Planetesimal formation occurs early in the evolution of a solar system, embedded in the circumstellar gas disk, and it is the crucial first step in planet formation. Their growth is difficult beyond boulder size, and likely proceeds via the accumulation of many rocks in turbulence followed by gravitational collapse - a process we are only beginning to understand. We have performed global simulations of the gas disk with embedded particles in the FLASH code. Particles and gas feel drag based on differential velocities and densities. Grains and boulders of various sizes have been investigated, from micron to km, with the goal of understanding where in the disk large planetesimals will tend to form, what sizes will result, and what size ranges of grains will be preferentially incorporated. We have so far simulated particles vertical settling and radial drift under the influence of gas drag, and their accumulations in turbulent clumps.