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O.Kaario August 18, 2000 08:37

Mean temperature with moving piston
Hi, has anyone computed engine case with moving piston and combustion, and plotted the mean temperature during the exhaust stroke (exhaust port/valves not yet open) ? I have obtained interesting some point during the exhaust stroke the mean gas temperature starts to rise!?

Hassaneen August 18, 2000 10:24

Re: Mean temperature with moving piston
I've just finished the compression stroke and I don't yet have any concrete suggestion but check the boundary condition, heat transfer process, specific heats and entropy change. By the way, in my case, when the code start to Activate the first layer in the expansion stroke it gives an error of negative density in more than 100 cells and terminates the job. Do you have any clue for this. Thanks.

Hassaneen August 21, 2000 08:05

for O.Kaario
Dear Kaario, please read my previous message because I have a question for you. Thanks

O.Kaario August 21, 2000 09:46

Re: for O.Kaario
I'm sorry but I do not know what might have cause the negative densities. I would first check the cell addition/deletion routines as well as the *.commands file (vertex movement).

Hassaneen August 21, 2000 10:32

Re: for O.Kaario
I have checked the events routines and the .cgrd file several times and I found nothing wrong with it. It made it safely until 189 degrees crank angle but I don't know what happen when it activates the first layer in the expansion stroke. Do you use (step) or (ramp) in the Transient module for the load step??. Also, do you control the relaxation factors?? Thanks any way for your help. =Ahmed=

Hassaneen August 22, 2000 03:23

Re: for O.Kaario
would you please send me your EGRID routine (case.cgrd) to see if I have any thing wrong in my routine. Thanks =Ahmed=

Hassaneen August 22, 2000 12:13

Re: for O.Kaario
Please Kaario, if you can forward me your EGRID (.cgrd) routine . Thanks

James Johnson August 22, 2000 19:56

Re: Mean temperature with moving piston
You may find that there will be no problem if you restart the run before it crashes and you decrease the time step. Decreasing the piso underelaxation could also help.

Hassaneen August 23, 2000 02:34

Re: Mean temperature with moving piston
James, thanks for the suggestions. I've tried to decrease the pressure correction under-relaxation so many time but it seems like it doesn't accept it and go back to the default when you use the Transient module (case.trns).

Hoimyung Choi August 23, 2000 05:23

before activating layers.....
If STAR-CD event user want to activate cell layers, he should first deactivate coresponding layers in reverse-activating order. So, some negative time events are needed. Did you know that?

Hassaneen August 23, 2000 05:52

Re: before activating layers.....
all these cells were deactivated during the compression stroke.

O.Kaario August 23, 2000 06:31

Re: for O.Kaario
Here is the *.commands file (the *.cgrd file only reads 'ifile *.commands')

evfi conn file.evn

evflag prep off grid uparm newxyz

evlo upto event evex

cset news cran 1 70801 1200

cset subs active

*get live ncset

vgen live + 1 -1271 76261 77491 41 0 0 ypst / live 0

*defi noexec

vgen live + 1 -1271 76302 + i 77491 + i 41 0 0 ypst / live 0


*set i 0 1

*loop 1 40

cset news deact

vset news cset

vmod vset f f ypst

Hassaneen August 23, 2000 08:09

Re: for O.Kaario
Thank you very much for the file. As a matar of fact I'm using the other command found in tutorial 13.1 but I will try to use yours. Do you have any recommendition for the relaxation factors, the differencing schemes or the maximum number of cells to be Activated or Deactivated. Thanks

O.Kaario August 23, 2000 09:08

Re: for O.Kaario
I have used PISO relaxation 0.7 - 1.0 (all have worked nicely (!)) and MARS for all variables. I have left some ten (horizontal) cell layers to the mesh at tdc.

Hassaneen August 23, 2000 09:42

Re: for O.Kaario
Excuse me, one more and may be last thing: the length of the time step (dt) does it have too much effect in the stability? in my case it's equivalent to one degree crank angle whatever the speed is. Thanks

O.Kaario August 23, 2000 09:59

Re: for O.Kaario
I have used 4e-5s dt which in my case gives appr. 0.5 degree stepping.

Hassaneen August 23, 2000 11:48

Re: for O.Kaario
in my case, the vertices at bottom of the 1st layer at BDC follow the YPST up to the TDC, is that correct??.

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