Many-core architectures are promising hardware to design hard real-time systems as they are based on simpler and thus more predictable processors than multi-core systems. However, the worst-case behavior of the Network-on-Chip (NoC) for both inter-core and core to external memories or peripherals communications must be established. Several NoCs targeting hard real-time systems, made of specific hardware extensions, have been designed. However, none of these extensions are currently available in commercially existing NoC-based many-core architectures, that instead rely on wormhole switching with round-robin arbitration. In this paper, we thus demonstrate three properties of such NoC-based wormhole networks to identify worst-case scenarios and reduce the pessimism when modeling flows in contentions. We then describe and evaluate an algorithm to compute Worst-Case Traversal Time (WCTT) of flows that uses these properties. In particular, our results show that the pessimism can be reduced up to 50% compared to current state-of-the-art real-time packet schedulability analysis.