Adaptive Time Stepping and Physical TIme
 

Hi All

I have a doubt regarding the Physical time of the simulation. In my CFX log .out file I can see something called simulation time within each Iterations and it has 1e-3 and 1e-5 two values and I am not sure what exactly it is !! And also I am using adaptive time stepping so it shows current time step and next time step.

A picture is word a thousand words ?

You mean

SIMULATION TIME: 1.00E3-03 (1.00E-5)

The first number is accumulated (global for all runs) simulation time, second number in parenthesis is accumulated (current run) simulation time for this run.

Hi
So this is how it looks..And how will I get the physical time as I want to see how many times the flow has passed by the domain. And since its adaptive time stepping the time step size is different at each iterations.

Have a look at the solver variables in the CFX reference guide. That describes all the available variables to track these things.

At the time step shown, your simulation has advanced up t = 1.6725E-03 [s] since you started the original simulation, stopped and restarted (possibly several times).

For this specific restart, the simulation has advanced only 1.8882E-05 [s]

Above I have assumed you have not changed the time solution units; therefore, time is written in [s].

So how can i check the real physical time ? I just want to check how many times my flow passed by the domain !!

I do not understand your question.

As far as I understand the simulation time is the physical time of the calculation. For a standard transient run, what other meaning can it have?

There is the CEL variable (t), or (Time) and you can a monitor expression and see it is progressing, or create an expression that uses it to compute the quantity of interest.

So that means..my real time is that simulation time what we see..so my flow inside the domain was passing by so much time. For example to make it clear..I have my domain 1m long and inlet velocity of 15m/s then for the flow to go through starting from inlet to outlet it takes 15 seconds ..in this case to check the LES result I should atleast have my flow 3 to 4 times pass by the domain that means 15*3 = 45 real seconds... So I was asking this how to calculate this real flow time..does cfx give that information ? For each iteration it might take 1e-5 seconds but am asking about the real time and not the computation time. Did I made you clear... ??

It appears you are doing a simulation with translational periodic boundaries for inlet and outlet. If you want to accumulate time as you suggest then the flow is no longer periodic, so these boundaries are not suitable.

But based on what you are trying to do, can't you just say a typical residence time is 15s, so if I run it for 45s then on average the fluid will have gone through my domain 3 times. It is approximate, but isn't that good enough?

No I am just doing a pipe flow with pressure inlet and outlet . But its an LES so I want to run it for some physical time which is equal to 5 times the flow from inlet to outlet so that I can consider my results to be meaningful (This was based on some paper that to consider your LES results the flow have to pass through the domain 4 to 5 times) And my channel is 250 mm and inlet velocity is 2 m/s according to this my simulation should run for 1 to 2 seconds real time..This is what I was looking for in the simulation. And since my time step is adaptive one..I cant tell exactly till what iternation number i can run it to get real time of 2 seconds

Then just run your simulation for a specified time, not number of iterations. So just run it for 2s total time.

Oh I dint know thats possible but is it possible when we restart the case every 24 hours ? Will it restart from the previous time ?

Restart what? What as 24 hours got to do with it? I have no idea what you are talking about.

Please explain your questions if you expect us to have any hope of answering it.

Sorry that you couldnt understand my question. Basically when I am running a computation on cluster which is limited to 24 hours as the job time, the simulation will stop after 24 hours. So when I am doing LES I need to restart my simulation from the current iteration further for 24 hours. So basically I create a .res file and then initialise the simulation with the .res file and then the simulation runs for 24 hours. So in this case the iteration starts from the current one. So when I give time lets say in 24 hours if my simulation ran for 0.75 seconds (Physical time) and I need to restart it from 0.75 seconds.Will it do that or not was my question.

If you set the simulation to run for 10s simulation time and you stop it at 7 seconds, then if you restart it using the previous simulation as an initial condition it will run the remaining 3 seconds to give a total simulation time of 10s. The result should be almost indistinguishable from if you ran it as a single run - almost..... (Some features do not restart perfectly and will result in a kick to the results. This kick is insignificant for most cases).

Yeah now you got it..Exactly...Thanks a lot ...I was guessing the same.. But thanks a lot.

Furthermore you can see the time you simulated already (total time simulated) in the picture you sent. It is the "simulation time". The number in the brackets is the physical time simulated in your current run. The value right next to "simulation time" is the total time you already simulated over all your 24 hour runs.