# COMSOL navier-stoke equation help

1. Jan 4, 2008

### gganendra

Hi everybody, newbie here...

im doing a modelling work with comsol and have a problem of solving using navier-stoke equation for the boundary. is it possible to discretize the boundary condition into several parts and apply different condition in each segment?

thank you beforehand,

2. Jan 5, 2008

### PerennialII

Hi gganendra,

yes - that is possible. Probably the easiest way is to do it like you described, divide the (original) boundary to multiple new boundaries and apply different conditions to each new segment. Alternatively, in Comsol you can define the boundary condition as a function of coordinates along the boundary (you can use either a global coordinate system, or then a local coordinate system going along the boundary) which gives you freedom to define pretty much whatever kind of bcs you desire (non-piece-wise mean).

3. Jan 5, 2008

### gganendra

oh so it is possible....because in comsol script (i prefer script rather than multiphysics, it gives more freedom), each boundary is given an indices (fem.bnd.ind field), and each indices apply to each line (2d, or face, 3d) for example in a rectangular geometry and i cant divide the line to several parts

okay then i will try the coordinate system... thank you

4. Jan 5, 2008

### PerennialII

yeah, if you don't want to modify your model working with coordinate systems is pretty straightforward (or logical expressions -- there is nothing preventing writing the bc as a combination of (something>something) or if(cond,true,false) where the conditions are coordinate dependent. Step functions can also be a nice aid. And actually also defining the bc as a tabular expression [the ability to define functions in a tabular fashion which comsol then interpolates ] where the expression spans the boundary is a pretty elegant way of doing it). If you "partition" existing boundaries by re-defining the geometry then you can naturally apply your conditions straight on those, but that requires some "operating" on the model.