boundary condition in the case of dynamic condition

Question

sir, 
 I am learning the dynamic features of PLAXIS 2D. for this I am using PLAXIS 2D tutorial manual by Bentley. Here, I am confused in the dynamic boundary condition given in tutorial 14 dynamic analysis of generator on the elastic foundation . why Xmin, Ymax has none boundary while Xmax and Ymax has viscous boundary? can we take all the boundary viscous?

Vasileios Basas · Answer

Hi Shailja, 
 The difference between dynamic boundaries is explained here: https://communities.bentley.com/products/geotech-analysis/w/wiki/45984/on-the-use-of-dynamic-boundary-conditions 
 The main difference between the " None " and the " Viscous " option is that the former is a fully reflective boundary, and the latter consists of viscous dampers that absorb the incoming wave energy. 
 In the example you have mentioned, we are creating a half-symmetric model, which means no damping along the line of symmetry (Xmin). On the other hand, the Xmax and Ymin boundaries are realistically placed at some distance from the region of interest. Therefore, if you had a fully reflective boundary condition ("None") on those sides, that would imply a reflection of the outward propagating waves back into the model, which does not allow the necessary energy radiation. Theoretically, you could use a larger model to minimize the issue of having reflective boundaries since material damping will absorb most of the energy in the waves reflected from distant boundaries. However, this solution leads to high computational costs. As such, the alternative is to use nonreflecting (or absorbing) boundaries.