Design and optimization of mixing processes of fibrous suspensions require a comprehensive quantification of functional relations between the substrate properties, the stirring parameters and the mixing result. Experimental tests have shown that the velocity distribution in stirred systems is significantly affected by the fibrous particles. Particle size and particle shape exert essential influence on the flow velocities. Therefore, the use of simplified particle-free model systems for the replication of mixing processes of fibrous suspensions is not or of limited suitability.