external aerodynamics-hybrid volume meshdma.ing.uniroma1.it/users/paciorri/external_aero.pdf ·...

BETA CAE Systems S.A. 1 ANSA v.14.x Tutorials

Tutorial

EXTERNAL

AERODYNAMICS

HYBRID VOLUME MESH

Table of Contents

1. Introduction ................................................................................................................................... 2 1.1. Prerequisites ......................................................................................................................... 2 1.2. Case description ................................................................................................................... 2 1.3. Data files ............................................................................................................................... 2

2. Read the ANSA database ............................................................................................................. 3

3. Create the wind tunnel box ........................................................................................................... 9

4. Connect the vehicle to the box .................................................................................................... 23

5. Generate a triangular surface mesh ........................................................................................... 42

6. Examine and improve the quality of the surface mesh ............................................................... 45

7. Create and examine the surface mesh of the box ...................................................................... 47

8. Create the boundary layers and volume mesh ........................................................................... 50

9. Conclusion .................................................................................................................................. 60

EXTERNAL AERODYNAMICS – Hybrid Volume Mesh


1. Introduction

This tutorial presents in detail all the steps needed to generate a hybrid volume mesh for external aerodynamics applications. Only half of the vehicle is modeled, taking advantage of symmetry conditions. Note that for the purpose of this tutorial the mesh will not be created to such a detail that would be required for a highly accurate simulation, but instead only some techniques are demonstrated.

The steps described in this tutorial are:

Read the ANSA database containing the geometry of the vehicle.

Create the geometry of the wind tunnel box.

Connect the vehicle to the box.

Generate a variable density triangular surface mesh for the vehicle.

Examine and improve the quality of the surface mesh.

Generate a triangular mesh for the box.

Create penta layers from selected regions of the vehicle.

Mesh the remaining volume with tetra elements.

Examine the quality of the volume mesh.

Obtain information regarding the mesh (number, quality) and the vehicle (frontal area).

Output the mesh in Fluent format.

The resulting mesh consists of around two million volume elements.

1.1. Prerequisites

Reading chapter 1 of ANSA for CFD Brief User Guide is recommended in order to obtain a familiarization with the ANSA interface and terminology.

1.2. Case description

The geometry of the vehicle is shown below:

1.3. Data files

The file required for this tutorial is external_aero.ansa.

The results can be found in the file external_aero_surf.ansa.



2. Read the ANSA database

Start ANSA (e.g. by typing ansa64.sh for the 64 bit Linux version) and from the pop-up launcher select the CFD option so that

ANSA begins with the default CFD layout.

Please, refer to the PDF document “ANSA for CFD Brief User Guide” for a description of the CFD layout.

Activate File>Open to read the ANSA file.

Alternatively, click on the Open icon which is located on the left up corner of the ANSA window:

The File Manager window appears. Navigate and select the file external_aero.ansa.

Press Open.

Note: If you wish to see the name of each function you can click with the right mouse button on any toolbar.

Choose the “Show labels” option.

Press “Apply to all”, if you want to have an effect on all toolbars.

Now the label of each button appears below every icon.



The geometry of the vehicle is read into ANSA and should appear as shown on the left.

Notice that the model appears quite coarse.

This is because the current Resolution settings (or assigned element length) have a big value.

Activate the MEASURE

function from the Utilities toolbar:

Pick with the left mouse button two Hot Points to get a measurement, as an indication of the size of the vehicle. The distance is displayed on the screen and in the Text Window (on the left down corner of the ANSA window).

Press ESC to exit from this window.



Click on the Settings button

and select Settings>Resolution.

Change the value of the Perimeter Length to 30 mm and press OK.

This ensures that all visible Faces (and correspondingly Macro Areas in MESH menu) are assigned an initial element length of 30 mm.

Press OK in the appeared Warning

window.

Now, the shape of the vehicle is represented in more detail.

Switching to the MESH menu temporarily, you can see the assigned initial element length along the Perimeter Segments of the Macro Areas.

Note: Note that, while you zoom out, some perimeter nodes may disappear. This is due to the Detail on Demand effect setting

(F11). When you zoom in, you begin to see the perimeter details, when you zoom out, you see less.

To change the Detail on Demand effect setting, push the F11 key and go to the Presentation Parameters tab.

At the bottom scroll the slide bar to the left end in order to deactivate this visualization effect and press OK.



Switch back to the TOPO menu.

The geometry of the vehicle has already been cleaned up topologically and saved in the ANSA database that you have opened.

To confirm that, the Checks function can

be used.

Activate the Checks>Geometry function from the Tools toolbar

Deactivate the Single Cons check, since the existing Single Cons are the free edges of the geometry.

Press Execute.

Under the Status bar there is a green check mark indicating that the geometry is clean.

Alternatively, deactivate the Double

CONS visibility flag from Faces Draw toolbar, at the bottom of the screen.

Only red CONS appear along the open side of the symmetry plane, as expected. No yellow (double) or cyan (triple or more) CONS appear anywhere.



