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?
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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 example). 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. /BATCH /COM,ANSYS RELEASE 5.3 UP071096 15:49:44 12/16/1996 /input,start,ans ,/home3/staff/jmt1129/,,,,,,,,,,,,,,,,1 /show,x11 *dim,dval,array,9 *dim,sval,array,9 dval(1)=.01 sval(1)=1e6 dval(2)=.1 sval(2)=1e6 dval(3)=1 sval(3)=1e6 dval(4)=.01 sval(4)=1e5 dval(5)=.1 sval(5)=1e5 dval(6)=1 sval(6)=1e5 dval(7)=.01 sval(7)=1e4 dval(8)=.1 sval(8)=1e4 dval(9)=1 sval(9)=1e4 *do,ii,1,9,1 parsav,all /clear /gopr parres refl=1e%ii% /prep7 et,1,42 et,2,19,,,,1 n,1 n,101,100 fill n,201,,1 n,301,100,1 fill mat,2 ngen,2,1,101,301,200 nmodif,101,99.9 nmodif,301,99.9 e,1,2,202,201 *repeat,100,1,1,1,1 !e,1,11,211,201 !*repeat,10,10,10,10,10 ex,1,1e6 dens,1,1 nuxy,1,.3 damp,1,0.001 ex,2,1e6 dens,2,1 nuxy,2,.3 !mp,damp,2,50 /TITLE, REFLECTIVE PROBLEM, DAMP=%dval(ii)%, STIFFNESS = %sval(ii)% mat,1 esel,s,,,1,75 emodif,all esel,all mat,2 e,101,102,302,301 type,2 real,2 mat,3 e,101,301 r,2,,,,sval(ii) mp,damp,3,dval(ii) !*.005 !betad,0.001*ii fini !/debug,-1,,,1 /solu antype,tran !qslv,pcg,1e-5 !d,101,ux !d,301,ux d,1,uy d,101,uy outres,all,10 time,1e-10 solve kbc,1 f,1,fx,.5 f,201,fx,.5 nsub,1000 time,1 solve fini /show,reflect,f33 /POST26 FORCE,TOTAL SHELL,TOP LAYERP26,0 ESOL,2,1,0,S,X,SX1 FORCE,TOTAL SHELL,TOP LAYERP26,0 ESOL,3,21,0,S,X,SX21 FORCE,TOTAL SHELL,TOP LAYERP26,0 ESOL,4,41,0,S,X,SX41 FORCE,TOTAL SHELL,TOP LAYERP26,0 ESOL,5,61,0,S,X,SX61 FORCE,TOTAL SHELL,TOP LAYERP26,0 ESOL,6,81,0,S,X,SX81 NSOL,7,1,U,X,UX1 NSOL,8,21,U,X,UX21 NSOL,9,41,U,X,UX41 NSOL,10,61,U,X,UX61 plvar,2,3,4,5,6 plvar,7,8,9,10 FINISH *enddo |
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