Non-reflecting Boundary with MMALE and Initial Stress depth

[R. D.]

Dear All,

I want to simulate a wave propagation throughout a medium (e.g. soil, which also has in-situ stress). The boundaries at the sides and bottom should let the wave pass through (i.e. no reflection) as they are meant to simulate infinite medium.

I am using MMALE element formulation and the soil "must" have initial stress defined and equilibrium should be maintained if no load is applied. 

The first option that came to my mind was non-reflecting boundary, however, it exhibits issues when it is combined with conventional spc boundaries. The non-reflecting boundary itself does not include the spc since as I understood it deals with velocity rather than displacement, therefore, it cannot maintain the initial stress.

I do not know if there is any other method available in LS-DYNA. 

Thank you in advance for your help.

Best Regards,

Reza Daryaei

[l...@schwer.net]

1\ I am not sure why you want to use the MM-ALE solver for a wave propagation problem, but be advised I think the *BOUNDARY_NON_REFLECTING only works with 2D & 3D Lagrange solids.

 

2\ The *BOUNDARY_NON_REFLECTING feature is only intended to work with elastic waves.

 

3\ You could surround your MM-ALE mesh with a few layers of Lagrange solids and then use *BOUNDARY_NON_REFLECTING on the Lagrange layers.

 

4\ I am not sure how you intend to implement the in situ soil stress, but if you intend to use dynamic relaxation you will need to constrain the boundaries from displacements normal to the surfaces. This will cause a conflict with the use of *BOUNDARY_NON_REFLECTING. One idea I have suggested in the past, but never tried, is to constrain the outer most layer of Lagrange solids and apply *BOUNDARY_NON_REFLECTING to the layers of Lagrange segments just inside this boundary. If you try this approach, please let me know if it works.

 

                --len

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[James M. Kennedy]

Dear Reza,

 

*BOUNDARY_NON_REFLECTING

 

Purpose: Define a non-reflecting boundary. This option applies to continuum domains

modeled with solid elements. For geomechanical problems this option is important for

limiting the spatial extent of the finite element mesh and thus the number of solid

elements.

 

Remarks:

 

1. Non-reflecting boundaries defined with this keyword are only used with three

dimensional solid elements. Boundaries are defined as a collection of segments,

and segments are equivalent to element faces on the boundary. Segments are

defined by listing the corner nodes in either a clockwise or counterclockwise order.

 

2. Non-reflecting boundaries are used on the exterior boundaries of an analysis

model of an infinite domain, such as a half-space to prevent artificial stress wave

reflections generated at the model boundaries form reentering the model and

contaminating the results. Internally, LS-DYNA computes an impedance matching

function for all non-reflecting boundary segments based on an assumption of linear

material behavior. Thus, the finite element mesh should be constructed so that

all significant nonlinear behavior is contained within the discrete analysis model.

 

Sincerely,

James M. Kennedy

KBS2 Inc.

August 18, 2021

 

 

From: ls-d...@googlegroups.com [mailto:ls-d...@googlegroups.com] On Behalf Of R. D.
Sent: Wednesday, August 18, 2021 7:50 AM
To: LS-DYNA2 <ls-d...@googlegroups.com>
Subject: [LS-DYNA2] Non-reflecting Boundary with MMALE and Initial Stress depth

 

Dear All,

--

[R. D.]

Dear All,

Thanks for the answers and sorry for a relatively late Response :))  I wanted to let you know that confining the ALE with Lagrangian elements and applying the non-reflecting boundary on lagrangian elements works!
Therefore thank you Len for the suggestion.

The reason I am using MMALE: "it is a geotechnical penetration problem where the penetrator - in this case a pipe-shaped pile - is installed in the soil using an impact force. 

I am using the same material as ALE for the Lagrangian part which is Soil. I still have challenges regarding initialization of stress, since I do not use dynamic relaxation rather *INITIAL_STRESS_DEPTH command. 

The stress initialization is still tricky in conjunction with non-reflecting boundary condition in Lagrangian elements. I have read in the manual that if dynamic relaxation is used the non-reflecting boundary is turned momentarily into fixed constraint and in the transient analysis a force is applied on the segment to ensure the correct initialization. 

One alternative is also to assign a different material to the lagrange part with very high resistance so that it does not deform under high soil pressure at bottom of the soil.

I will try to post my experiences here for the future engineers who want to simulate similar problems.

Best regards 
Reza

[SUMAN KUMAR]

Dear all,

Thank you for discussing this problem. Thanks to Len for the suggestion and Reza for the confirmation. It maybe helpful for me to mitigate the reflection problem in MM-ALE based blast modelling.

Regards,

Suman.