Activate Shadow display mode from

the Drawing Styles toolbar

and the Double visibility flag

from the Faces Draw toolbar

and rotate the part to view it better.

Switch to PID display mode to view the Faces of the vehicle colored according to their Property ID (PID).

Switch to FLUENT deck, so that the

Property list provides the Boundary Condition types for the Fluent solver.

Then switch back to TOPO menu.



Click the PID icon from the

Containers toolbar:

to open the Properties list window.

There are 14 different properties for different areas of the vehicle.

Press ESC to exit the Properties List window.

Switch back to ENT (Entity) display mode

Select File>Save as and save the file as

“external_aero.ansa” in your local

directory

Esc



3. Create the wind tunnel box

You will now create the wind tunnel box. First some 3D Points will be created, which will be used to create a Face that will represent the

“road” PID.

Activate the Points>New>Num. Input function.

The Numerical Input window opens. Type in the following coordinates:

0 0 0

12000 0 0

12000 -3000 0

and press OK.

The 3D Points are created and are displayed as cyan squares. To be able to view them, activate

the POINTs visibility flag button from the

Auxiliaries toolbar (if it is not already activated).

Press the F9 key to fit the whole

model in the screen.

Open the Model Browser from the Parts icon.

(Containers toolbar)

In the Model Browser window, there are two

Parts, named “vehicle” and “wheels”. The

3D Points were placed in one of the existing Parts.

Create a new Part to separate them, by activating New [Part].

F9

New

Part



During the New Part Creation, we can edit its Name in the respective field by typing “box” in the field and pressing ENTER.

The new Part appears as an EMPTY thumbnail in the Model Browser. You will now move the 3D Points from Part

“wheels” to Part “box”.

Activate the Set Part button.

Select the 3D Points with the left mouse button from the screen. The selected points are highlighted. Confirm with middle mouse button. The message “Select PART” appears in the Text Window.



Double click on the EMPTY Part thumbnail.

The 3D Points have been relocated from the “wheels” to the “box” Part.

The thumbnail icons of both Parts are updated.

Note: In ANSA, all entities (Faces, Points, elements, etc.) must belong to a Part in the Model

Browser. So, when you create new entities the Model Browser will open.

Right click on the Part “box” and select the

option Set Current.

The “box” Part is now the Current Part.



Finally, left click on the Utilities button and activate the flag Current Part>Use always.

As long as this flag is active, all new created entities will be put automatically to the Current Part, without any notification.

Press ESC to exit the Model

Browser.

Deactivate Shadow mode, to speed up

the step that follows next.

You will now create a planar Face for the road of the wind tunnel box.

Activate the Surfaces>Plane function by left-clicking on the Surfaces label.

Select the three 3D Points shown on the left to define a temporary Working Plane. The order of selection is important for the orientation of the Face to be created. As soon as the third point is selected, the mouse cursor appears in 2D mode with a thin hairline cross following it. This is for indicating the defined Working Plane.

Select with the right mouse button the opposite diagonal points shown.

Note: Right mouse button selection implies the

selection of existing point positions, in this case the previously defined 3D Points.

The Properties List window appears, in order

to assign a PID to the new Face. Press the New button to create a new PID.

1

1

3

2

2

Esc



A new Shell Property window appears. Type the name “road” in the Name field and press

OK.

Back in the Properties List window, the last created PID is marked. Double-click on it with the left mouse button to select it.

The Face is created.

Switching to the MESH menu

temporarily, you can see that the new Face has the same Resolution (element length) of 30. (Activate the MEASURE function from the Utilities toolbar and pick with the left mouse button two consecutive Hot Points on the “road” to verify) This is a very small value for the “road” Face

and should be increased.



Activate the Perimeters>Length

function.

Select the Macros option from the Selection

Window, select the Macro Area corresponding to the “road” with the left mouse-button and confirm with the middle mouse-button.

The Length Parameters window appears,

indicating the currently assigned element length.

Type in a value of 400 and press ENTER.

The new length is assigned on all Perimeters of the selected Macros Area.

You can now switch to TOPO

and activate Shadow mode.

Length



Now, disable the Shadow mode for clarity

reasons.

and the Points visibility flag because the

3D Points are not necessary for the moment.

Now, you will create the other sides of the box, using a functions combination.

Open the Settings dialog box

(Windows toolbar)

Go to Settings>TOPO and activate the

selection: “Perform TOPO in GEOMETRY functions” Press OK. Activate the Transform [Copy] function

(Utilities toolbar)

The Copy Entities selection window opens.

Keep the Entities selection mode. Select with the left mouse button the “road” Face and confirm with middle mouse button.

Switch to the Rotate tab. Select with the left

mouse button the two Hot Points shown to define the rotation axis (according to right hand rule). The axis is displayed graphically on the screen and the coordinates are filled automatically in the Rotate window.

