Force fluent to hold only last timestep

ansys_matt · August 23, 2014, 12:05

Hello,

I am running a transient flow problem and it seems fluent keeps every timestep in RAM, which eventually causes a out of memory problem. It fills 32 GB holding all the previous timesteps in RAM.

Since there is absolutely NO reason why fluent needs the first timestep in RAM when solving for the 1000th timestep, I would think one could tell ANSYS to only hold say, 5 timesteps at a time. I am already having fluent export the data to a file (under solution activities), but is there any way to tell fluent to NOT hold all timesteps in memory while marching forward in time?

Thanks

ansys_matt · August 23, 2014, 20:36

Does anyone know of a more active forum for discussing ansys fluent?

If not, how about somewhere to go where experts help newbs? I have posted 3 times and gotten no response whatsoever!

Thanks!

sescobar · August 24, 2014, 09:52

Hello,
Based on my experience with Fluent it does not store all the previous time steps in RAM. Depending on your temporal discretization scheme it will store the primitive variables for one or two previous time steps.
What is the set up of your simulation? Number of cells in the mesh? Do you have particles in the flow? How about species and chemical reaction?

ansys_matt · August 24, 2014, 22:07

Hello sescobar,

I am running methane-air 1 step combustion, no particles, 30000 cells. My task manager showed fluent adding about .1GB every iteration until ram was full. Once ram was full the solver quit with an unable to malloc error.

So what I did was add 5 GB of virtual memory in windows 7 (it previously had no page file at all). Next time I ran the exact same case, the 32 GB filled up just like before, but then suddenly dropped to 3 GB while the solution kept going. It finished without incident 10 hours later.

I changed absolutely nothing inside ANSYS, so I have no idea why this happened. Apparently it does store each time step (how else could we explain going from a base 3GB to 30 GB while it was running), but as long as there is virtual memory it just dumps the ram when full. Strange.

August 23, 2014, 12:05	Force fluent to hold only last timestep	#1
ansys_matt Member Join Date: Feb 2013 Posts: 60 Rep Power: 13	Hello, I am running a transient flow problem and it seems fluent keeps every timestep in RAM, which eventually causes a out of memory problem. It fills 32 GB holding all the previous timesteps in RAM. Since there is absolutely NO reason why fluent needs the first timestep in RAM when solving for the 1000th timestep, I would think one could tell ANSYS to only hold say, 5 timesteps at a time. I am already having fluent export the data to a file (under solution activities), but is there any way to tell fluent to NOT hold all timesteps in memory while marching forward in time? Thanks

August 23, 2014, 20:36	Any more active forum?	#2
ansys_matt Member Join Date: Feb 2013 Posts: 60 Rep Power: 13	Does anyone know of a more active forum for discussing ansys fluent? If not, how about somewhere to go where experts help newbs? I have posted 3 times and gotten no response whatsoever! Thanks!

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August 24, 2014, 09:52		#3
sescobar New Member Join Date: Jun 2011 Posts: 22 Rep Power: 14	Hello, Based on my experience with Fluent it does not store all the previous time steps in RAM. Depending on your temporal discretization scheme it will store the primitive variables for one or two previous time steps. What is the set up of your simulation? Number of cells in the mesh? Do you have particles in the flow? How about species and chemical reaction?

August 24, 2014, 22:07		#4
ansys_matt Member Join Date: Feb 2013 Posts: 60 Rep Power: 13	Hello sescobar, I am running methane-air 1 step combustion, no particles, 30000 cells. My task manager showed fluent adding about .1GB every iteration until ram was full. Once ram was full the solver quit with an unable to malloc error. So what I did was add 5 GB of virtual memory in windows 7 (it previously had no page file at all). Next time I ran the exact same case, the 32 GB filled up just like before, but then suddenly dropped to 3 GB while the solution kept going. It finished without incident 10 hours later. I changed absolutely nothing inside ANSYS, so I have no idea why this happened. Apparently it does store each time step (how else could we explain going from a base 3GB to 30 GB while it was running), but as long as there is virtual memory it just dumps the ram when full. Strange.