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-   -   Inlet Problems (http://www.cfd-online.com/Forums/star-ccm/93684-inlet-problems.html)

 Union747 October 23, 2011 23:35

Inlet Problems

Hi Everyone,

I'm fairly new to STAR CCM+, so this may be a very basic question:

I'm setting up what I think to be a very basic flow model. I've created a quarter cylinder shape in solidworks and using symmetry planes it will basically model a "marble in a wind tunnel". I have set it up to have a velocity inlet and a pressure outlet and gave it simple boundary conditions of pressure at atm and a velocity of 1 m/s. When I create a scalar scene with velocity, the corners of the inlet have a higher velocity flow and some cells aren't producing a result (are "blank" or clear colored). I'm not sure if there's something simple that can be done to fix this, but it seems like something the program should be able to handle. I am simply looking for a uniform inlet, and there are issues at the cylinder walls.

- KG

 rwryne October 24, 2011 10:50

Quote:
 Originally Posted by Union747 (Post 329116) Hi Everyone, I'm fairly new to STAR CCM+, so this may be a very basic question: I'm setting up what I think to be a very basic flow model. I've created a quarter cylinder shape in solidworks and using symmetry planes it will basically model a "marble in a wind tunnel". I have set it up to have a velocity inlet and a pressure outlet and gave it simple boundary conditions of pressure at atm and a velocity of 1 m/s. When I create a scalar scene with velocity, the corners of the inlet have a higher velocity flow and some cells aren't producing a result (are "blank" or clear colored). I'm not sure if there's something simple that can be done to fix this, but it seems like something the program should be able to handle. I am simply looking for a uniform inlet, and there are issues at the cylinder walls. Please help if you can! :) - KG

I've seen the higher velocity problem when poor quality cells were present near a boundary. Though your geometry sounds simple enough that you shouldn't have any poor quality cells, have you checked?

Trimmed or poly mesh?

 Union747 October 25, 2011 11:53

Picture

Thanks for your response! Here is a screen shot of the inlet problems that I am having. As noted before, the rounded edge on the corner is a wall, and the top edge is a symmetry plane. I'm looking for a uniform inlet of 1 m/s, and I'm not sure why these conditions are occurring in the corners. The two pictures show the same situation with two different quality meshes. It is a poly mesh and the output didn't mention poor quality cells.

Any advice would be appreciated!! Thanks!

http://i54.tinypic.com/uue1h.jpg

 sieginc. October 25, 2011 14:13

How many cells are in your mesh? It doesn't look very fine. In your scalar scene, do you have auto range selected? If you set your own scale then its possible the values for the 'missing' cells are outside the the range you selected.

 Union747 October 25, 2011 16:26

I have about 136000 cells in my current mesh, but since this is just basically me learning how to use the program for other simulations it is fine enough for now.

I've attached some more photos of the situation. You were correct in that the range was just not large enough to view the velocities in those cells, so thank you! I am, however, still uneasy about the large velocities near the wall of the cylindrical "wind tunnel". Everything around the "marble" seems to be fine..

Any thoughts on why this might be happening??

http://i56.tinypic.com/rj2i3q.jpg

 abdul099 October 25, 2011 19:49

The values plotted in the scene are the velocities in the cell centroids. The inlet velocity fixes only the velocity at the cell face (at the inlet boundary), not the velocity in the centroid.
Have a look on the inlet velocity by adding the inlet boundary to a scalar scene, and you should see exactly the velocity given as input value.

Regarding the other weird look, how many iterations did you run the model? For me it looks like you've aborted right after the first steps. :confused:
What about refining the mesh in some steps (don't overact) and look if it will get better?
You should also look for the jumps in cell size at the walls. There are some flat prism layers connected to a big polyhedral cell. Try to get the transition a little smoother.

 Union747 November 7, 2011 01:56

Quote:
 Originally Posted by abdul099 (Post 329444) The values plotted in the scene are the velocities in the cell centroids. The inlet velocity fixes only the velocity at the cell face (at the inlet boundary), not the velocity in the centroid. Have a look on the inlet velocity by adding the inlet boundary to a scalar scene, and you should see exactly the velocity given as input value. Regarding the other weird look, how many iterations did you run the model? For me it looks like you've aborted right after the first steps. :confused: What about refining the mesh in some steps (don't overact) and look if it will get better? You should also look for the jumps in cell size at the walls. There are some flat prism layers connected to a big polyhedral cell. Try to get the transition a little smoother.
Hi again,

Thanks to everyone that has responded. I ran 1000 iterations on the simulation shown above, and I'm still having issues with the inlet.

Basically, I know the velocity near the walls is really high, just not why it's so high. I've decided that I must have inputted something wrong, being that this is a fundamental problem that the program should be able to handle. Does anyone have any idea what could cause this? Am I overlooking something simple?

Thanks,

KG

 ping November 7, 2011 06:37

when running a case for the first time i always look at two section plots scenes through important flow areas (eg a section plane thru the middle from inlet to outlet) - one with velocity vectors so you can see direction and magnitude (unlike your contour plot) and the other as pressure. these will tell you lots more than your current scene. something is wrong at the wall - change it to a symplane too to test. add the inlet boundary to the vector displayer to ensure the flow direction is correct. run from scratch again and watch the vector scene flow establish. try initialising the flow domain with your inlet velocity and start again. the 'missing' cells faces are probably due to clip settings in the scene - turn this off.

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