Hello and Welcome!
In this workshop we will demonstrate
how to use the Ansys Fluent Meshing Watertight Geometry Workflow
to generate a simulation-ready mesh
that can be used to study the external aerodynamics
of a generic aircraft geometry.
Specifically, we will focus on creating local mesh refinement zones
and adding boundary layers
with a specified first cell height.
Let’s get started!
Launch Ansys Fluent in meshing mode.
For this demonstration,
double precision is enabled,
and 4 processes are engaged for meshing.
Once Fluent Meshing launches,
select Watertight Geometry workflow.
In the Import Geometry task,
leave all the settings to default
and load the provided geometry file.
Here we have the fluid domain around a generic aircraft geometry.
Only half of the model will be used, with the central plane set to a symmetry boundary condition,
to reduce the resource requirement during the simulation.
Next, set ‘Would you like to add local sizing?’ option
to ‘Yes’ in the Add local sizing task.
We will be adding a Face Size control
to the trailing edge of the wing
to ensure this relatively small and sharp feature
is accurately resolved during meshing.
To do that, select ‘Face Size’
under Size Control Type
and set ‘Target Mesh Size’ to 0.5 mm.
Leave ‘Select by’ to label
and pick ‘wing_le’ and ‘wing_te’ from the list,
and select ‘Add Local Sizing’.
The leading-edge surface is also included to ensure
that the wing curvature is accurately resolved.
Next, we will add local refinement regions.
These regions are nothing but BOIs
that resemble the shape of the selected object.
Let’s now see how we can setup this task.
Right click on the Add Local Sizing task
and select ‘Create Local Refinement Regions’
from the ‘Insert Next Task’ menu.
For this workshop,
we will be using the offset surface local refinement region.
In the ‘Create Local Refinement Regions’ task,
change the ‘Type’ to ‘Offset’.
Set a mesh size of 2mm.
With ‘Select By’ set to ‘label’,
select the ‘fuselage’, ‘wing’, ‘wing-le’,
and ‘wing-te’ objects.
The specified mesh size will act as a soft sizing control
to create the surface mesh on the selected objects.
Since the aircraft is oriented in the x-direction,
we can leave the ‘Flow Direction’ to ‘X’.
In this workshop, we will set the ‘Defeaturing Size’ to 20mm,
which will be used to wrap the selected objects and create a rough shape.
Two types of BOIs can be created using the Offset surface method:
Boundary Layer Levels
and Wake Levels.
The boundary layer levels are surfaces created
by scaling the surface of the selected geometry
equally in all directions,
with respect to the centroid of the selected geometry.
Set the ‘Boundary Layer Height’ to 200 mm
and ‘Boundary Layer Levels’ to 2.
The boundary layer height defines the offset distance of the farthest boundary layer level
from the geometry.
The boundary layer levels parameter
defines the number of regions that are created within this height.
If more than one level is specified,
each level will have equal width
but a different mesh size.
Wake levels are like the boundary layer levels,
except that they are not equally scaled in all the directions.
Set wake levels to 2.
The offset distance for each of the wake levels is calculated
using the wake growth factor
and cross-wake growth factor,
which control the offset in downstream and cross-wake direction respectively.
We will leave these values to their default.
For both the boundary layer levels and the wake levels,
the mesh size parameter is used to define the sizing of each level,
which increases by a factor of two between successive levels.
Since we defined our mesh size as 2 mm,
the BOI sizing for the boundary layer levels will be 4mm and 8mm,
and for the wake levels will be 16mm and 32mm.
Click ‘Create Local Refinement Regions’ to finalize the setup
and execute the task.
In the ‘Generate the Surface Mesh’ task,
set minimum size to 1mm
and maximum size to 500 mm.
Set the Curvature Normal Angle to 12 degrees to make sure
all the curved features on the aircraft are accurately captured.
Click on Generate the Surface Mesh.
In the ‘Describe Geometry’ task,
select ‘The geometry consists of only fluid regions with no voids’
as the Geometry Type and leave the rest to default
and execute the task.
In the Update Boundaries task,
since the labels were predefined during the CAD phase,
Fluent automatically assigns the appropriate boundary condition
based on label name.
No changes are required here,
and we can continue to the next task.
Since there is only 1 fluid region in our model,
this is correctly identified as the fluid by the meshing algorithm.
No changes are required here either
and we can move onto defining the boundary layers.
Set ‘Add Boundary Layers?’ option to ‘Yes’.
We will use the last-ratio method to grow the boundary layer mesh from all the walls
in our fluid region.
Set ‘Offset Method Type’ to ‘last-ratio’.
In addition to providing the option of ‘Transition Ratio’
that allows the user to control the size difference
between the last layer of boundary layer mesh
and the core mesh,
it is also possible to define the ‘First Height’
which allows the user to control y+ value of the mesh.
Set this input to 0.05 mm.
To ensure the boundary layer flow behavior is accurately captured,
it is common practice for external aerodynamics simulations
to use a high number of boundary layers.
A value of 10 is a good starting point.
Select ‘Add Boundary Layers’ button to execute the task.
Now we are ready to generate the volume mesh.
In this demo, we will use the ‘poly-hexcore’ Fill With method
to fill the core of the fluid volume
with hexahedral cells,
which are connected to boundary layer mesh
through polyhedral cells.
The min cell length and max cell length values
are automatically populated based on the previously
defined local and global sizing controls
and are sufficient for our purpose.
Hit the Generate the Volume Mesh button to create the volume mesh.
The generated mesh has a total of approximately 1.9 million cells
with a mesh quality of 0.15.
Looking at the mesh, the boundary layer levels,
and wake level BOIs created using the local refinement region task
are clearly distinguishable from the rest of the mesh
due to the finer cell sizing used in these regions.
That brings us to the end of this workshop.