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June 24, 2011, 18:02 
sloshing mean kinetic energy not constant

#1 
New Member
Marco S.
Join Date: Oct 2010
Posts: 8
Rep Power: 8 
Hi,
I am analysing linear sloshing. I have the problem, that the mean kinetic energy does not get stable. I use for the momentum advection:second order monotonicity preserving. Can I also use first order? What is the difference and how can I get a stable simulation. I wait for your help! Thank you 

June 27, 2011, 20:13 

#2 
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michael barkhudarov
Join Date: Mar 2009
Posts: 332
Rep Power: 10 
First and second order advection methods should both be fine in this case.
I assume it is the timeaverage mean kinetic energy that does not settle to a steadystate, right? The instantaneous value would oscillate because of the sloshing movement. Is the total fluid volume more or less constant? 

June 29, 2011, 04:32 

#3 
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Marco S.
Join Date: Oct 2010
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Yes that is right. The time averaged mean kinetic energy does not settle to steady state.
I tryed to change a lot of values in Flow. I found out, that the change of the time step controls values brings the simulation to steady state. My values are: a) Initial time step: 0.0001 Minimum time step: 0.0000001 Maximum time step: 0.002 With this parameters the simulation do not reach steady state b) Initial time step: 0.001 Minimum time step: 0.00001 Maximum time step: 0.05 This values brings the simulation to steady state! I dont know which values are the best for my simulation. My simulation is with noviscosity, noturbulence, no heat transfer. It refers to the potential theory. What do you mean by total fluid volume? I have a constant fill level. I also want to ask, how can I see that my simulation was a succes? By the time aerages mean kinetic energy? Another question is: I chose SI Units and get for example for the Force 0,0040. Is that Newton? Because that is too small. Normally it has to like 5 Newtons. Thank you very much for helping me! 

June 29, 2011, 20:08 

#4 
Senior Member
michael barkhudarov
Join Date: Mar 2009
Posts: 332
Rep Power: 10 
Yes, the FLOW3D force output would be Newtons in SI units. Not sure why it is so small. It should basically be equal to the weight, right?
Adding viscosity would help to reach steadystate. Even if your fluid level is constant, the total volume may change due to numerical errors. FLOW3D output the total fluid volume as a function of time. Make sure it is more or less constant. En error of <1% is normal. To check the correctness of a simulation you can check 2d and 3d plots, forces, slosh amplitude and so on and make sure it makes sense. Other than that, if the solver is converging, no error messages pop up from the solver, then you problably have good results. using constant time step size may make results more accurate, but using a smaller one does not always make sense. If you do use a smaller time step, make sure to tighten pressure itereation convergence by setting EPSADJ=0.1 or 0.01 (the default is 1.0). The minimum time step size does not have any effect on the results. 

June 30, 2011, 05:57 

#5 
New Member
Marco S.
Join Date: Oct 2010
Posts: 8
Rep Power: 8 
Hallo,
the Force is in SI Units and I found my mistake :) It was my density. I had the default setting of 1. And water has 1000kg/m3. My simulations are now stable. The problem was the too small initial time step size. I had dt=0,00001. Now I have dt=0,0001. I have also change max.time step from 0,002 to 0,01. That works very good. My question is, does this have a big influence on the results? My way was by trial and error. I lost 1 week, but now it works great with best results. Everything makes sense. Both 1st and 2nd Order Momentum advection works fine. I had another question to the Volume of Fluid advection. I have only water with a free surface. Should I choose Split Lagarngian or One Fluid, with free surface. What do you think? I thank you a lot. 

July 1, 2011, 17:51 

#6  
Senior Member
michael barkhudarov
Join Date: Mar 2009
Posts: 332
Rep Power: 10 
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