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How many modes should I use when analyzing an RFlex body?
In fact, here is no limit to the number of modes that you can use in the analysis of an RFlex body. Theoretically, using more modes will increase the accuracy of the results.
However, the scale is not linear.
In other words, increasing the number of modes from 30 to 300 does not increase the accuracy by 10 times.
The mechanical properties of a flexible body are affected by low frequency modes by 95% or more. If it is possible to represent the properties correctly with low frequency modes, it is unnecessary to use high frequency modes. In short, we recommend selecting the proper number of modes based on engineering knowledge. For those who still have difficulty determining the number of modes to use, here are some useful tips.
If you are unfamiliar with frequency characteristics of the part:
(Number of modes) = (number of constraint modes) + α
Where α is the number of normal modes. This number is greater than the number of constraint modes.
The number of constraint modes is determined by the number of interface nodes.
- (Number of constraint mode) = 6 x (Number of interface node)
- RBE independent node, Shell element node, Beam element node
- (Number of constraint mode) = 3 x (Number of interface node)
- (Solid element node)
In a model that contains a shell element node and 4 interface nodes, there will be 24 constraint modes (24 = 6 x 4). In this case, we recommend using a little more than the number of constraint modes as the number of normal modes for the analysis ((24) + (5 to 10)). These guidelines are useful for novice users. If you are a more experienced user, see the following description.
◾ If you are familiar with frequency characteristics of the model:
If, based on the above recommendations, you have decided to use 50 modes and after the 40th mode the frequency is higher than 10,000 Hz.
If the minimum integration step is 0.0001 sec, then the analysis can represent a maximum frequency of 10,000 Hz.
In this case, it is meaningless to analyze modes with frequencies above 10,000 Hz.
Therefore, we recommended analyzing only modes 7 to 40, which have frequencies below 10,000 Hz. Now, if the 50th mode in the above example has a frequency of 400 Hz, then you may need more than 50 modes to include a mode with a frequency above 10,000 Hz and improve the accuracy of the analysis.
However, this doesn't mean that you must increase the number of modes until the frequency reaches 10,000 Hz.
You must determine the optimal number of modes for analysis, remembering that more modes take longer to analyze.
The following table summarizes the above description:
The above cases only provide a few guidelines. To select the correct number of modes for the model, you must consider the characteristics of the model, the purpose of the simulation, and many other aspects.
Note that, in RecurDyn/RFlexGen, you must set the value manually in the No. of Normal Modes field.
