Experimental Study of Unsteady Evolution of Dense Particle-Liquid Flow down a Rough-Bed Inclined Channel Using a Refractive Index Matching Technique

The commonly observed dense particle-liquid flows in natural environments, are crucial for understanding various geophysical hazards. Under conditions of high particle concentration and strong interphase coupling, such flows exhibit pronounced structural layering and substantial temporal variability. In the present study, the refractive index matching technique is employed to develop an inclined flume system that enables direct visualization of internal structures for granular flow. The stratified configurations and unsteady evolution under two representative inclination angles are examined. Vertical profiles of particle concentration, velocity, shear rate, and granular temperature are extracted and analyzed. The results indicate that the evolution proceeds through four stages: front, quasi-steady, fluctuation, and decline. The inclination angle significantly influences flow structure and evolution. At low inclination, the flow gradually transforms from a dual-layer of granular laminar flow-friction flow to a single-layer configuration, where the granular laminar flow thickness occupies 54%, 64%, and 100% of the flow depth during the quasi-steady, fluctuation, and decline stages. In contrast, at high inclination, a dual-layer of collision flow-friction flow structure is maintained throughout the whole flow, where the collision flow thickness occupies 44%, 50%, and 67% of the full depth in quasi-steady, fluctuation, and decline stages, respectively. The mesoscopic layer structure and its evolution are also found. We also discussed the reason for the difference. Findings of this study provides the evolution of stratified structures in dense flows, offering experimental evidence and physical insights for the dynamical modeling of particle-liquid flows.