Experimental measurement of magnetic signatures induced by ion dynamics in seawater jet flows

As underwater stealth technology continues to advance, the effectiveness of conventional acoustic detection is becoming increasingly constrained, motivating the development of non-acoustic detection techniques based on electromagnetic signatures. This study presents a systematic theoretical, experimental, and numerical investigation of magnetic fields generated by ion dynamics in conductive seawater jet flows. A multiphysics coupling model incorporating hydrodynamics and electromagnetics was first established. A high-fidelity experimental platform was then developed using a nonmagnetic recirculating water tank and a high-sensitivity cesium optically pumped magnetometer to measure a pulsed jet with a modulation frequency of 0.216 Hz. To extract weak signals from background noise, a signal-processing framework combining pure-water reference measurements with minimum-variance unbiased estimation was proposed. Both the measurements and the numerical simulations showed a pronounced non-monotonic spatial evolution of the induced magnetic field: the signal amplitude increased downstream, reached a maximum of about 0.1 nT in the near-field region, and then decayed rapidly in the far field. Further flow-magnetic coupling analysis revealed that the induced magnetic field was primarily concentrated in the high-speed jet core within the strong shear layer, where the conductive fluid most effectively interacted with the ambient geomagnetic field. The subsequent attenuation of the magnetic signal was governed by radial spreading of the jet pulse, kinetic-energy dissipation, and weakening of the local velocity gradient. These results demonstrate the feasibility of detecting hydrodynamically induced wake signatures of non-ferromagnetic underwater targets using high-sensitivity magnetic measurements, and provide both experimental support and theoretical guidance for non-acoustic underwater target detection.