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08-24-2009 07:39 AM

Want to do a non linear analysis, but inertia relief is not a feature

here. It can be solved by setting the Matrix stabilization MSTAB=1, but

this requires a balanced loading which I wonder how to set up.

PROBLEM:

Want to do analysis on a free falling object the moment it hits the water.

Known variables: pressure on the surface caused by the object hitting

the water surface.

Acceleration is unknown.

This might be a trivial question, but I do not have much experienced

with NX Nastran yet and I need to find out the best and most correct way

of doing this. Am greatful for any tip.

Helge M

2 REPLIES

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08-24-2009 08:30 AM

Hello Helge,

You are correct that SOL 601 does not have an inertia relief capability. But you

could try solving the response dynamically by applying the pressure load to the

free-free structure. The acceleration from the pressure loading will be determined

as part of the solve. The strucuture will undergo large displacement because it

is not restrained, so you may want to ignore those results and instead focus on

the stress results that are due to the relative displacement over the structure.

Alternatively, you could do a dynamic impact analysis. Here you model both the structure

and the water. We don't have a water material model, but you could try using a hyperelastic

material model instead - both water and hyperelastic materials are considered incompressible.

Model the water as a large block just below the structure. Restrain the boundaries

on all sides of the block except the sureface where impact occurs. Model the structure

in a position ablove the block moments before impact and apply an initial velocity

that corresponds to the velocity at impact. Then of course, set up a contact condition

between the structure an the water. The impact pressure and acceleration are them

calculated as part of the dynamic solution. You could compare the computed pressure

results with that of pressure data you currently have to make sure they are similar.

You could adjust the hyperelastic material properties to make the pressures more

similar if needed. The max stress will occur in the moments after impact and I think

you could stop the simulation after that point.

Mark

=?ISO-8859-1?Q?Helge_Mj=F8vik?=

>This problem can be solved in linear analysis using the inertia relief.

>Want to do a non linear analysis, but inertia relief is not a feature

>here. It can be solved by setting the Matrix stabilization MSTAB=1, but

>this requires a balanced loading which I wonder how to set up.

>

>PROBLEM:

>Want to do analysis on a free falling object the moment it hits the water.

>Known variables: pressure on the surface caused by the object hitting

>the water surface.

>Acceleration is unknown.

>

>This might be a trivial question, but I do not have much experienced

>with NX Nastran yet and I need to find out the best and most correct way

>of doing this. Am greatful for any tip.

>

>Helge M

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08-27-2009 02:58 AM

Thank you for the good tips.

Will try it out.

Helge

Mark Donley wrote:

> Hello Helge,

>

> You are correct that SOL 601 does not have an inertia relief capability. But you

> could try solving the response dynamically by applying the pressure load to the

> free-free structure. The acceleration from the pressure loading will be determined

> as part of the solve. The strucuture will undergo large displacement because it

> is not restrained, so you may want to ignore those results and instead focus on

> the stress results that are due to the relative displacement over the structure.

>

> Alternatively, you could do a dynamic impact analysis. Here you model both the structure

> and the water. We don't have a water material model, but you could try using a hyperelastic

> material model instead - both water and hyperelastic materials are considered incompressible.

> Model the water as a large block just below the structure. Restrain the boundaries

> on all sides of the block except the sureface where impact occurs. Model the structure

> in a position ablove the block moments before impact and apply an initial velocity

> that corresponds to the velocity at impact. Then of course, set up a contact condition

> between the structure an the water. The impact pressure and acceleration are them

> calculated as part of the dynamic solution. You could compare the computed pressure

> results with that of pressure data you currently have to make sure they are similar.

> You could adjust the hyperelastic material properties to make the pressures more

> similar if needed. The max stress will occur in the moments after impact and I think

> you could stop the simulation after that point.

>

> Mark

>

>

>

>

>

>

> =?ISO-8859-1?Q?Helge_Mj=F8vik?=

>> This problem can be solved in linear analysis using the inertia relief.

>> Want to do a non linear analysis, but inertia relief is not a feature

>> here. It can be solved by setting the Matrix stabilization MSTAB=1, but

>> this requires a balanced loading which I wonder how to set up.

>>

>> PROBLEM:

>> Want to do analysis on a free falling object the moment it hits the water.

>> Known variables: pressure on the surface caused by the object hitting

>> the water surface.

>> Acceleration is unknown.

>>

>> This might be a trivial question, but I do not have much experienced

>> with NX Nastran yet and I need to find out the best and most correct way

>> of doing this. Am greatful for any tip.

>>

>> Helge M

>

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