A strong turbulent field produced by particle clouds was simulated in a numerical calculation based on the DSMC method. The flow studied was a gas-solid flow in a vertical channel. Particles were initially distributed homogeneously. Under the effects of particle-fluid and particle-particle interactions, the particle distribution gradually becomes inhomogeneous and finally leads to formation of particle clouds. These clouds are a kind of large obstacles for fluid motion, and produce large scale fluid turbulence.