Particle–fluid interaction forces as the source of acceleration PDF invariance in particle size

• Acceleration PDF invariance extends to component forces acting on the particles.
• The components invariance is explained analytically.
• Lift is as significant as drag, suggesting a fluctuations-based driving mechanism.

The main goal of our research is to understand the forces responsible for particle suspension in turbulent flow, taking into account the complicated nature of turbulence and the interaction of turbulent flow with particles. Recent observations indicate that the acceleration (or the total force) probability density functions (PDFs) of relatively small spherical particles is invariant to the particle diameter. We prove the postulated role of particle/fluid interaction forces in maintaining suspension using unique dataset of Lagrangian information of flow tracers and inertial particles. The data was obtained using the three-dimensional particle tracking velocimetry (3D-PTV) method, applied simultaneously to obtain velocity and acceleration data along trajectories of flow tracers and inertial particles. Through the use of the particle’s equation of motion, the magnitude and direction of forces are derived and the components representing the turbulent flow compared to the components due to the flow-particle interactions. We demonstrate that the invariance of PDFs of the total force extends to the orthogonal components of forces and add an analytical model that explains the phenomenon. Furthermore, we show that the lift forces in turbulent flow are important contributions for both tracers and inertial particles.