Why You SHOULD be Running Dynamic Simulations

Let’s discuss the difference between the linear solver and the dynamic solver with a case study. Take the example of an engineer slamming his head on his desk after getting poor simulation results from his linear solver.

In scenario #1, the engineer lightly places his head on his desk then proceeds to slowly press down with his body weight. The force value, F, represents the maximum amount of force he can transmit through his neck.

In scenario #2, the engineers head starts a distance of 2 feet from his desk. The engineer proceeds to accelerate his head towards his desk with the aforementioned force value, F. After he comes into contact with the desk, his neck continues to transmit the force until his head comes to a complete rest against the surface.

Now, in both scenarios, the engineer’s neck transmits the same amount of force, F. However, the second scenario will produce higher levels of cranial stress at specific instances in time. The higher stress is related to momentum. The engineer recognizes this and proceeds with scenario #2 until the desired level of masochistic indulgence is achieved.

If we were to simulate these events, design scenario #1 could be adequately achieved with either the static solver or the dynamic solver and the results would be the same. However, if we were to run scenario #2 with a static solver, we would get the same result as we would with scenario #1. Obviously, the static solver has limitations.

The linear solver only sees the last moment in time; when things are at rest. Thus, we have two data points, the beginning and the end.

On the other hand, the dynamic solver is aware of the time in-between the beginning and the end. As a result, we are left with a much more complete picture.

For a moment, let’s sidestep the slamming head on desk approach, as this practice is no longer necessary. Let’s use the example of a cantilever beam with a weight of one hundred pounds suspended from the end.

For the linear study, let’s consider the case of loading the end very slowly. The results of this simulation are shown below.
The maximum displacement is 58 mm.

For the dynamic study, let’s consider that the weight will take .05 seconds to reach its maximum value of 100 pounds and will then level out. The maximum displacement results of this simulation are shown below.
The maximum displacement is 93 mm. This is an enormous jump from 58mm. Obviously, we’d want to study the dynamic simulation results over the static.