Biomedical Engineering Reference
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functions will model up to the viscous sublayer (
y
+
< 5) while others will model up
to the log layer (
y
+
> 30), which means that the wall-adjacent cells must exist in the
correct sublayer by determining its
y
+
value. The wall function loses its accuracy
for certain cases such as flows with severe adverse pressure gradients, strong body
forces and rapidly changing fluid properties near the wall. For these cases the LRN
approach with a fine near-wall mesh needs to be used.
5.3.6
Turbulence Modelling Approach
Defining the Problem
The task of resolving a turbulent flow through the selection
of a turbulence model, a compliant mesh, and additional boundary conditions has
many options and can appear daunting to a new CFD user. This section aims to
provide an overview of some techniques and approaches to complete this task. The
first step is to determine the required level of complexity of the CFD model in
relation to the available resources. Figure
5.17
summarises the questions that need
to be addressed to help define the problem. Initially the physical problem can be
estimated by the amount of flow physics that need to be captured (e.g. is the flow
indeed turbulent?), and the level of accuracy required. Then one must decide on the
modelling requirements which involve selecting the appropriate turbulence model
and should be based on the established results and best practices for the same class
of problems. This leads to the appropriate mesh generation where there is a direct
correlation between the mesh size, the amount of flow physics needed, and the
complexity of the selected turbulence model. Finally, the computational resources
and time required by these choices must be available for the given problem.
Fig. 5.17
Turbulent modelling considerations
Selecting a Turbulence Model
The selection of an appropriate turbulence model
will be determined by the level of accuracy required and the available time and
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