Page 193 - Process Modelling and Simulation With Finite Element Methods

180 Process Modelling and Simulation with Finite Element Methods

5.2.1 Heating from above
We could save time by just altering our buoyant convection example for
the hot wall-cold wall problem by changing the boundary conditions. However,
in order to visualize convective rolls in two dimensions (hexagonal cells are
a 3-D phenomenon), we need to have an aspect ratio of about 3:l for
width:height of the layer or greater. So we will walk through the problem set
up here.
Launch FEMLAB and in the Model Navigator, select the Multiphysics tab.

Model Navigator
Select 2-D dimension
Select Physics modes+Incompressible Navier-Stokes >>

0 Select ChE +Convection and conduction >>
OK
Since we anticipate heavy computing requirements ahead, we will forego the
streamfunction post-processing luxury.
Pull down the options menu and select Add/Edit constants. The AddEdit
constants dialog box appears.

AddEdit Constants
Name of constant: Pr
0 Expression: 1
Name of constant: Ra
Expression: -1
APPlY
OK

Ra=-1 makes the heating from above, coupled with the setting below for Fy.

Pull down the Options menu and set the grid to (0,l) x (0,l) and the grid
spacing to 0.1,O.l. Pull down the Draw menu and select Rectangle/Square and
place it with unit vertices [-1.5,1.5] x [0,1].
Now for the boundary conditions. Pull down the Boundary menu and select
Boundary Settings.