  As a smaller grain, which is attached on larger one, is growing, it pushes
also the larger one and other grains in its surrounding. In a simulation of
similar system, repulsive force such as contact force based on linear
spring-dashpot model can not accommodate this situation when cell growing rate
is faster than simulation time step, since it produces sudden large overlap
between grains that makes unphysical result. An algorithm that preserves system
linear momentum by introducing additional velocity induced by cell growth is
presented in this work. It should be performed in an implicit step. The
algorithm has successfully eliminated unphysical overlap.
