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bongirs

Why no initial acceleration is available in RADIOSS?

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Hello,

 

I see that in BC Manager I can set initial velocity, imposed velocity, imposed acceleration but no initial acceleration.

Why is this so?

Also, what is the difference between initial and imposed velocity?

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Hi Bongirs,

When an initial loading condition is provided, the condition will be constant throughout. Whereas for an imposed condition it will be a forced condition where it is possible to ramp the load with respect to time.

The major disadvantage of the constant acceleration type of loading is that over a larger time range only small displacements are generated resulting in small forces. But for a constant velocity applied, the accelerations and the inertia forces result from nonlinear effects only. And so, high velocity and a linearly increasing displacement can be achieved from the start.

 

Also please refer the below link to download free Ebook (Crash analysis with Radioss). http://www.altairuniversity.com/free-ebooks-2/free-ebook-crash-analysis-with-radioss-a-study-guide/ where we talk about this topic with an interactive video.

 

 

 

 

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Thank you George for your answer!

So I understand that the initial velocity is set at t=0 to the selected nodes. The nodal velocities may change over the course of the simulation if the nodes face contact or if there are nodal forces.

For imposed velocity, the velocity of selected nodes is maintained at specified value no matter what. So if they come in contact with another body, there will be increase in contact stress and even result in complete body motion to satisfy the imposed velocity. I was just wondering in which scenario could this be useful?

 

I didn't quite understand the part about initial acceleration.

What I understood from your reply is that if the simulation time is small then the acceleration would have negligible effect?

The problem is that if for say drop test, the object is large. The nodes that come in contact will experience additional stress due to the acceleration of nodes that are still in air.

I don't know how much effect that has though...

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Hi Bongirs,

 

Example for an initial velocity problem:

Like you want to simulate a drop test, so to avoid the free fall simulation time, we can calculate the velocity just before the the object hits the grounds and put the same in the initial velocity boundary condition. With gravity applied to the model, the velocity will be take care by gravity during the remaining time.

 

Example for imposed displacement problem:

Like you want to obtain a load Vs displacement graph where the load is being applied trough an actuator on the object. It is generally used for quasi static simulation and imposed velocity is given as a function of time similar to what applied to an actuator in a physical test.

 

Instead of applying initial acceleration, if we know the mass of the object, we can directly applying a force=mass* accln .This force can be given with varying time so if you need to apply initial acceleration, give force at t=0 and then make it zero for rest of the time. I think this will consider the initial accln.

For acceleration, we can use force equation but same does not seems feasible for velocity and displacement. Thus,I think we have velocity and displacement BC's but not initial accln.

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