||April 25, 2009 11:10
Lening D is correct - if your Jacobians aren't all positive it may just be a simple matter of flipping the I-J indices on your grid. Keep in mind that in order to do this, you have to have created a 2D block (in other words, only blocks can have their computational coordinates changed).
Other causes of bad Jacobians are the obvious issue of grid line crossing. In this case, any of the diagnostic functions in the Examine command can be used to check this (e.g. Area, skewness).