Type in a value of 90 degrees (positive,

according to the right hand rule) in the angle field and press the + button. The Transformation Options window opens.

2

1



Select the “Auto-Offset” and the new Face

will belong to a new Property with ID the first unused of the database. However, it will still use the same name with the selected face that was copied. Press OK.

You can change its name through the Properties List.

The new Face is previewed in the specified position. Press Finish at the Copy Entities

window to confirm the creation of the face.

The selected Face is copied and rotated to the new position. Topology has been applied automatically at their common CONS, which appears yellow.

Since the Transform [Copy] function is still activated, select with the left mouse button the

Face shown on the left and confirm with middle mouse button. The Copy Entities window opens again. Keep the Entities option.



Switch to the Translate tab. Select with the left

mouse button the two Hot Points shown to define the translation vector. The components and magnitude are filled automatically.

Press the + button.

The Transformation Options window opens

next, where you should keep the option Auto-Offset for PID and press OK.

This will again ensure that the new Face will belong to a new Property with ID the first free unused ID of the database.

The new Face is previewed in the specified position. Press Finish at the Copy Entities

window to confirm the face’s creation.

The top Face is created.

The function Transform [Copy] is still

activated. So, select with the left mouse button the Face shown and confirm with the middle mouse button. At the Copy Entities window select the Translate tab.

1

2



Select with the left mouse button the two Hot Points shown to define the translation vector. The components and magnitude are filled, automatically.

Press the + button.

Again, the Transformation Options window appears, where you should keep the Auto-Offset option for PID and press OK.

Press Finish in the Function Selection window and ESC to exit from the function.

Note that the last two Faces are not connected topologically with the remaining box (red CONS).

Activate the Faces>Topo

function and make with the left mouse button a box selection of the CONS shown on the left. Confirm with middle mouse button

1

2

Topo



Topology is applied automatically and the common CONS change to yellow. Finally, you need to create two Faces for the “inlet” and “outlet”.

Activate the Faces>New function and the New Face

Options window opens. Activate the PLANAR option and from the

Feature selection bar

activate the Loop button.

Select with the left mouse button a red CONS

at the “inlet”. The remaining CONS are also

selected automatically (because the Loop Button is activated). Confirm with the middle mouse button. The Properties list window opens automatically to assign a PID to the new Face. Press the New button to create a new PID.

A Shell Property window opens. Type the name “inlet” in the name field, select type

“velocity-inlet” in ZONE_TYPE and press OK.

Note: If you cannot see the option “velocity-inlet” in the drop down list of Zone types, ensure that the FLUENT deck is activated.

Back in the Properties List window,

the new PID is highlighted. Double-click on it to give this PID to the face. When the confirmation window appears, click Yes to accept the created surface.

Press ESC to exit the function.

Note: If you create one or more new faces in a row, without exiting the Faces>New function, all of

them will be assigned automatically to the last selected PID.

New



Activate the Faces>New

function and the New Face Options window opens. Having the Loop button still activated, select one of the CONS highlighted in the left figure and confirm with the middle mouse button. Accept the created surface and the Properties List window opens.

Create a new PID named “outlet” and

select as ZONE_TYPE “pressure-outlet”.

Press OK and double click on the name outlet

in the Properties List. Press ESC to exit the function.

The wind tunnel box is now closed.

New



Having finished with the creation of the new Faces, press the Parts button to access the

Model Browser.

Click on the Utilities button and deactivate the Current Part>Use always flag, as you no

longer need it. Press ESC to exit the Model Browser.

Open the Properties list window. Notice that there are four PIDs with the name “road”.

This is because the three sides of the box were created by copying, rotating and translating the base “road” Face.

Although the next Faces received a different ID (offset by 1 each time), they all share the same name. You will now correct this by assigning a proper name to each face.

Switch to PID

and Shadow display mode

and activate the visibility of the Cross Hatches

Note: The PID colors of the faces you created may differ from the tutorial description pictures,

because ANSA gives a random color to every new PID. If you prefer to modify them, follow the procedure explained below.



While the Properties List window is still open (if it is not, activate it from the PIDs button or by clicking Ctrl+P),

pick with the left mouse button the top Face from its cross hatch. Notice that the PID of the picked Face is identified and marked in the list.

Note: If the HighLight button is also active, then

the Property is also highlighted on the screen.

While the PID is marked, press the EDIT button

to access its Shell Property card. Change the Name field from “road” to “top”.

Optionally, you can press the Color Edit button

to open the color palette window and specify a color of preference. Press OK to confirm.

Perform similar actions for the side and symmetry Faces. You should end up with the PIDs shown on the right:

Additionally, set the zone type of “top”, “symmetry” and “side” as symmetry, because you want

to impose symmetry Boundary Conditions.

Note: It is important to set the Boundary Condition type at this stage. During the meshing of the

model later on, a specific element length will be imposed to the sharp edges that will be detected based on the Boundary Condition type of each PID. A Perimeter will be identified as Sharp Edge only if it connects two Faces of Wall BC type, which form a sharp angle. If a Wall intersects with a Symmetry Type PID, then the angle between this wall and its symmetrical will be considered during Sharp Edge detection.

