Abstract: The ever increasing computing time, and the continuous developments of efficient numerical methods, have enabled breakthroughs in the field of turbulent particle suspensions: It has become possible to perform simulations around hundreds of thousands, or even millions of small spherical particles, where the flow conforming the surface of each sphere is directly solved. These first-principles, ‘brute-force’ simulations allow us to address fundamental questions concerning, for instance, the validity of widely-used point-particle methods for small inertial particles, and the mechanisms for turbulence modulation under one two or four-way-coupling conditions. We will present the ingredients which enabled us to perform these ambitious simulations with reasonable computational resources by today’s standards. Then the main part of the seminar will focus on the mechanisms for turbulence modulation that were unveiled by these simulations. We show that, for small solid mass fractions, the flow dynamics resemble those of single-phase flow with slightly larger wall shear. Conversely, for larger mass fractions, the flow shows strong turbulence attenuation, still with a net drag increase. We will explain the observations in light of the stress budget simplified for the dilute, two-way coupling limit.