A Study of Dam Break Wave of Thixotropic Fluid: Bentonite Surges down an Inclined plane

Thixotropic fluids are commonly used in the construction industry (e.g. liquid cements, liquid concrete, drilling fluids), industrial applications (e.g. muds, paints) and the food industry (e.g. liquid dairy products, ketchup). Related applications include some forms of mud flows and debris flows, pasty sewage sludges and some wastewater treatment residues. Thixotropy is the characteristic of a fluid to form a gelled structure over time when it is not subjected to shearing and to liquefy when agitated. A thixotropic fluid is a non-Newtonian fluid with a viscosity that is a function of both shear rate y and instantaneous state(s) of structure of the material. Such a fluid exhibits a reversible time-dependent decrease in apparent viscosity under shear rate and a gradual recovery when the shear stress is removed. This report describes a basic study of dam break wave with thixotropic fluid. A dam break wave is a sudden release of a mass of fluid in a channel. This type of flows has not been studied to date with thixotropic fluid, despite its practical applications : e.g., mudflow release, concrete tests including L-Box and J-Ring for self-consolidating concrete testing, paint applications. Theoretical considerations were developed based upon a kinematic wave approximation of the Saint-Venant equations for a thixotropic fluid down a prismatic sloping channel. The thixotropic fluid model of COUSSOT et al. (2002a) was used since it describes the instantaneous state of fluid structure by a single parameter. The analytical solution of the basic flow motion and rheology equations predict three basic flow regimes depending upon the fluid properties and flow conditions, including the initial degree of jamming of the fluid : (1) a short motion with relatively-rapid flow stoppage for relatively small mass of fluid, (2) a fast flow motion for a large mass of fluid, or (3) an intermediate motion initially rapid before final fluid stoppage for intermediate mass of fluid and intermediate initial rest period To. Physical experiments were performed with bentonite suspensions. Systematic experiments showed four types of flows. For small bentonite mass concentrations and short relaxation times To, the fluid flowed rapidly down the slope and spilled into the overflow container (Flow Type I). For intermediate concentrations and rest periods, the suspension flowed rapidly initially, decelerated relatively suddenly, continued to flow slowly for sometimes before complete stoppage (Flow Type II). For large mass concentrations and long rest periods, the mass of fluid stretched down the slope, until the head separated from the tail (Flow Type III). The last flow pattern (Type IV) corresponded to an absence of flow for large bentonite concentrations and long rest times. Quantitative informations were documented in terms of the final fluid thickness, wave front position, wave front curvature, side profile of the wave front during motion and after stoppage, as well as the flow motion immediately after gate opening. Some freesurface instabilities are also discussed and illustrated. It is believed that the present study is the first theoretical analysis combining successfully the basic principles of unsteady flow motion (i.e. Saint-Venant equations) with a thixotropic fluid model, which was validated with large-size systematic laboratory experiments. It is the belief of the writers that, for such complex systems this kind of approach, combining both rheology and fluid dynamics, is necessary to gain new insights of these complicated flow motions.