When you have finished with the name and zone type modifications, press ESC to exit the Properties List window.

Save the file from File>Save.

Esc



4. Connect the vehicle to the box

Back in ENT view mode with Shadow and Cross Hatches deactivated.

The vehicle is not connected to the box along the symmetry plane, as the red CONS indicate.

Activate the NOT visibility function

from the Focus toolbar.

and select the Faces as shown to remove them from visible and isolate the vehicle and the symmetry plane. Press ESC to exit the function.

You will now cut the symmetry plane by projecting the red CONS that form the outline of the vehicle.

To facilitate selection of red CONS deactivate temporarily the visibility of Double CONS.

Activate the function Cons>Project

Project



Select with box selection all the red CONS and

confirm with the middle mouse button.

Next, in order to select the Face of the symmetry plane, activate the visibility of Cross Hatches.

Select with the left mouse button the cross hatch of the symmetry plane Face.

Confirm with middle mouse button.

The red CONS are projected and cut the symmetry plane Face.

Deactivate Cross Hatches visibility and activate the Double visibility.

Notice that, along the cut, you can now see the red CONS of the vehicle overlaid with the yellow CONS of the cut on the symmetry plane Face.

Activate the function Faces>Topo

and select with box selection the CONS shown

on the left.

(Obviously not all the CONS of the vehicle have to be selected; only those where the vehicle meets the symmetry plane Face, but selecting more does not cause any problem).


Ensure no red CONS remain visible.

Topo



Topology has been applied and the vehicle is now connected to the box. The cyan CONS indicate that more than two Faces are connected on this boundary.

You should delete the Face of the symmetry plane that lies inside the vehicle.

Activate the Faces>Delete

function.

Select with the left mouse button the cyan CONS which is shared by the vehicle roof and the symmetry plane Faces.

The Delete Face Preview window opens, indicating that three Faces are connected to the selected CONS. The first of the three is highlighted.

Press Keep (or the K key from the keyboard) to

keep the Face of the roof.

The second of the three Faces is highlighted.

Press again Keep to keep the large Face of the

symmetry plane.

The third Face is highlighted.

Press Delete (or the D key of the keyboard) to

delete this Face because this is inside the vehicle.

Delete



The Face is deleted and now all CONS are yellow because they are shared by two Faces only.

Press ESC.

Press ALL from the Focus group to bring all the

Faces back to visible.

The volume appears closed. Deactivating for some seconds the visibility of Double CONS, to

perform a check, leaves a blank display.

Still, one additional check remains to see if there are any intersections.

Switch to the MESH menu.

Next activate the Checks>Penetration [Intersections] function.

A Template window appears.

Press Execute, so that ANSA checks for

intersecting Macros.



Intersections appear in red in the list, indicating that ANSA has detected problematic areas. Select the entries by pressing CTRL and left

mouse button.

Right-click on the selected entries and press Show Only to isolate them.

The tyres contact patches are not properly connected to the road.

They must be trimmed and connected to the road.

Press ESC to exit from the Checks window.

Switch back to TOPO menu.

Activate Shadow.

In order to trim the tyres contact patches, the Faces>Intersect function will be used.

However, in order for Boolean Operations to work, you have to set the correct orientation first.

Activate the Faces>Orient

function

Orient



Activate the Faces>Intersect

function.

The Face Intersection window opens.

Select the Union option from the Options List

window at the lower right corner of the screen.

Activate the PID region selection

from the Feature selection menu bar.

With the First group radio button active, first

select the front tyre and then the back tyre. The Faces are highlighted in purple. Confirm with middle mouse button.

Next, switch to the Second Group and select

the road Face. The Face is highlighted in blue.


Intersect



ANSA asks which faces to delete.

Check if the correct faces are chosen.

Press OK.

If the chosen faces are not the correct, you can select them manually.

The tyres are now properly connected to the road. There should not be any cyan CONS visible.

Later on, you will create boundary layers from the vehicle and the wheel surfaces.

However, the angle between the road and the tyres is very sharp, thus the layers creation may not be possible around the area where the wheels lie on the road.

Activate the AND function

from the FOCUS group

and switch to PID selection mode

in the Feature Selection toolbar, so that you make selections based on PIDs. Select with the left mouse button the visible Faces of the tyres. ANSA brings to visible all the Faces that belong to the selected PID. Press ESC.



Activate the Faces>Orient

function

Now the Faces are properly oriented. deck the visibility of Cross Hatches.

Switch to PID view mode to see the Faces

colored by their PID

Activate the Faces>Fuse [Proximities] function.

The Fuse window opens.

Make sure that the Translate option is selected

in the Options List window.

Activate the PID region selection mode.

With the First Group radio button active, select

the front tyre.

Confirm with the middle mouse-button.

