user subroutine DISP : fix the boundary when a variable reaches the prescribed value

[wkxez]

Hi I am using the subroutine DISP.

I defined a variable P used to determine when to fix the boundary.
if (KSTEP .EQ. 1) then

IF((COORDS(3) .EQ. 0.0000) .AND. (JDOF .EQ. 1) .AND. (P .GE. 0.10)) then
U(1)=0.0000000000
!write(*,*) 'P_conver',P_conver
END IF
IF((COORDS(3) .EQ. 0.0000) .AND. (P .GE. 0.10) .AND. (JDOF .EQ. 2)) then
U(1)=0.0000000000
END IF
ENd if

I defined a surface for the boundary condition. P changes with time. At some time, if p>=0.1 , I want to fix the x, y freedom of this surface at current poisition.
I tried to use this code. But it seems the boundary is totally fixed.
Could you help me check the problem?
Thanks a lot!