The present study describes the potential intrusive effects of CREC-GS-Optiprobes on the flow pattern in gas-solid downflow units. Granular flows and their interactions with the CREC-GS-Optiprobes are calculated using a CPFD (Computational Particle Fluid Dynamic) numerical scheme in three dimensions, using particle clusters. Once particles dispersed in the column, the numerical simulations revealed that these particles formed a relatively homogeneous suspension with cluster formation and solids accelerating until reaching a fully-developed regime. CPFD simulations showed that two axially aligned CREC-GS-Optiprobes, when placed in the fully developed flow regime, introduce minimum perturbations in the gas-solid suspension. Therefore this CREC-GS-Optiprobe creates a highly illuminated focal region where hydrodynamic parameters can be measured without significant flow disturbance. Thus, this study supports the application of fiber optic probes; such as in the case of the CREC-GS-Optiprobes, which can provide undisturbed axial particle cluster velocity, axial cluster slip velocities, cluster size and solid fraction measurements at strategically selected locations in the gas-solid downer unit. (C) 2012 Elsevier B.V. All rights reserved.