Simcenter STAR-CCM+ CSYS Transform operation to keep initial position of the Center of Mass in a DFBI simulation

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The aim of this article is to provide a procedure for automatically updating the position of the Center of Mass when an initial rotation/translation is applied to a body.


It is not unusual to change the initial position of a body, when running a simulation where the DFBI solver is used. 
Just as an example one could think about the initial trim or sinkage of a ship or the initial position of a free falling lifeboat. 
There are several other applications where this requirement is needed but in this article we'll just consider the simple example of a free falling object. 


By default the position of the Center of Mass (CoM), in the DFBI node, is defined with respect to the Laboratory Coordinate System:

In the image below it is possible to see the Lab Coordinate System (on the left) and the Body 1 C-Sys (in the centroid of the Body), automatically created by the DFBI solver and placed in the same position of the CoM.  

Now, let's change the initial position of the Body via the Transformation operation. In this case a rotation of 6 deg around the Y axis of the Laboratory CSys is applied:

Comparing the two images, the Body has been correctly re-positioned (Grey body is in the original position & red body is in the updated position). However, the Body 1-CSys remained in the original position (coincident with the CoM of the grey body). This of course will have an effect on the dynamic behaviors of falling object as the Center of Mass effectively results in a different relative position with respect to the Body. 
A possible non-efficient solution would be a manual (or automated via java) calculation of the new coordinates of the CoM. This can clearly introduce the possibility to make mistakes.

Instead here we can demonstrate the recently introduced Transform operation feature. This allows you to specify a coordinate system to be rotated together with the Body. 
In this case the selected Coordinate System is the Laboratory:


This feature creates a copy of the specified coordinate system, rotated together with the Input Parts selected. 


This "Transform" coordinate system is linked to the Transform Operation and it will be automatically updated every time the operation is modified. 

At this point, it is fundamental to link this coordinate system to the CoM and this can be done by making sure that it is used in the definition of it under the DFBI node:


With this configuration, if the same rotation of 6 deg is applied, the result will be:

The image shows that the Body 1-CSys has now moved together with the body. However the axes are still aligned with the Laboratory CSys. This impacts the definition of the Moment of Inertia. 

In order to improve this result, the same "Transform" CSys created by the Transform Operation has to be used to define the Orientation of the Body, checking the Initial Values folder of the DFBI node:


This produces the following update of the Body 1-CSys:


It is important to highlight that the procedure described here works well even if a translation and a rotation are simultaneously defined (in the same Transform operation):


At this point, you are free to apply the Transform operation as many times as needed, without being worried about the correct position of the CoM. 
This procedure is suitable for design sweeps studies too. 

How to define a moving coordinate system
Simulation Operations: how to implement a re-meshing procedure to reduce the cell count in an Overset case with DFBI motion

KB Article ID# KB000049566_EN_US



Associated Components

Design Manager Electronics Cooling In-Cylinder (STAR-ICE) Job Manager Simcenter STAR-CCM+