/s).
Postprocessing>Fluxses> Mass flow rate
Hope,,,,it would be good:)