The effect of particles on flow instability and turbulence is a longstanding topic of research either by experiments or simulations. I will talk about the specific case of finite size particles in two distinct configurations: flow around a cylinder and dispersion in turbulent Couette and Poiseuille flows. Based on numerical simulations, I will show how particles trigger hydrodynamic instabilities and influence turbulent properties of the carrying flow. I will comment on the intricate interplay between particle accumulation and the transition from laminar to turbulent flow. These two generic configurations help a better understanding of the complex interaction between the migration of particles and flow dynamics.