Fuse

Orient



Automatically, the Second Group Selection is activated. Select the road.

Confirm with the middle mouse-button.

The Virtual Translation window opens.

Specify a “virtual” translation of the First Group (the tyre) to -20 mm along the Z-direction and press OK.

ANSA is going to create new Faces, so the Properties List window opens. In this case, you should create a new PID for the new Faces, so press the New button.

A new Property card opens. In the Name field type “interior_front_patch”, set the

TYPE to “interior” and press OK.

Note: If you are not able to see the option “interior”, activate the FLUENT Deck.

Back in the Properties List window, the new PID is highlighted. Double-click on it to select it.

Next the Model Browser opens. Double-click on the “wheels” Part to assign them there.



ANSA performs the “virtual” translation of the tyre by -20 mm, calculates the intersection and creates Faces to close this gap.

The new Faces appear in a new PID color.

De-activate Shadow to observe the small

enclosure that has been created around the tyre contact patch.

Repeat the same procedure for the rear tyre and create a new PID for the new Faces, name it “interior_rear_patch” and set its type

“interior” as well.

Note: Inside these small volumes, layers will not be created. These areas are given automatically different PIDs in order to separate them from the main Faces, road and tyres.

If this was not performed, layers would not be able to grow around the common area of the road and tyres, because a very sharp angle is formed between them.

Zoom in to the front tyre.



Activate the NOT function from the Focus

toolbar

and ensure that you are in the PID mode in

the Feature Selection toolbar,

so that your selections are based on the faces PIDs. Select with the left mouse button the patches created by the FUSE function and click the middle mouse button to exit the NOT function.

After hiding the patches, all the road and tyres’ faces are visible. Now, the faces covered by the patches will be assigned to different PIDs.

Activate the Faces>Set PID

function.

Activate the ENT selection mode.

from the Feature Selection toolbar

With the left mouse button select the faces shown on the left. Make sure that you select all faces perimetrically.

If you accidentally select different faces, you can de-select them with the right mouse button.

Confirm with the middle mouse button.

Set PID



Next the Properties List window appears. Press the New button in order to create a new

property for the tyre Faces, from which no layers will be grown.

A new Property card opens. In the Name field type “tyre_front_no_layers” and press

OK.

Back in the Properties List window, the new PID is highlighted. Double-click on it to select it.

The tyre Faces are put to the new PID.

Next, do the same procedure for the rear tyre.

Select the tyre Faces that are covered by the patch and assign them to a new PID named “tyre_rear_no_layers”.

Finally, do the same thing for the road Faces.

Zoom in again to the front tyre.

Activate again the Faces>Set PID function

and the ENT selection mode from the Feature

Selection toolbar.

Set PID



With left mouse button select the road Face covered by the patch.


The Properties List window appears. Press the New button in order to create a new property for

these road Faces, from which no layers will be grown.

A new Property card opens. In the Name field type “road_no_layers” and press OK.

Back in the Properties List window the new PID is highlighted. Double-click on it to select it.

In the same manner, put the rear tyre road Face to the PID named “road_no_layers”.

In the end, you should see a result similar to the one shown in the above two figures for the two different tyres.



Switch to ENT view mode.

from the Drawing Styles toolbar

Press ALL from the FOCUS group.

Activate the Faces>Orient to

assign uniform orientation to all visible Faces.

Note: If the orientation does not appear as shown on the left (gray color), press once again the Faces>Orient function.

Deactivate Shadow and Cross Hatches.

The final step consists of defining some refinement areas.

Activate the OR function of the Focus group

and select the common CONS of the symmetry and road Faces, so that all of them are selected with one click.

Middle-click to exit from the OR function.

You will now create some 3D Points that you will use to mark the extremities of the refinement areas.

More specifically, these points will be used to define the Size Boxes for mesh refinement.

Orient



Activate the Points visibility flag to view the

created 3D Points.

Activate the Points>New

function.

The Numerical Input window opens. Type in the following coordinates (use copy-paste from here):

2600, 20, 1650

8000, 20, 1650

12020, 20, 1650

2600,-1650, -20

12020, -1650, -20

and press OK.

Note: the points extend further out of the box by 20mm, because it is important to ensure that they include the area.

Note: The 3D points are not connected to the Faces, they are only geometrical point positions.

Activate the FLUENT deck.

New



Next, deactivate the visibility of the Macros, so

that only the 3D Points are visible.

Activate the SIZE BOXES>NEW

function.

Switch to “Nodes” selection mode and “Ortho” box type.

and pick the four points shown on the left.

Press the middle mouse-button to end the selection. Toggle around the box types with the key button “2” and if the highlighted box is the desired one confirm with the middle mouse button the creation of the box.

Then, the Size Box card opens.

Type in as name “around_car” and set

Max length surface=50 Max length volume=50 Note: you can define different length for the size box surface and different for the size box volume. In general, you can define Max length volume=0 and let ANSA calculate the maximum length for volume mesh.

Press OK to confirm.

