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What is usTangentialForce

I would like to ask what usTangentialForce represents in the EDEM. Additionally, could you please clarify why usTangentialForce is not zero when the coefficient of restitution is set to 1?

Thank you very much.

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Alice Sun

Could you please provide a more detailed explanation?

Stephen Cole

Hi, there is some discussion on this here:

https://community.altair.com/discussion/64787/edem-what-is-usnormalforce-in-edem-contact-model-api

The damping tangential force is related to the coefficient of Static Friction, the Restitution isn't included in the equation.

Regards

Stephen

Alice Sun

Is the tangential damping force given by this equation?
When the coefficient of restitution is equal to 1, β becomes zero.
In that case, shouldn’t the tangential damping force be zero?

Alice Sun

Is the tangential damping force given by this equation?
When the coefficient of restitution is equal to 1, β becomes zero.
In that case, shouldn’t the tangential damping force be zero?

Stephen Cole

Hi, you're correct in that the above goes to 0 with restitution of 1 however tangential energy dissipation has two mechanisms.

The first (viscous damping) is given in the equation above however for the second if the tangential force exceeds the Coulomb limit, sliding occurs and energy is dissipated through friction, regardless of the restitution coefficient.

You can see an example in the code here:

https://community.altair.com/discussion/35246/edem-api-contact-model-example-hertz-mindlin

In ChertzMindlin.cpp under:

    void dampingTangentialForce(const CSimple3DVector& relVel_t,

                                const NCalcForceTypesV3_4_0::SInteraction& interaction,

                                const CSimple3DVector& F_n,

                                const CSimple3DVector& F_t,

                                const double tangentialStiffness,

                                const double sqrtEquivMass,

                                const double dampingRatio,

                                CSimple3DVector& tangentialOverlap,

                                CSimple3DVector& F_td,   

                                CSimple3DVector& newF_t) 

    {

        double ft2 = F_t.lengthSquared();

        double fn2 = F_n.lengthSquared() * interaction.staticFriction * interaction.staticFriction;


        if (ft2 > fn2)

        {

            newF_t = F_t * sqrt(fn2 / ft2);

            tangentialOverlap = newF_t * (-1.0 / tangentialStiffness); //slippage has occurred so the tangential overlap is reduced a bit


                                                                       //at this point we get energy loss from the sliding!

            F_td = newF_t;

        }

        else

        {

            //at this point we get energy loss from the damping!

            F_td = relVel_t * (-2.0 * sqrt_5_6 * dampingRatio * sqrt(tangentialStiffness) * sqrtEquivMass);

            newF_t = F_t + F_td;

        }

    }

}

This is covered in the "if (ft2 > fn2)" block. If the material is sliding energy may still be lost due to the friction.

Regards

Stephen

Alice Sun

Thank you for the reply.