  We study the turbulence induced in the dust layer of a protoplanetary disk
based on the energetics of dust accretion due to gas drag. We estimate
turbulence strength from the energy supplied by dust accretion, using the
radial drift velocity of the dust particles in a laminar disk. Our estimate of
the turbulence strength agrees with previous analytical and numerical research
on the turbulence induced by Kelvin-Helmholtz and/or streaming instabilities
for particles whose stopping time is less than the Keplerian time. For such
small particles, the strongest turbulence is expected to occur when the
dust-to-gas ratio of the disk is ~C_eff^(1/2) (h_g / r) ~ 10^(-2), where C_eff
~ 0.2 represents the energy supply efficiency to turbulence and h_g / r ~ 5 x
10^(-2) is the aspect ratio of the gas disk. The maximum viscosity parameter is
alpha_max ~ C_eff T_s (h_g / r)^2 ~ 10^(-4) T_s, where T_s (<1) is the
non-dimensional stopping time of the dust particles. Modification in the
dust-to-gas ratio from the standard value, 10^(-2), by any process, results in
weaker turbulence and a thinner dust layer, and consequently may accelerate the
growth process of the dust particles.
