# Interface type 7: how to calculate STFAC if ISTF=1?

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Hi everybody

i'm simulating a quarter sphere entering in a tank with water; i modelled the water using SPH elements.

between the surface of sphere and particles of water i use a type7 interface.

i have tryed two ways:

with ISTF=1 i set a STFAC=40000Pa

with ISTF=0 i set STFAC=0.1 (it is a scale factor to be multiplied by the young modulus of the sphere material, i have wood with E=14500MPa so by 0.1 it is 1450MPa as interface stifness) it comes out a more larger stifness interface respect the previous one.

All the rest of model is the same between the two simulations

the simulation with lower STFAC seems to be less viscous than the one with bigger STFAC.

I'm i wrong?

Dose some know why this is the behaviour of water? What does interest the interface stifness with this kind of behaviour?

Thank you to all

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Hi Damir,

From your inputs, with ISTF= 1 the contact stiffness is less than ISTIF=0.

I assume the first runs faster compared to the later, isn't it?

When the contact stiffness is small there is a chance of parts/elements penetration into each other. I assume this could be the reason for the viscous behavior?!!

I am not really sure about the math behind, but as the penalty increases (stiffness factor) more noise can be seen on SPH.

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Hi Prakash

Yes, with ISTF=1 i input a STFAC of 40000Pa and the simulation is large faster than ISTF=0 and STFAC=0.1.

In Fact Nodal time step can be seriously affected if penetration is large as the online help says.

It says also that

"Moreover, there is a viscous damping acting on the rate of penetration. The contact force is then computed as:

"

so incrasing stifness interface we increase the viscous damping acting on slave nodes, and it results in the viscous

behaviour of water.

Could it be the reason?

thanks

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I think we are almost there.

From the above eqaution, For a constant velocity (of rigid body) contact force is directly proportional to damping coefficient ("C").

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Yes, we are almost there...

So if i use ISTF=1, i need a low stifness interface (STFAC), for low down the dumping coefficient and increase the time step.

But is there a way to calculate this stifness interface? How can i decide between 1MPa, 40000Pa or 400Pa...?

May i refer to a characteristic quantity, like sphere water entering velocity, to calculate the stifness, and if is so, is there

a formula to use?

thanks

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Maybe using Hertzian contact theory?

I am not sure. maybe a trial and error method is more economical.

Start with 100.. see the contact. Not good, increase a zero and go on..

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Great!!!

But if i had not  experimental data witch to refer and compare qualitative or quantitative data,

i cannot procede with trial and error method.

I'm i wrong?

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You are right,

You need some or the other data to start off with.

If don't have any data you may have to rely on theory like Hertzian contact theory or others... OR you may to take reference from similar past simulations.

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