Multi-physics coupled simulation for thermal safety of high-altitude parachutists considering dynamic microenvironment

In extreme high-altitude parachuting, parachutists operate in a dynamic microenvironment. Thermal safety assessment relies on accurate acquisition of key parameters such as convective heat transfer coefficient h_c and ambient temperature T_a. The core thermal process governing the parachutist's thermophysiological response is the convective heat transfer between the human body surface and the high-speed airflow. Prevailing studies rely on static environmental assumptions and oversimplify the coupled effects of aircraft wake, ambient wind, and body motion. This leads to over-conservative clothing insulation design and insufficient thermal response prediction accuracy. To address this limitation, this study establishes a multi-physics coupled analysis framework integrating kinematics, CFD, and thermoregulation model. Through CFD simulation and data interpolation, vector superposition of aircraft wake and ambient wind field is achieved, and the complex aerodynamic environment during airdrop exit is quantified. A strategy based on angle of attack and Re segmentation is proposed to dynamically calculate the h_cfor each human body segment during descent. Comparisons with experimental results at −35 °C and 10 m/s demonstrate that the proposed CFD-coupled method reduces the skin temperature prediction error by 0.3 °C compared with the traditional empirical h_c model. Experimental and mechanistic validation is also conducted on core assumptions such as the constant clothing thermal resistance in high-velocity dynamic environments. In line with ISO 11079 occupational cold exposure safety thresholds, the minimum clothing thermal resistance required to maintain skin temperature above 15 °C and core temperature above 35 °C is 1.1 clo under the specific HAHO scenario at 10,000 m altitude, −80 °C ambient temperature, and 5500 m parachute deployment altitude. This represents a 43.6%–54.2% reduction compared to literature-reported values (1.95–2.4 clo). This study provides an accurate theoretical basis and a versatile quantitative tool for thermal safety assessment and protective clothing lightweight design in extreme environments beyond high-altitude parachuting.