Formula-SAE Car Wing Repair External Aerodynamic Simulation in STAR CCM+

OBJECTIVE

To identify the types of errors which can be classified & repaired in STAR CCM+ & then perform a surface repair on one of the FSAE (FORMULA SAE) car front wing applying the surface repair methodology.

THEORY
Basically, there are six types of errors which are checked whenever we perform surface repair:

a.) Pierced Faces:

• A pierced face is a face that is intersected by one or more edges of another face.
• If an edge pierces a face, the surface diagnostics colour the face red in the graphics window. If an edge is piercing a face, the surface diagnostics colour the edge red.

(fig 1: the image above shows the red coloured pierced edges)

b.) Face Quality Error:

• Face quality is a measure of similarity between a face & the ideal face shape ( an equilateral triangle).
• The surface diagnostics tool calculate face quality as the ratio of the in-circle radius to the circum-circle radius, multiplied by a factor of 2.
• The in-circle of a face is the largest circle that fits inside the face, while the circum-circle is the circle that passes through the three points of the face.

(fig 2: the image above shows the yellow coloured edges which lead to face quality error)

NOTE:

• Face quality ranges from 0 to 1.
• `F_q = (2*r_i) / r_c` this function defines the value of face quality.
• For `F_q=0` it represents a line & there is no in-circle radius.
• For `F_q=1` it represents an equilateral triangle.
• Faces for which `F_q < 0.01` are poor in quality.
• In surface repair mode, poor quality faces are coloured in yellow, when you activate poor quality face diagnostic.

c.) Face Proximity Error:

• Face proximity considers the distance between two faces.
• Face proximity ranges from 0 to 1, with low values indicating the following problems:
  --> Folds in the surface, for example, "V" or "Z" shaped kinks in the surface causing localized overlaps.
• Faces that have face proximity below 0.05 (by default) are considered to be close proximity faces.
• In surface repair mode, close proximity faces are coloured in orange when you activate the close proximity faces diagnostic.

(fig 3: the image above shows face proximity error on surfaces coloured orange)

d.) Free Edges:

• A free edge is the one that is joined to only one face. Two joined faces share two vertices for a common edge.
• The surface diagnostic colours free edges green in the graphics window.
• Double-clicking a free edge selects all connected free edges.
• We can fix free edges by filling or patching the hole using the surface repair tools.
• There are instances when you do not need to fix free edges like - "Baffle Interface".

e.) Non-manifold edges:

• A non-manifold edge is an edge that three or more other faces share.
• Example: An interior surface joined to an exterior surface has non-manifold edges where the interior & exterior faces share an edge.
• The surface diagnostics colour non-manifold edges blue in the graphics window.
• An understanding of how non-manifold edges are calculated is required when we are diagnosing multiple parts.
• We can fix non-manifold edges by deleting the faces that are causing the additional edges, using the surface repair tools.

(fig 5: the image above shows the non-manifold edges coloured in blue)

f.) Non-manifold vertex:

• A vertex is non-manifold if any of its connected faces have no other link with the rest of its connected faces, apart from the vertex.
• Example: The diagnostics identify a non-manifold vertex when one surface joins another at a single vertex.

(fig 6: the 2 vertices shown in blue colour in the above figure are non-manifold vertex and as per slide-out panel there are in total 4 non-manifold vertices in above model)

-->And as per the STAR CCM+ functionality, the geometry repair tool divides the geometry into below-mentioned 8 types of categories & then function upon them effectively while performing surface repair:

DIAGNOSTICS CATEGORY	DESCRIPTION
	Indicates errors that are associated with edges that cross each other and faces that self-intersect. In most cases, the Geometry Repair tool can fix these types of error by merging edges and vertices to remove the self-intersection.
	Indicates errors that are associated with poorly defined or incomplete sets of points that define a spline
	Indicates errors in the relationships between entities in the geometry model. This type of error can also indicate that the model is corrupt. The Geometry Repair is likely not able to repair corrupted geometry. In this case, you are advised to return to the native CAD package to see if the errors can be repaired.
	Indicates errors that are associated with loose tolerances of vertices, edges, and faces in the geometry. Loose tolerances can result in loss of data/definition of the geometry. For example, a high tolerance can prevent a gap from being recognized as a gap and ultimately stopping the gap from being closed. If the Geometry Repair tool cannot repair tolerance issues, you are advised to return to the native CAD package and look at the tolerance settings and tighten up the tolerance settings there.
	Indicates errors that are associated with overlapping faces. This type of issue could be due to tolerance issues in the geometry model.
	Indicates errors that are associated with fundamental errors in the entity definition. For example, faces that have edge loops that are defined in the opposite direction to other edges in the geometry.
	Indicates errors where the geometry contains non-manifold vertices or non-manifold edges. A vertex is non-manifold if any of its connected faces have no other link with the rest of its connected faces apart from the vertex. An edge is non-manifold if it is shared by two or more faces.
	Indicates errors that are associated with the assembly definition, and the parts and instances within a model. This type of error is typically caused by poorly defined shells and solids. Another cause could be issues with instancing and missing information from the parts and assemblies.

PROCEDURE

(fig 7: Loading the file named wing2.sim)

(fig 8: Importing wing1.X_B file in this wing2.sim, we will obtain the above image where we can see 15 body parts in wing1 under the geometry tree & wing2 model also)

(fig 9: Entering into the surface repair we come across the number of errors which belong to various categories as can be seen in the slide-out panel)

(fig 10: under the global tab we enter into the merge imprint tool to see how faces overlapped each other & why pierced face errors are being created, this is where we see that by entering the tolerance value of 1e-04 m, and not modifying other things we are not able to remove pierced faces.)

(fig 11: entering into the hollow mode & hiding the meshes we see a clear view of overlapping surfaces & for this, we select the resulting mesh type as conformal under advanced settings & the pre-processing as boolean subtract.)

(fig 12: selecting the suitable advance imprint options & then clicking on next we will be able to merge 1 pair of overlapping surfaces, which when we perform on all i.e., by selecting imprint all we get a completely merged error-free model as shown in the image below)

(fig 13: Now, we see after applying imprint all, all the 18 pairs have been worked upon & the pierced face errors have been brought down to 9)

(fig 14: this is the mesh view of fig 12, and in the output box we can see it is written that all 18 pairs have been successfully merged & no overlapping region is left)

(fig 15: Now, we see there are 3 non-manifold edges error which can be removed by deleting overlapping faces one on the other)

(fig 16: one more non-manifold edges error reduced)

(fig 17: Due to deleting faces some free edges are also being created, which will be addressed after removing all non-manifold edges)

(fig 18: Now, the free edges were removed by creating a closed-loop using them, & we can see in the slide-out panel that our )

(fig19: the further pierced face errors are removed by deleting the extra generated faces & in order to not defeature the model we then create a closed-loop back in that region uniformly.)

(fig:20 all pierced face errors, free edge errors, non-manifold edge errors, non-manifold vertex errors are removed)

(fig 21: An error-free FSAE front wing model is obtained as above)

(fig 22: this wing when simulated completely with the formula car, will have a velocity effect as shown in the above image)

CONCLUSION

So, as per our objective, we have studied the various types of error classification in STAR CCM+ & also performed the surface repair on the FORMULA SAE car front wing.