Page 25 - Process Modelling and Simulation With Finite Element Methods

12 Process Modelling and Simulation with Finite Element Methods

menus and dialogue boxes with their content, limiting the number of models that
can be effectively discussed.

0.2.1 k-E model of a turbulent static mixer
Figure 0.1 shows the Model Navigator, which permits selection among pre-
determined application modes, setting up user-specified “multiphysics”
combinations of application modes, access to the models listed here by the user,
and access to the Model Library, which houses many manyears of solved and
explored models contributed by the FEMLAB user community and by the
COMSOL development team. In this subsection, we will walk through the
“turbulent static mixer,” which is modelled as a complicated 2-D geometry with
the popular k-E model of turbulence [2]. This model can be found by selecting
the Model Library tab in Figure 0.1, which brings up the Model Library dialogue
page, whose menu tree is traversed in Figure 0.2. We arrive at the turbulent
static mixer model with illustration and short blurb description. Selecting the
OK button brings up the FEMLAB GUI with the geometry, model equations and
boundary conditions completely specified. Figure 0.3 shows the postprocessing
screen that the model was stored with in the file “static-mixer.mat.” MAT files
are the binary format for efficient disk storage of MATLAB variables. Those
created by FEMLAB have the complete state of the FEMLAB environment
saved. The postprocessing screen shows a color density plot of surface velocity
of the last solution executed. Let’s find out what situation that was, in terms of
model equations and parameters.
The model is specified in a series of dialogue boxes. Traversing the pull
down menus from left to right will show the pertinent specifications. Now pull
down the Options menu, with the Add/Edit Constants choice highlighted as
shown. The Options menu allows specification of a local database of constants,
coupling variables, expressions, and differentiation rules, as well as specifying
the display scales. Here, we only need to view the constants. Figure 0.5 shows
the Add/Edit Constants dialogue box. We can see that rhof=l and n~f=10-~ are
specified. Note that these are pure numbers, i.e. no units are specified. It is up
to the user to employ a consistent set of units for his models, or to specify the
model in dimensionless form with dimensionless control parameters. This is not
necessarily a trivial task.
The next pull down menu over is the Draw menu. Here we will only switch
to Draw mode, which then takes over the display. Figure 0.7 shows the grey
composite geometry CO1 that was constructed when the model was originally
created. Note that the Draw toolbar has replaced the postprocessing toolbar on
the left. We could use these tools to enter new geometric primitives. CO1 is a
simply connected single domain, but we are not limited to either a single domain
or to simply connected domains. FEMLAB accepts these graphic primitives,
along with Boolean set theory operators (union and intersection) to construct