  The conditions allowing particle suspension in turbulent flow are of interest
in many applications, but understanding them is complicated both by the nature
of turbulence and by the interaction of flow with particles. Observations on
small particles indicate an invariance of acceleration PDFs of small particles
independent of size. We show to be true the postulated role of particle/fluid
interaction forces in maintaining suspension. The 3D-PTV method, applied for
two particle phases (tracers and inertial particles) simultaneously, was used
to obtain velocity and acceleration data, and through the use of the particle's
equation of motion the magnitude of forces representing either the flow or the
particle interaction were derived and compared. The invariance of PDFs is shown
to extend to the component forces, and lift forces are shown to be significant.