Press ESC to exit from the function.

New



The box is created.

In case you wish to edit the Size Box properties activate SIZE BOXES>LIST

Right-click on the “around_car” Size Box entry and select Edit.

After making any modifications, close the Size Box card.

Open the Model Browser window

Create a New Part, name it “Size_Boxes”.

Then click on the Set Part button and select the

Size Box you have created.

Confirm by pressing the middle-mouse button.

Double-click on the empty part in the Model

Browser window.

Press ESC to exit.

Note: Ensure that the Box visibility flag is active

to visualize the blue Size Box.

Also, activate the visibility of the box cross hatches.

In order to have a different max length around the car and in its wake, the box will be split at the location of the intermediate point.



Activate the SIZE BOXES>SPLIT

function

and from the Split Options window select the Project option.

First pick the intermediate point along the Box edge and afterwards click on an edge of the box.

The Box is split in two.

Now, the second box must obtain the proper values.


Activate the SIZE BOXES>LIST

function.

The Size Box list window opens.

1 2

Split

List



Pick the box from the screen.

Its entry is highlighted in the list.

Right-click and select Edit.

In the card that opens assign a name “wake”.

Set values:

Max length surface=100

Max length volume=100

Press OK.

Now, the Size Boxes have correct values.

Press ESC to exit the list window.




5. Generate a triangular surface mesh

You will now mesh the model. First, you will focus on the car and, after that you will mesh the wind tunnel box.

Open the Model Browser (Ctrl+R).

Four Parts are visible. Select the

“Size_Boxes”, “vehicle” and “wheels” Parts

(box selection), right-click and select Lock.

Press ESC to exit the Model Browser, activate

Macros visibility mode

and then make all visible by selecting ALL.

ANSA keeps visible only the locked Parts of the Model Browser.

De-activate the crosshatches of the Size Box.

Switch to MESH menu.

Note: Notice that some Perimeter Segments of the vehicle have element length larger than the assigned value of 30 which was assigned at the beginning of the tutorial.

When the vehicle was connected with the symmetry plane by the TOPO function, during the automatic pasting of the two participating CONS through the TOPO function, the order of pasting was random. So, the element length of the resulting yellow CONS was received randomly from the length of one of the two initial red CONS.



To correct this, activate the Perimeters>Length function and

select with box selection all the Macros of the vehicle.


Type 30 in the Input window that opens and press enter.


Now, you have a uniform initial element length.

Next, you will create a curvature dependent variable size triangular mesh, but first you need to specify the correct length and nodal spacing for all Perimeters.

Activate the Perimeters> Spacing [Auto CFD]

function.

Select all the Perimeters of the model with a box selection and confirm.

The CFD Spacing Parameters window opens.

Copy the values shown on the left and Press OK.

With this function, you can control the growth rate, feature angle, min/max length on all visible Perimeters and the imposed element length on all “sharp edges”. As “sharp edge” is considered the perimeter between two Macro Areas, when these Macro Areas form with each other a sharp angle.

The user specifies the “Sharp Edges limit“ (e.g. 20

o) and ANSA finds all the “sharp edges” (here,

the edges between Macros with a supplementary angle greater than 20

o) and

assigns to their perimeters a length of 15 mm.

Length

Spacing

Auto CFD



ANSA applies suitable length and nodal biasing on all visible Perimeters, taking into account also the Size Boxes (as they are visible).

Zoom in to examine how the curvatures and sharp edges have been refined.

Note: The size boxes are taken into account only if they are visible. Otherwise, the perimeters numbering as well as the surface and volume mesh is not controlled by them.

Activate the Mesh Generation>CFD [Visible] function.

ANSA meshes the Macros with a curvature dependent mesh taking into account also the size boxes limitations.

Activate Shadow for better visualization.

Deactivate the visibility of Size Boxes.

Note: Again it is important to point out that the Sharp Edges were refined with an element length of 15 mm, however the Perimeters that connect the vehicle with the Box are not refined even though the symmetry plane face forms a sharp angle with the faces of the vehicle. This happened because the “symmetry” PID is defined as a symmetry BC Type and sharp edge detection works only on Perimeters between two Walls.

CFD

Visible



6. Examine and improve the quality of the surface mesh

Quality check of the elements will be performed according to Fluent EquiArea Skewness.

Press the F11 key to access the Presentation Parameters window.

In the Shells tab:

De-activate the warping option.

Keep as default the remaining parameters.

Switch to the Solids tab:

De-activate the negative volume option.

Keep as default the remaining parameters.

Switch to HIDDEN view mode.

from the Drawing Styles toolbar

All elements exceeding the skewness value of 0.5 are colored in green. The number of violating elements is reported in the legend as OFF.

F11



Select Shell Mesh>Reshape [Advanced].

In the pop-up window, keep the Violating option and expand level 2.

Press OK and confirm with middle mouse

button. ANSA performs local Perimeter Join operations and Reconstruction to fix the problems. This affects only the areas around the problematic elements.

