Experimental investigation of viscoplastic fluid flow in stenosed circular pipes using Laser Doppler Velocimetry

Abstract

This study reports an experimental investigation of steady viscoplastic flow through stenosed circular pipes using Laser Doppler Velocimetry. A dedicated hydrodynamic bench was developed to compare Newtonian water with a viscoplastic Carbopol gel flowing through pipes exhibiting three levels of axisymmetric constriction (57%, 86%, and 94% reduction in area). Rheological characterization confirmed the suitability of a modified Casson-type model for describing the Carbopol solution, which exhibits a measurable yield stress and shear thinning behavior. The local velocity measurements revealed that recirculation regions downstream of the stenosis become significantly longer and more persistent in viscoplastic flow than in water, especially for moderate to severe constrictions. The global hydraulic analysis further demonstrated that, unlike the Newtonian case, the viscoplastic fluid may experience a markedly reduced flow resistance in the post-stenotic region, sometimes even below the reference value of an unobstructed pipe, highlighting a counter-intuitive stabilizing mechanism. Turbulence measurements confirmed this trend, with substantially attenuated fluctuations in the non-Newtonian flow. The results provide experimentally validated trends and correlations that improve the prediction of pressure losses in complex geometries and contribute to a deeper understanding of flow stabilization mechanisms relevant to both biomedical and industrial transport systems.