  We study the viscous effects on the interaction between the coplanar Be-star
disc and the neutron star in Be/X-ray binaries, using a three-dimensional,
smoothed particle hydrodynamics code. For simplicity, we assume the Be disc to
be isothermal at the temperature of half the stellar effective temperature. In
order to mimic the gas ejection process from the Be star, we inject particles
with the Keplerian rotation velocity at a radius just outside the star. Both Be
star and neutron star are treated as point masses. We find that the Be-star
disc is effectively truncated if the Shakura-Sunyaev viscosity parameter
alpha_SS >> 1, which confirms the previous semi-analytical result. In the
truncated disc, the material decreted from the Be star accumulates, so that the
disc becomes denser more rapidly than if around an isolated Be star. The
resonant truncation of the Be disc results in a significant reduction of the
amount of gas captured by the neutron star and a strong dependence of the mass
capture rate on the orbital phase. We also find that an eccentric mode is
excited in the Be disc through direct driving due to a one-armed bar potential
of the binary. The strength of the mode becomes greater in the case of a
smaller viscosity. In a high-resolution simulation with alpha_SS=0.1, the
eccentric mode is found to precess in a prograde sense. The mass capture rate
by the neutron star modulates as the mode precesses.
