Flow of a yield-stress fluid over a cavity: Experimental study of the solid-fluid interface

- Benchmark experimental data on the flow of a viscoplastic fluid above a cavity are reported.
- Flow structure consists of a flowing layer sandwiched between two unyielded zones, one static in the cavity and one mobile above.
- Shear rate is non-monotonous in the flowing layer.
- The flow self-organizes to partially smooth out the existence of the cavity.

This paper presents an experimental characterization of the flow of an elasto-viscoplastic fluid (Carbopol) over a dead zone constituted of the same material. The studied configuration consists of a closed rectangular channel with a cavity in its base. A curved solid-fluid interface forms into the cavity, separating a yielded flowing layer above from an unyielded dead zone below. The hydrodynamics of the flow is investigated by means of high-resolution optical velocimetry (PIV). We focus in particular on the velocity profiles and shear-rate evolution in a quasi-longitudinal flow domain located around cavity mid-length. Our measurements show a non-monotonous evolution of the shear rate, which increases from zero at the solid-liquid interface, passes through a peak (sometimes leveling off at its maximum value), and returns to zero in a plug zone sufficiently far above the cavity. Two main flow zones can be distinguished: a Poiseuille zone, in which velocity profiles are the same as in the flow over a rigid wall, and a boundary layer ensuring the transition with the dead zone. Hence, consistently with our previous work (Luu et al., 2015 [9]), the flow self-organizes to partially smooth out the bottom perturbation. The characteristic thicknesses of the flow zones are shown to evolve with cavity length and hydrodynamic properties of the incoming flow. The study also points out the influence of elastic effects, notably on the shape (asymmetry) of the solid-fluid interface.