  Recent high resolution images of spiral galaxies show wide varieties of
features including nuclear spirals in the central parts. Some of the galaxies
show grand-design nuclear spirals. The morphology of grand-design spirals can
be further divided by the openness of the arms: tightly wound ones with winding
angle of around 3$\pi$ radian and open spirals with winding angle of around
$\pi$ radian. Based on hydrodynamical simulations, we have investigated the
mechanism responsible for the openness of nuclear spirals. Since the gas flow
in the nuclear region is mainly governed by the central mass concentration near
the nuclei and the sound speed of the gas, we have examined various models with
different mass concentration represented by the mass of the central black hole
and different sound speeds. We found that the tightly wound spirals can be
formed when the mass of the black hole is large enough to remove the
inner-inner Lindblad resonances and sound speeds lie between 15 - 20 km/sec.
Thus, the presence of the tightly wound nuclear spiral could imply the presence
of relatively massive black hole in the center.
