hi! "warning:non-positive volumes exist".anyone know why and how to deal with it? thanks in advance!

That means there's a problem with your mesh. If you're 3D, then you've gotta go back to the actual mesh and find the problem. If you're using gambit, go to the mesh commands, volume mesh, and pick check mesh, then you can check each volume to find out which one has the bad elements, and then you can go from there to fix it. This is the same for a standard 2D mesh, except you would go to surface mesh instead of volume mesh when trying to find the bad elements. Also, the button all the way at the bottom and to the right on the screen brings up the mesh visualization tools. You can play around with the options to find the bad elements this way too (but it takes a while for meshes over 1,000,000... not sure how big your case is).

If you're using the 2D axisymmetric model, I believe you will get this error if you have any mesh below the x-axis. In fluent go to grid->translate and use this tool to make sure that your axis of symmetry lies on the x-axis and that the entire mesh is above the x-axis.

If this doesn't answer your q, then you might have to post some more details of your model (2D, 2D axisymmetric, 3D, your meshing program, etc...).

Hope this helps. Jason