A non-invasive experimental technique which enables optical penetration into the bulk of a flowing liquid-solid concentrated suspension was employed to study such flows. Matching of refractive indices of the continuous and dispersed phases was used to establish optimal conditions for the quality of the optical signals and can be used to measure concentration in the Bowing dispersion. Laser-Doppler anemometry was applied to measure velocity profiles in a rectangular duct and to detect velocity fluctuations in the viscous flow of the concentrated suspension induced by the particles presence. The existence of a net drift of particles in a concentrated suspension is demonstrated.