Activate the Macros>Freeze/Un

function.

Select with the left mouse button all the Macros of the vehicle to freeze them. Press ESC to exit the function.

The mesh now appears in blue, indicating that the Macros are frozen. This status protects them from accidental deletion or modification by any action.

The mesh is also frozen, so that we can proceed with meshing of the wind tunnel without modifying the existing perimeter nodal distributions along the common boundaries of the vehicle and the tunnel.


Freeze/Un



7. Create and examine the surface mesh of the box

Having completed the vehicle mesh, you will now focus on the box. Activate the Model Browser.

The “Size_Boxes”, “vehicle” and “wheels”

Parts are currently locked (yellow). Select them, right-click on them and Unlock them using Mark As>Not Locked.

The Parts are now unlocked. Select the “box” and “Size_Boxes” Parts,

right-click on them and select Mark As>Locked.

The Parts are marked in yellow. Press ESC to exit the Model Browser.

Press ALL from the Focus Group.

Deactivate the HIDDEN view mode

and activate the Box visibility flag.

Only the tunnel and size boxes are visible.



Activate the Perimeters>Spacing [Auto CFD] function.

Select all visible Perimeters with box selection and confirm. Type in the values shown on the left and press OK.

Use the Mesh Generation>CFD [Visible] function to

surface mesh the wind tunnel. Examine the result of the refinement Boxes. Switch to Shadow mode.

Open the Model Browser, select the

locked parts, right-click on them and select Mark As>Not Locked.

Then press ESC to exit and press ALL to

examine the whole model.

Spacing

Auto CFD

CFD

Visible



Examine the final mesh and save it.

The surface mesh is complete. Save the FILE from File>Save.

The status of the model can be retrieved for comparison from the database “external_aero_surf.ansa” located in the same path as the original starting file.



8. Create the boundary layers and volume mesh

Now, the Boundary layers will be created from the vehicle and the road. Note: Layers are going to grow from the gray (positive) side of the mesh. So, if the orientation of the mesh is not the one shown on the left, please proceed with switching the mesh orientation using the function:

Macros >Orient [Visible].

Use the function as many times as it is required to achieve the orientation shown here.

Ensure also that the interior tyre contact patches (front and rear) have uniform orientation! If not, use the Macros>Orient [Select] function,

activate the Feature Selection:

PID Region

and select one face of each tyre contact patch. Confirm with the middle mouse button.

Press ALL to bring the entire model to visible.

Note: During the layers’ creation procedure the wind tunnel must be visible for the layers to auto connect with the car as well as with the tunnel!

Activate the Volumes>Layers [Create] function.

In the first tab, define the Basic parameters.

Set as first height 1 mm and select “Absolute” mode. Number of layers: 3 Additional outer layers: 3 Do not activate any option for problematic areas treatment. Note: It is always recommended for the first run

the Exclude/Collapse Problematic areas flag to be deactivated. If ANSA detects a problem it will stop and report its position. The reported problematic areas will be put in a set, accessible by the Sets button. In case the Exclude/Collapse option is active, the problematic areas will be excluded and the layers creation procedure will go on.

Orient

Layers



Go to the last tab, to adjust the Growth Controls settings. Set a “Proximity check factor” of 0.3.

Keep all the other options in the other tabs as default. Press OK to proceed.

In the next window, select from which PIDs you want to grow layers from. To accomplish this, the user has to either check one by one the PIDs or use the SHIFT button to highlight them and then activate the “Grow” check box of one PID. Automatically, all highlighted PIDs will be checked. Select the PIDs shown on the left to grow layers from. The “top”, “side”, “inlet”, “outlet”, “tyre_front_no_layers”, “tyre_rear_no_layers” and “road_no_layers” will be excluded from layer generation Press OK.

ANSA begins the layer creation, but stops with the message: “Problematic areas detected!!! A new set containing them is generated. END”

If you click on the SETs button on top

of the screen, you will find a set with the points marking the problematic areas on the surface mesh.

Right-click on it and select Show Only.



Zoom in to the area of one of the wheels and its wheel arch and examine it. ANSA detected a proximity problem there and it could not grow the layers, respecting the proximity check factor. Note: This can be fixed in several ways:

1. refine the surface mesh locally 2. set a lower proximity check factor 3. assign a very small value for First

Height in the Layers window, so that ANSA is allowed to locally squeeze the layers more and avoid the problem.

Here, the Collapse problematic areas option will be used for demonstration reasons to solve the problem. Press ESC to exit the SETs menu.

Press ALL and F9 to bring the whole model to

visible.

Ensure that all entities are visible (FE-mode and Macros). Otherwise, the layer sides will not

auto-connect!

Activate again the LAYERS [Create] function.

This time, activate the Squeeze option for the

problematic area treatment. Note: With the Squeeze flag ON, there will be a gradual reduction of the layers height, depending on the geometry. In this way the volume elements that will be grown later can fit in the problematic area. An example of the Squeeze effect on the problematic areas can be seen below. There is enough space left for volume elements between the layers of the wheel and wheel arch.

