Rapid urbanization has led to a growing number of environmental challenges in large parts of China, where the Yangtze River Delta (YRD) urban agglomerations serve as a typical example. To evaluate the relationship between environmental sustainability gaps and urbanization in 26 cities of the YRD, this study revisited the environmental sustainability assessment (ESA) by combining the metrics of environmental footprints and planetary boundaries at the city level, and then integrated the footprint-boundary ESA framework into decoupling analysis. The results demonstrated considerable spatiotemporal heterogeneity in the environmental sustainability of water use, land use, carbon emissions, nitrogen emissions, phosphorus emissions and PM_2.5 emissions across the YRD cities during the study period 2007–2017. Decoupling analysis revealed a positive sign that more than half of the 26 cities had achieved the decoupling of each category of environmental sustainability gaps from urbanization since 2014, especially for nitrogen and phosphorus emissions. On the basis of ESA and decoupling analysis, all the cities were categorized into six patterns, for which the optimal pathways towards sustainable development were discussed in depth. Our study will assist policy makers in formulating more tangible and differentiated policies to achieve decoupling between environmental sustainability gaps and urbanization.

Based on the theory of turbulent jet in fluid mechanics and considering the compressibility of mixed gas, a leakage model of indoor natural gas is established. Based on the RNG k-ε turbulence model, a coupled numerical model of indoor natural gas diffusion with double leaks was established by using Species component module, considering the diffusion efficiency of multi-components and accompanying energy conversion. The change trend of CH4 concentration field, density field and velocity field at different diffusion time is analyzed, and the development law of leakage diffusion process and the coupling characteristics of double leakage flow field are revealed. The results show that the core region, the initial segment and the main segment of the jet field are formed by the diffusion of natural gas. At the same time, when the distance between the two leaks is close, the coupling effect of the two is strong at the beginning of the leakage, and the stronger one has entrainment effect on the weaker one, which changes the basic form of the jet flow field.