Global thermodynamic, transport-property and dynamic characteristics of the Venus lower atmosphere below the cloud layer

This study investigates the global characteristics of the Venus lower atmospherebelow the cloud layer. Starting from available data regarding species composition,temperature and pressure, the general thermodynamic and transport properties ofthe Venus lower atmosphere are first computed at altitudes below the cloud layer.The thermodynamic and transport properties are validated with known data. Athermodynamic stability analysis for a variety of potentially existing mixtures isconducted to highlight the sensitivity of the thermodynamic regime to the variationof the species molar fractions. The limits of thermodynamic stability are representedin the thermodynamic phase diagram of the mixture, and thus regions ofphase stability and instability are determined. It is found that the Venus loweratmospherethermodynamic regime is located in the stable, single-phase regime,with supercritical conditions occurring in the lower few kilometers, thereby validatingad-hoc expectations which have never envisaged the existence of a two-phaseregime at those altitudes. Using thermodynamic and transport-property informationthus computed, several non-dimensional numbers are calculated. The Prandtlnumber is first evaluated. Then, the characteristic Reynolds number is estimatedfor different assumed length scales, showing that, independent of the length scale,the entire lower atmosphere below the cloud deck is in a fully-turbulent flow regime.Finally, the speed of sound and the Mach number are computed as function of altitude.The information obtained from all these calculations fills a scientific void,and may be crucial for the design of landers on the Venus surface.

本研究旨在探究金星云层下方低层大气的全局特性。首先基于已公开的大气组分构成、温度与压强数据，计算云层以下各高度处金星低层大气的通用热力学与输运性质，并结合已知实测数据对上述热力学与输运性质开展验证工作。针对多种潜在存在的大气混合物，本研究开展热力学稳定性分析，以阐明热力学状态对组分摩尔分数（molar fractions）变化的敏感性。热力学稳定性极限可通过混合物的热力学相图进行表征，据此可明确相稳定与相不稳定区域的分布。研究发现，金星低层大气的热力学状态整体处于稳定单相区，仅在下方数公里范围内存在超临界条件，这验证了此前从未被设想过的“该高度区间不存在两相区”这一特设预期。基于上述计算得到的热力学与输运性质数据，本研究计算了多个无量纲数：首先对普朗特数（Prandtl number）进行评估；随后针对不同假定的特征长度尺度估算特征雷诺数（Reynolds number），结果显示，无论选取何种长度尺度，云层以下的整个低层大气均处于完全湍流流态。最后，计算得到声速与马赫数随高度的变化关系。本研究通过上述一系列计算填补了一项科学空白，其成果对金星表面着陆器的设计可能具有关键指导意义。