An experimental study of the solid–liquid interface for an elasto-viscoplastic fluid flow in benchmark geometry: 4 to 1 planar contraction

This paper presents an experimental study of the flow of an elasto-viscoplastic fluid (Carbopol) in a rectangular duct with an abrupt contraction of an aspect ratio of 4:1. At the corners of the contraction, a dead zone appears with no recirculation zone. The hydrodynamics of the flow is studied using high-resolution laser Doppler velocimetry (LDV). We focus, in particular, on velocity and shear rate profiles within and above the dead zone. The position of the solid–liquid interface is provided for different flow rates. The velocity profiles show a line where they overlap at the same point, known as the “slip line.” The position of this line is independent of the flow rate, but its velocity evolves with the flow rate, for which we have proposed a correlation. The position of this line aligns with the level of contraction for all flow rates. Shear rate profiles show a non-monotonic evolution: from zero at the solid–liquid interface to a peak, then decreasing to zero again in the plug zone at the core of the pipe, indicating the presence of different layers between the solid–liquid interface and the slip line. The line of maximum shear rate is independent of the flow rate, suggesting that the width of the zone between the slip line and this line remains constant at all flow rates. The study is completed by establishing correlations between the friction factor (f) and the Euler number (Eu) as a function of the generalized Reynolds number Regen upstream, downstream, and at the level of contraction.