- **CFX**
(*http://www.cfd-online.com/Forums/cfx/*)

- - **sliding grids
**
(*http://www.cfd-online.com/Forums/cfx/19741-sliding-grids.html*)

sliding grids
Hi,
I have a problem when I try to use sliding grids; by the way, the simulation of the centrifugal pump produce high residual mass. this cause the end of the simulation. have you an idea ? Thank you very much! Giuseppe |

Re: sliding grids
It is difficult to understand your problem at this stage. Would you be more generous with your information. When does this occur, immediately or...?
What are your: -speed -mesh -time step -underelaxation factors -descritization schemes (spatio-, temporal-) etc..?? regards Nyatoto |

Re: sliding grids
Hi, Nyatoto
The simulation diverge immedyately, after 7-8 iterations. I have 1400 rpm , my model of rotor has structured mesh with 4 elemet for each arc with an angle of ten degrees. the fluid was water I used : >>FIXED TIME STEPPING TIME STEPS #NSTEP * #DT with #CALC PI=3.1415926; DEG=PI/180.0; #ENDCALC /* ... ASTEP IS GRID MOVEMENT ANGLE PER TIME STEP ... */ #CALC NSTEP = 144; MAXIT = 50; CRIT = 1000; ASTEP = 2.5*DEG; #ENDCALC /* ... WORKING VALUES ... */ #CALC RPM = 100; OMEGA = RPM*2.0*PI/60.0; REVOLT = 2.0*PI/OMEGA; DT = REVOLT*ASTEP/(360.0*DEG); #ENDCALC i used default under relaxation factors and the options: THREE DIMENSIONS BODY FITTED GRID UNMATCHED GRIDS CARTESIAN COORDINATES TURBULENT FLOW ISOTHERMAL FLOW INCOMPRESSIBLE FLOW TRANSIENT FLOW TRANSIENT GRID furthermore i have: >>FALSE TIMESTEPS U VELOCITY 0.8 V VELOCITY 0.8 W VELOCITY 0.8 :>REDUCTION FACTORS K 0.55 EPSILON 0.55 :>DEFERRED CORRECTION K START 0 K END #MAXIT EPSILON START 0 EPSILON END #MAXIT I ran two simulations with the following boudaries conditions: 1) inlet + outlet with inlet velocity of 3,5 m/s 2) simulation all circuit No inlet No outlet I hope for some help Thanks in advance! Giuseppe |

Re: sliding grids
Hi Giuseppe,
Is your RPM 1400 or 100? Please check your FC file. Let me first assume that everything is right with your geo file (BC, grid,etc). Your false time steps appear to be too high, it is generally a good practise to start with low false time steps (if you need them). In your case I would like to know what your under relaxation factors are! These too can initially be set very low, sometimes the default ones may be too high, expecially if your flow is highly turbulent. I assume you are using k-e model, which will INITIALY give you huge errors if you have a flow with high swirl (I suspect your model has this) and the solver ambitiously tries to get fast solution with high under relaxation factors. Another thing is the discretization schemes: You may start with, say upwind or Hybrid. For time scheme I prefer fully implicit. I suggest you try under relaxation factors in the range of u,v,w=0.1-0.3 or lower, P can be as high as 0.5 or 0.7, k,e about 0.3-0.5. You can increase these later as you wish. similarlt try smaller times steps to begin with. Lastly, select your out put monitor position wisely. Hope this helps Nyatoto |

All times are GMT -4. The time now is 14:05. |