Startup flow with and without wall slip in a pipe plugged with weakly compressible complex fluids—A new insight

The present article explains the nontrivial synergetic effect of wall slip, compressibility, and thixotropy in a pressurized flow startup operation of various structured fluids. Opposite to intuition, experimental and numerical simulations suggest that the wall slip (adhesive failure) facilitates gel degradation (cohesive failure), revealing a new flow startup mechanism. The irreversible thixotropic rheological model, along with the static slip-based model, is utilized to describe the structural degradation kinetics in the bulk phenomenon and the near-wall phenomenon, respectively. The near-wall transient variations in axial velocity or strain evolution and the initial pressure propagation mechanism along the axis of the circular pipe explain the essence of the aforementioned synergy. Finally, a comparative study of the effect of wall slip on the pressure propagation mechanisms and startup flow of generalized Newtonian fluids, viscoelastic based thixotropic fluids, and viscoelastic solids is also performed. Wall slip can convert no-steady-state thixotropic elasto-viscoplastic flow cases into a steady-state fluid flow, whereas it causes viscoelastic solids to move with a slip velocity. Additionally, our study revealed that stick-slip phenomena occur at an acoustic time scale. It requires a compressive wave to travel with information of stick position to the outlet and again back to the inlet, concomitantly causing the release of additional fluid, thereby converting the stick to a slip regime. Conventionally, stick-slip phenomena were mainly associated with nonlinearity. Hence, our study opened a new direction. Finally, concomitant with the experimental observations, we found that stick-slip phenomena disappear when the fluid is uniformly compressed and a steady state is reached.