Layers



Additionally, move on to the Growth Controls tab, and change the Minimum layer aspect to 0.01. This will allow the algorithm to squeeze the layers to a point that their height is 1% of the base length. Press OK.

Select the same options as before in the second window and press OK.

This time, ANSA grows the layers because it is allowed to squeeze the elements in the area between the wheel and the wheel arch. In text window you see the following message: Generating Layer 1 Generating Layer 2 Generating Layer 3 Generating Layer 4 Generating Layer 5 Generating Layer 6 END

Take some time to examine how the layers have grown and how they were automatically connected to the wind tunnel. Save your file from File>Save.



Select any volume from the list to visualize it on the screen.

Note: As you noticed, after the layers generation, yellow spots appeared. These yellow spots indicate that Macros are connected with FE-Mode elements. In this case, the FE-mode “top_cap_fluid” PID that is created after the layers generation, is connected with the symmetry plane which is meshed Geometry. Deactivating the visibility of Macros, you will see the layers that were generated.

Activate the VOLUMEs>DEFINE [Auto Detect] function.

Press OK keeping the default options.

ANSA detects four volumes and the Volumes List window opens.

Note: If you closed accidentally the Volume window, activate Volumes>List to reopen it.

Define

Auto Detect



Activate the function Volumes>Mesh Vol.>Tetra Rapid

Select the main Volume and confirm with middle mouse-click. ANSA meshes with the Tetra Rapid algorithm. Note: Mesh Vol. allows the definition of the desired settings at the Options List window that appears at the bottom right corner of the ANSA window when the function is activated.

Select the other two volumes on the tyres and confirm with middle mouse-click. All Auto-detected Volumes are now volume meshed. Press ESC to exit the function.

Open the Property List window.

There are 4 “fluid” Zone Types. Assign all volumes to the same PID and rename it as fluid_zone. Press ESC.

Mesh Vol.



Activate the function Volumes>Set PID

and with a box-selection select all four volumes.

Confirm with middle mouse button. Double-click the “fluid_layers” property to put everything there.

Open the Property List window.

Holding the CTRL button select all 3 PIDs with zero Number of Elements. They are empty so they will be deleted. Right-click on one and select Delete. Then press OK. Rename the “fluid_layers” PID to “main

volume”. Press ESC to exit the Property List.

Next, press the Compress button to remove the remaining features of the deleted volumes.

Press OK.



Next, open the Volumes>List,

select a volume and press Info to get the volume stats shown on the left. Press ESC twice to exit the Info and Volume List window

Press All

Switch to Hidden view mode.

Notice that there are some solid elements reported as OFF.

Select Volumes>Improve> Fix Quality [Visible].

The quality of the elements is improved and there are no more OFF elements.

Note: By selecting Fix Quality [Visible], ANSA

makes the Quality Improvement based on specific options and criteria. In order to see and change these options, before selecting the Fix Quality function, click on the Mesh Parameters

icon and go to the Fix Quality tab if you want to change the Shell Mesh options.

List

Improve

Fix Quality



Go to the Volume Mesh tab, if you want to change the volume mesh options. Here, the user can define how much the nodes of the solid elements will be moved, either between PID bounds or from the external bounds. By selecting the Unconstrained option the movement can be unlimited. Finally, you can change the Quality criteria by pressing the F11 key.

Enable the Shadow mode.

De-activate the visibility of Macros and FE-mode mesh.

Press F1 for top view.

Make an OR selection (in ENT mode).

Rotate the model to inspect the volume mesh.



Press ALL.

Ensure Geometry, FE-Mode and Volume visibility flags are active.

Activate the Compress function from the

Utilities toolbar to remove any unused nodes and properties.

Save the file. Select File>Output>Fluent.

Type a name for the exported file and press OK.

Then, ANSA will open the Fluent Output Parameters window, where you will choose the type of the file, which part of the model will be output (all the model, only the visible etc) along with other options. Select the “Model” output option as shown on the left and press OK.

ANSA supports the output of *.msh file types as well as their compressed form *.msh.gz.



9. Conclusion

In this tutorial, you have followed all the basic steps to create a hybrid volume mesh for external aerodynamics.

Mesh generation, applying curvature and sharp edges refinement, was achieved through the use of ANSA automated techniques without manual effort by the user. Through the use of two functions (Perimeters> Spacing [Auto CFD] and Mesh Generation >CFD) the user is able to prepare a mesh suitable for any CFD case and geometry, by simply defining a few parameters, such as minimum and maximum element length, growth rate and feature angle.

Shell and Volume Mesh Quality Improvement is also performed fully automatically using the Shell Mesh >Reshape [Advanced] and Volumes >Improve>Fix Quality functions.

external aerodynamics-hybrid volume meshdma.ing.uniroma1.it/users/paciorri/external_aero.pdf ·...

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