Suppression mechanism of porous media coatings on wavefront steepness behaviors inside high-speed maglev tunnels

The initial wavefront triggered by the entry of maglev trains into tunnels continues to steepen nonlinearly during its propagation. The micro-pressure waves (MPW) released as the initial wavefront propagates to the tunnel exit can induce sonic booms and cause building structures resonance. This study proposes porous media coatings (PMC) installed on the tunnel to alter the propagation characteristics of the initial wavefront. The effects of porosity and geometric parameters of the PMC on the wavefront propagation characteristics are discussed. The research reveals that as the porosity, thickness, coverage angle, and streamwise range of the PMC increase, the friction effect is enhanced accordingly. By intensifying the friction effect, the PMC can continuously reduce the wavefront gradient magnitude and increase the wavefront thickness. The maximum reductions in wavefront gradient magnitude and MPW amplitude are 77.7% and 66.3%, respectively. The PMC transforms the MPW profile from triangular waveforms to Gaussian-shaped waveforms, extending its duration by 33.3%. Additionally, the acoustic Reynolds number is proposed as the criterion to assess whether the wavefront steepens or spreads during propagation. A critical acoustic Reynolds number (Rea) of 60 is identified, where wavefronts with Rea greater than 60 steepen during propagation, and vice versa. This study confirms the inhibitory mechanism of the PMC on wavefront steepening, even causing it to gradually spread. The findings provide significant insights for the future design of maglev tunnels.