Surface wrapper is an automated geometry preparation tool available in Simcenter STAR-CCM+ that generates a clean, manifold and water-tight body from a dirty input geometry. This articles summarizes the utilization of the different settings available to improve wrap quality. 

Target and Minimum size:

The surface wrapper uses an oct-tree method which generates hexahedral cartesian mesh. The mesh is hexahedral cells (Octants) are generated based on the target size. The cells are recursively split where they intersect the input geometry with the smallest size being equal to the minimum size. 
 

Volume of Interest:

When wrapping the geometry different volumes can be extracted as the resulting wrap. Different options available under default controls for volume of interest are listed below. 

Contact Prevention:

The contact prevention setting allows users to preserve/resolve a gap between two surfaces that is not resolved by the minimum surface size. The contact prevention can be applied between all the selected input surfaces with "One Group Contact Prevention" or between a combination of two sets of surfaces with "Two Group Contact Prevention". A "volume of influence" setting can be used to specify a custom zone in which the contact prevention is applied. 

Contact prevention overrides the minimum surface size limit. It should be applied carefully only where needed as it can increase the computational expense for the wrapper. 

The example below shows three blocks with a gap of 0.1mm. The example shows the wrapper result with different settings to showcase where the contact prevention is applied. 

Gap Closure:

This setting helps to avoid unwanted leakages in the geometry. It closes gaps or holes that are bigger than the target surface size. There are three methods available for gap closure. The gap closure size must be greater than and not equal to the gap/hole size the user is trying to close. 

1. Size: User specifies a gap closure size. Any hole that is smaller than the gap closure size will be closed. 

2. Seed Point: User specifies a seed point location. Any hole that leaks to the volume which contains the seed point will be closed.

3. Both: Both checks are performed to close the holes. 

The example below shows a simple scenario with two boxes connected with cylindrical channels with diameter of 10mm hole. The wrapper is run with the Volume of Interest set to "Largest Internal Volume". Different wrapper settings are used to help understand the working of the gap closure.

 

Wrapper Defeature:

The surface wrapper performs a projection of the wrapped surface back to the feature curves of the input geometry. This is done to preserve important features of the geometry. However, in certain instances this can produce a poor quality wrap and defeaturing a particular feature curve may be desired to improve wrapper quality. In the wrapper feature setting, the user selects surfaces that they wish to defeature as an input. The final wrap is not projected back to the feature curves of the selected surfaces. 

The best use case for wrapper defeature are sharp corners or cusp features. A very fine wrapper size is typically required to resolve these sharp corners, if the size is not small enough the wrapper does a poor job of capturing the geometry and the projection back to the original surface leads to "needle" type features. In instances where the sharp corner is not important for the fluid flow analysis, the wrapper defeature can be used improve the quality. 

The example below shows how the wrapper defeature can be used to improve wrap quality near a sharp corner. 

 

Partial Wrap:

The partial wrap setting is used to preserve the original geometry of user selected surfaces. The selected surfaces are preserved and stitched in with the rest of the wrap. This setting is used to capture intricate geometric details that may require very fine wrapper settings to capture properly.

The electronic cooling example below shows the use of the partial wrap. The entire geometry is wrapped (shown in grey), while the original geometry is preserved for the cooling fan and the CPU heat sink (shown in orange).