A user would like to get advise on how to specify a non-reflective
boundary condition in a structural wave-propagation problem. According
to the Release 5.3 Elements manual, this can be accomplished with a
surface effect element (i.e., SURF19) using MP,DAMP together with the
input of SURT and EFS real constants. When the user tries to do this,
he gets reflections. What's wrong?
The use of the surface effect elements to absorb energy works as
documented in the manual. However, there are several things that
need to be understood in order to use this feature.
1. A specified boundary condition (D command) at the reflective
boundary will negate the effect. The surface effect element with
the elastic foundation acts between the non surface element and the
ground and replaces the specified displacement at that location. The
inclusion of the specified displacement prevents the surface from
deforming and from absorbing energy.
2. Large values of damping will act as a rigid boundary and will reflect
an energy wavein exactly the same manner as a fixed boundary. This
may be verified by selecting some elements and specifying a large
damping value for them (for example, the last 25 element in the attached
3. The amount of damping will need to be tuned to the problem by
adjusting the value of stiffness and material damping in the element.
The example below demonstrates the use of surface effects elements to absorb energy at a boundary. This example was created for use with ANSYS 5.3
PLEASE NOTE that the example below was updated in August 2005 to use the latest elements SOLID182 and SURF153. The updated version is attached, and should be used with later releases of ANSYS.
/COM,ANSYS RELEASE 5.3 UP071096 15:49:44 12/16/1996
/TITLE, REFLECTIVE PROBLEM, DAMP=%dval(ii)%, STIFFNESS = %sval(ii)%