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Nodal Time History in Local Coordinate System


In several simulations, the response of a nodal point (displacement, velocity, and acceleration) is helpful to be output in a user-defined local coordinate sytem. LS-DYNA offers several options to enable the local system output of nodal time history data and here are some examples of it. All nodal time history data is output to either NODOUT or NODOUTTF (971 and beyond) only.

1. ELEMENT_SEATBELT_ACCELEROMETER
This is one of the most commonly used feature where a nodal rigid body is first defined use three nodes such that N1 and N2 form the local X-Axis and N3 lies in the XY plane. With this information, LS-DYNA first computes the cross product of N1N2 and N1N3 to determine the local Z and this step is followed by a cross-product of local ‘X’ and local ‘Z’ to determine the local ‘Y’. The node N1 is then referenced in *DATABASE_HISTORY_NODE whose output is then output in a local coordinate sytem.

2. *DATABASE_HISTORY_NODE_LOCAL – REF = 0.0
Alternatively, one can reference a local coordinate sytem using *DEFINE_COORDINATE_NODES to create a local coordinate system and reference this id in *DATABASE_HISTORY_NODE_LOCAL keyword which will then cause the nodal output in local system. Using this option, the paramter REF plays an important role in determining how the local system data is output. To illustrate this, lets consider a simple rail crush example (as shown below) in which a node time history is output at the initial and final state in the NODOUT file.

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When using *DATABASE_HISTORY_NODE
When using the non-local option of the history output, LS-DYNA, outputs the nodal time history in the global system. This is the default behavior for all non-accelerometer nodes. This is shown in the following figure.

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When using *DATABASE_HISTORY_NODE_LOCAL with REF = 0.0
In this case, the local system is fixed in space at all times and the nodal output is the with respect to the local system which does not translate or rotate based on the local system. This is shown below.

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When using *DATABASE_HISTORY_NODE_LOCAL with REF = 1.0
In this case, the local system rotates based on the nodes that is used to define this (must use *DEFINE_COORDINATE_NODES) to account for any change in the orientation of the system. This is shown below.

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When using *DATABASE_HISTORY_NODE_LOCAL with REF = 2.0
In this case, the local system both translates and rotates based on the three nodes that are used to define the coordinate system. This update results in a relative output of the nodal time history data wrt to the local system. This is illustrated below.

(Click image to enlarge)

Notes
For a collection of nodes to be output in a local system, you can use *DATABASE_HISTORY_NODE_SET_LOCAL.

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