Drag and stability of objects in a yield stress fluid

The drag force exerted on objects in a yield stress fluid was measured when the velocities become infinitely slow. In these quasi-static conditions, yield stress effects are predominant. Particular care was paid to determining the yield stress and checking interface conditions when characterizing these fluids from a rheometric standpoint. Drag coefficients could then be determined for interesting objects of various shapes. The important role of fluid-object interface effects was also highlighted. On the basis of these results, a stability criterion is proposed for the object in the fluid, to estimate the yield stress needed to balance buoyancy forces.