  The arising of turbulence in gas-dynamic (non-magnetic) accretion disks is a
major issue of modern astrophysics. Such accretion disks should be stable
against the turbulence generation, in contradiction to observations. Searching
for possible instabilities leading to the turbulization of gas-dynamic disks is
one of the challenging astrophysical problems. In 2004, we showed that in
accretion disks in binary stars the so-called precessional density wave forms
and induces additional density and velocity gradients in the disk. Linear
analysis of the fluid instability of an accretion disk in a binary system
revealed that the presence of the precessional wave in the disk due to tidal
interaction with the binary companion gives rise to instability of radial modes
with the characteristic growth time of tenths and hundredths of the binary
orbital period. The radial velocity gradient in the precessional wave is shown
to be responsible for the instability. A perturbation becomes unstable if the
velocity variation the perturbation wavelength scale is about or higher than
the sound speed. Unstable perturbations arise in the inner part of the disk
and, by propagating towards its outer edge, lead to the disk turbulence with
parameters corresponding to observations (the Shakura-Sunyaev parameter $\alpha
\lesssim 0.01$).
