CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   STAR-CD (http://www.cfd-online.com/Forums/star-cd/)
-   -   error number 76 (http://www.cfd-online.com/Forums/star-cd/114491-error-number-76-a.html)

Tattaraaaaaaa March 12, 2013 03:25

error number 76
 
Hi All,

I just started simulating engine ports-flow with starCD. I made the model in es-ice, I have set-up all the boundary and solvers. The model hasnīt shown any mesh problem during the mesh check, but while It was running for the first time an error occurred:

error 76, negative densities fond at more then 100 cells.

Since It is my first real run, after the tutorials does anyone know how to overpass this error?

cheers

Ed_89 March 12, 2013 09:58

Have you tried using a debugger ?? I'd check the User guide for more information

Tattaraaaaaaa March 12, 2013 10:38

answer et Ed-89
 
Hi,

right now I have seen many debugfiles in the output of the run which crashed so I think is due to mesh problems. As a metter of fact I also have checked the worning and errors present in the star.info file of output and I have found bad splines definition in my model!

I guess the geometry has to be done more precisely with less splines.

Cheers

JOEROCKER March 22, 2013 05:29

Hi,
Even i get the same problem of negative densities. How to solve it? I checked the manual, it says "Check the result shown in the .ccmp file at the time the calculation crash occurred. Look for abnormal velocity magnitudes, or very high value of absolute pressure or temperature. The quality of cells associated with such abnormalities is generally not very high. You can also look at the .info file and search for cells associated with a high Courant number. Locate these cells in the es-ice model and fix the mesh." How to identify these cells, thje star.info file seems to be too big. Can u please help?

Ed_89 March 22, 2013 05:37

How far in does your simulation run ? what are you trying to simulate ?? if you can try and produce a vector plot in pro-STAR to conduct a visual check for velocity problems, if not try and check the mesh in the create results panel of es-ice, failing that you can try reducing the pressure under relaxation and your time step, if you had a simulation that run conduct post-processing on what little data you have to identify the problem (temp, velocity, pressure).

Because without having your files to look at I can't do any of this for you.

Ed_89 March 22, 2013 05:39

It could also be a mesh problem.. take your time and check your boundary conditions and initial conditions to ensure you have not made any silly mistakes there and let me know how you get on.

Ed

JOEROCKER March 22, 2013 05:51

Hi Ed, i'm trying to simulate an IC engine with methane as a fuel. The simulation starts at 348 degrees of crank angle. The simulation stops at 872 degrees. I had given an initial time step of 0.1 and later i tried to increase it to 0.05.

Ed_89 March 22, 2013 06:05

Have you read the best practices file ?? if you follow that as to your initial conditions etc, you should be fine, try post processing the data and check the mesh as I said at say 871 deg CA to 872 deg Ca for an interval 0.1, if the mesh has problems there you will spot them. If not drop your pressure under relaxation.

Ed

skylent March 22, 2013 07:30

if your engine speed is high enough, timestep=0.1deg, is always ok. maybe the mesh around 872 not very good. you can decrease the timestep to a half to decrease the courant number.

Quote:

Originally Posted by JOEROCKER (Post 415652)
Hi Ed, i'm trying to simulate an IC engine with methane as a fuel. The simulation starts at 348 degrees of crank angle. The simulation stops at 872 degrees. I had given an initial time step of 0.1 and later i tried to increase it to 0.05.


Tattaraaaaaaa April 12, 2013 08:37

Hi if can help for this discussion, even if I am very late in replying, I will describe you what I found.
In my case (I simulate flow into ICE ports during compression stroke) I have looked into the debug files and there where too many splines defined at the ports and valves inlet. For this reason I had very small cells creating problems.

Hope this information can help!

ktkim September 30, 2013 05:29

error 076 : Hi there, can you help me? i'm beginner.
 
Hi, I have problems error 076:negative density~ as you have.

I use v4.12, v4.16 and v4.16(esice)-v4.12(prostar)
and make 35~70thouand cells trimming models.

simulation of all version and model occur error 076 at any timestep(same case, condition, model ~ but randomly stop by error076).

So im sick of and very angry T.T

recently I found some to check error 076 in prostar before simulation.
Prostar program > Grids > Check grids > Face area

(as we check negative volume first when we make model.)

this checking 'Face area' show 'error 76 : Cell faces with very small surface areas found in the model.' in Error Summary.

How can I solve this problem?
today I try to simulate time step=0.05 at all CA.

Help, and I will reply too.


All times are GMT -4. The time now is 20:11.