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 June 16, 2018, 13:39 Solution does not advance with time: steady state too soon? #1 New Member   Noix Join Date: May 2018 Posts: 13 Rep Power: 8 Hello again. I'm trying to simulate a cylinder which fills ups with compressible gas. Selected ideal-gas as density method and I'm using the Density-Based Solver with a Courant number of 1. I managed to get my simulation up and running and the first time step looks great in terms of velocity and pressure, just what you would expect: However it does not seem to keep going after sucesive time step: the residuals don't go up like they should bewteen time step: they just keep going down with no big differences in the profile (exactly the same every time step!) Why does that happen? How can this be the steady state with such huge differences of pressure and speed?

 June 16, 2018, 15:05 #2 Senior Member   Join Date: Dec 2016 Posts: 152 Rep Power: 10 Hi, we are going to need a few things to begin to troubleshoot your problem. First: can you post a picture of the residuals with indicators of where the next time step is? It would be really cool to see what you are talking about with the continued trend. Second: if this is a transient simulation, what is your step size, and what happens in between each step? I know you said you are simulating filling, but how is this being filled? Are you moving the piston each time step, or is it stationary and you are just filling the cylinder? Third: this isn't as important right now, but it could be, what does your mesh look like? Noix_V likes this.

 June 16, 2018, 17:21 #3 New Member   Noix Join Date: May 2018 Posts: 13 Rep Power: 8 Hi, thanks a lot for replying. I should have explained my setup more. Basically, all I'm doing is filling a gas tank by using a pressure-inlet boundary condition. I've tried several setups to do this problem, in that last one I also used a high-pressure reservoir and watched how it flowed from the large reservoir to the small cylinder. I don't have a picture of the residuals right now (I'll upload it later if you think it's neccesary) but it's really simple: they just don't change when the next time step begins. However, there IS a small increase in the overall pressure (the blue areas), but I don't know if that's the real result. I don't have any references either. I am using variable step sizes. At first I select an extremely small one (0.0001) so the initial profile develops. When I'm using that step the residuals are like they should be (increasing when starting the next time step), but then they just decrease in time with no changes. I try to increase the step at that point but it does very little. These are the residuals in the early stages (small time step). https://puu.sh/AGwjj/7cf81fdfc0.png It takes a while but it stabilizes. Eventually. Here's the mesh. Honestly I don't know if I should refine the area where the initial flow starts. https://puu.sh/AGwh4/705862c9f3.png https://puu.sh/AGwhh/ab6f462703.png Thanks a lot for your help.

 June 17, 2018, 11:35 #5 New Member   Noix Join Date: May 2018 Posts: 13 Rep Power: 8 Raider: thanks a lot for helping me again! I'll try to answer point to point. First: The setup has no outlets, just an inlet. That's why it should be increasing in pressure constantly. I don't know how that affect what you said before. Second: point taken. How fine elements should be in general? Third: Flow should be laminar. There's a porous zone on the bottom cylinder and should be modelled with Darcy's law. Dimentions are a radius of 100 mm and an entrance of 15 mm, with 200 mm in height. It's fairly small. Fourth: It is. My professor asked me to do this problem on 3D, because then I should put aditional stuff in the geometry, but yeah, I think it would be better to do a 2D problem as well. I'll talk with him about that. I think it's not necessary at all to have the reservoir at the top, I just need to simulate the filling of the small cylinder, which should be possible by not using the big one at the top. Regardless, I would really like to do this 3D, or is it that hard to do? (honest question) Fifth: This is not exactly the same simualtion, but it's the same situation. (I'm horrible at this, right?) https://puu.sh/AGWH9/334da27d19.png Sixth: 10^-3. You're right. Seventh: About 5000. it does not converge. Bonus: Yes, because I'm using the ideal-gas model the energy equation is mandatory. Thanks a lot for your time! I would really want to know what considerations should I have with the mesh on a no-outlet situation, if you can reply that.

 June 17, 2018, 11:45 #6 New Member   Noix Join Date: May 2018 Posts: 13 Rep Power: 8 I really think I should tell you about the whole problem I'm trying to do. I'm trying to simulate hydrogen adsorption on a porous cylinder. My plan is to divide the geometry into two areas, an expansion zone (not porous) and a porous zone. I already have an UDF written for that purpose (and tested, it works) on a basic cylindrical geometry that SOMEHOW (I have no idea why) convergerges perfectly. This would be my mesh, is it acceptable? Only an inlet at the top, axis at the left side and the others are all walls. https://puu.sh/AGW7u/c44b0cfd7c.png Edit: i tried this. It diverges automatically (error at AMG solver) no matter how I set Courant, the time step or the URF. Thanks a lot again.

 June 21, 2018, 00:01 #8 New Member   Noix Join Date: May 2018 Posts: 13 Rep Power: 8 (Sorry for late reply!) Thanks for your help Raider. I'm going to try some more setups with the steps you've given me and I'll report back with what I get!