The coagulation of dust particles is a key process in planetesimal formation .
However , the radial drift and bouncing barriers are not completely resolved , especially for silicate dust .
Since the collision velocities of dust particles are regulated by turbulence in a protoplanetary disk , the turbulent clustering should be properly treated .
To that end , direct numerical simulations ( DNSs ) of the Navier Stokes equations are requisite .
In a series of papers , Pan & Padoan used a DNS with the Reynolds number Re \sim 1000 .
Here , we perform DNSs with up to Re = 16100 , which allow us to track the motion of particles with Stokes numbers of 0.01 \lesssim St \lesssim 0.2 in the inertial range .
By the DNSs , we confirm that the rms relative velocity of particle pairs is smaller by more than a factor of two , compared to those by Ormel & Cuzzi ( 2007 ) .
The distributions of the radial relative velocities are highly non-Gaussian .
The results are almost consistent with those by Pan & Padoan or Pan et al .
at low- Re .
Also , we find that the sticking rates for equal-sized particles are much higher than those for different-sized particles .
Even in the strong-turbulence case with ƒ \alpha -viscosity of 10 ^ { -2 } , the sticking rates are as high as \gtrsim 50 \% and the bouncing probabilities are as low as \sim 10 \% for equal-sized particles of St \lesssim 0.01 .
Thus , the turbulent clustering plays a significant role for the growth of cm-sized compact aggregates ( pebbles ) and also enhances the solid abundance , which may lead to the streaming instability in a disk .