CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   STAR-CCM+ (http://www.cfd-online.com/Forums/star-ccm/)
-   -   reversed flow on faces limited in 643 cells in region out (http://www.cfd-online.com/Forums/star-ccm/68429-reversed-flow-faces-limited-643-cells-region-out.html)

emanuele September 19, 2009 06:10

reversed flow on faces limited in 643 cells in region out
 
Hello everybody,
i am a newby and i dont know if introducing myself somewhere is required.
Anyway i am 27years old marine engineer from italy approaching for the first time on cfd planet.
I am asking for your help because any boat model i am trying to validate with cfd,return a message reporting what stated in the title of this thread "reversed flow on faces limited in 643 cells in region out"...
I tryed to change volume control dimensions up to 2,5 time boat length,width and depth...but the message won't disappear....although performing grid parameters of my multiphase.

So now i am trying with a second order solver...and the number of faces affected seem to get smaller iteration by iteration.
Can anyone explain me what's the meaning of the above mentioned message and what's the smartest solution to fix similiar problems without affecting solution and its transient? :confused:

i used velocity inlet and pressure outlet
http://img35.imageshack.us/img35/2462/mesh.th.png
Many thanks in advance

Maddin September 21, 2009 11:22

Check your conditions on the outlet boundary.
Sometimes it's normal because the flow reserves before it gets a straight flow.
When I use a Pressure Outlet as a Inlet I get this on every iteration ;)
To check the direction I use vectors on the boundarys.

emanuele September 21, 2009 11:29

Quote:

Originally Posted by Maddin (Post 230026)
Check your conditions on the outlet boundary.
Sometimes it's normal because the flow reserves before it gets a straight flow.
When I use a Pressure Outlet as a Inlet I get this on every iteration ;)
To check the direction I use vectors on the boundarys.

Thank you for your support....but i imagine reversed flow could affect convergence!isn't it?
I would like to try a polyedral mesh in order to check if multifaces volumes can manage turbulent flow in a most accurate way...actually i m going with trimmer and its refinement on free surface..

Maddin September 21, 2009 12:26

Is your problem 2D or 3D?
When this is a 2D problem then your region is wrong.
Show more pictures!

emanuele September 21, 2009 13:58

my problem is a 3d problem and it don't wanto to converge!!I checked several times what should be wrong but i discovered nothing....
i performed mesh diagnostics and everything seems to be ok,i enlarged my continuum but nothing....solution keep going in a wrong way.I'll show more pic as soon as i can!
i performed some trials without degrees of freedom in order to be sure it is not a prblem related to initial condition an moments of inertia...but it's the same..
:(

Maddin September 21, 2009 15:20

What settings to you use? Wall treatment? y+?
Bring more facts plz :)

emanuele September 22, 2009 10:19

ok,at the link below you can find my summary report :)...
thank you for your big interest!!

http://www.megafileupload.com/en/fil...umem-html.html

Maddin September 22, 2009 10:50

Look at your courant number, maybe you have so choice a smaller time step.

emanuele September 22, 2009 11:31

Quote:

Originally Posted by Maddin (Post 230150)
Look at your courant number, maybe you have so choice a smaller time step.

thank you!!i will manage to optimize it!;)

Maddin September 22, 2009 11:39

Tell me when it works, picture looks very nice. Could be a really nice animation :D

emanuele September 25, 2009 05:27

ok...error found...i was using velocity modulus..At inlet flow direction is "inlet to outlet"...and you do not need to specify it using "-" or "+"...just use modulus of velocity
In continum you should specify everytime the direction of the flow....i was using positive velocity at inlet and positive velocity in continuum bu the direction of my cordinate system was with x axis directed in the same direction of boat advancing motion,so,to define velocity in continum i must use a negative value because water come forth my boat...differently i was experiencing vertices waves...the hell..:)

Maddin September 25, 2009 06:17

When I understand you right you didn't had seen/known that the velocity in the initial conditions is according the laboratory coordiante system?

Ok when I look at your picture know I see it.
X-axle shows to the left so you must work with negativ flow when the flow direction is from left to right :)

emanuele September 25, 2009 06:22

Quote:

Originally Posted by Maddin (Post 230511)
When I understand you right you didn't had seen/known that the velocity in the initial conditions is according the laboratory coordiante system?

Ok when I look at your picture know I see it.
X-axle shows to the left so you must work with negativ flow when the flow direction is from left to right :)

yes!but at inlet,flow direction is utomatically defined inlet to outlet..isn't it?:o

Maddin September 25, 2009 06:43

Under Physics Conditions of the Boundary you can set the flow direction :)
You can also give a vector for the flow or "normal to boundary" or a angle.

emanuele September 25, 2009 06:47

ok...the solution is:
be sure x axys si oriented as flow motion direction :) ...it is easyer...


All times are GMT -4. The time now is 22:40.