[AHF]

Hi
i have a simple question.
its possible to find the place that negative volume occur ?
in my simulation everything is fine but suddenly error about negative volume . i find out the place of negative cell
it is possible ?

[Prikane]

Hello AHF,

When you get negative cells, you should go to Graphic (under RESULTS panel) and go on MESH, DISPLAY MESH, choose fluid area. Zoom to your most sensitive area and you will see, where negative cells occur.

Hope I helped you, best regards!

Prikane

[D.M]

Hi,
it realy depends on your geometry and the task you want to accomplish.

1. if u are using layering method, after u face with the error "the negative cell volume deteced" press ok and go to graphics and mesh menu, u should be able to see the negative cell volume easily and it mostly appears cause of your time step size, try to decrease the time step and if u got time issues try to decrease the time step by UDF just 10 or 5 steps before the negative cell volume detected and after that increase the time step again (i mean by UDF that u have just wrote).

2. if u are using remeshing method, the negative cell volume mostly apears around the boundries, so check the mesh near the boundries, it mostly apears cause your mesh is not small enough near the boundries, keep in mind that remeshing method cant increase the nodes of your mesh on the boundries (i mean if your wall has 10 nodes, remeshing method can't increase it to 15 nodes during the meshing!! i checked it my self and it seemed to be as i said and i dont know if im right!!), so if you are nearing an object to your boundries, cell skeweness goes higher and higher near the boundries and after a while u will face the "negative cell volume detected" so u should fine the mesh near your boundries.(i dont know if im right or not but this is as far as i knew)

regards.

[AHF]

Originally Posted by Prikane

Hello AHF,

When you get negative cells, you should go to Graphic (under RESULTS panel) and go on MESH, DISPLAY MESH, choose fluid area. Zoom to your most sensitive area and you will see, where negative cells occur.

Hope I helped you, best regards!

Prikane

Originally Posted by D.M

Hi,
it realy depends on your geometry and the task you want to accomplish.

1. if u are using layering method, after u face with the error "the negative cell volume deteced" press ok and go to graphics and mesh menu, u should be able to see the negative cell volume easily and it mostly appears cause of your time step size, try to decrease the time step and if u got time issues try to decrease the time step by UDF just 10 or 5 steps before the negative cell volume detected and after that increase the time step again (i mean by UDF that u have just wrote).

2. if u are using remeshing method, the negative cell volume mostly apears around the boundries, so check the mesh near the boundries, it mostly apears cause your mesh is not small enough near the boundries, keep in mind that remeshing method cant increase the nodes of your mesh on the boundries (i mean if your wall has 10 nodes, remeshing method can't increase it to 15 nodes during the meshing!! i checked it my self and it seemed to be as i said and i dont know if im right!!), so if you are nearing an object to your boundries, cell skeweness goes higher and higher near the boundries and after a while u will face the "negative cell volume detected" so u should fine the mesh near your boundries.(i dont know if im right or not but this is as far as i knew)

regards.

Thank you, all advises were useful. But another question , i use UDF (define_grid_motion) to reposition the node on the wall .( it void to negative volume) but the main problem is looping over the node ordering . i mean how does fluent check the node ? i use f_node_loop(f,tf,n) but for example the fisrt node in looping that it checked is not a first node or last node in geometry . how i can looping over nodes in order to first node to last node ؟

thanks