Non-Invasive liquid viscosity characterization in Fluid-Filled pipes using zero group velocity guided wave resonances

Non-invasive monitoring of liquid viscosity within sealed pipelines is important for industrial process control, yet conventional techniques require fluid sampling or sensor insertion. This study uses Zero Group Velocity (ZGV) guided-wave resonances in fluid-filled elastic pipes for viscosity characterization from external pipe-wall measurements. A 6 × 6 characteristic equation for axisymmetric wave propagation in a hollow cylinder containing a viscous Newtonian fluid is derived by incorporating viscous shear waves in the fluid domain. Perturbation analysis identifies two viscosity-dependent signatures, namely frequency shift and amplitude attenuation , and shows that amplitude attenuation, governed by boundary-layer shear dissipation, is the most sensitive to viscosity. Experimental validation using an electromagnetic acoustic transducer (EMAT) to excite the L(0,4)-type ZGV resonance near 569 kHz on an aluminum pipe filled with water-glycerol mixtures spanning viscosities from 8.9 × 10^-4 to 1.412 Pa·s yields an empirical semilogarithmic calibration curve with R² = 0.9951, an RMS theory-experiment deviation of 0.45 dB over an 11.5 dB dynamic range, and a repeatability-limited inversion precision of about ± 10 % for the tested conditions. The couplant-free EMAT configuration avoids transducer mass loading and allows consistent resonance tracking, with the measured peak frequency varying by only ± 0.2 % across the tested fluids.