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Effects of CO_2 Concentration on Second Mode Instabilities

机译：CO_2浓度对第二模式不稳定性的影响

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Transition prediction is a crucial part of the hypersonic vehicle design process, not only to understand the thermal protection system requirements but also to accurately predict aerodynamic flight characteristics. Increasing the understanding of how chemistry affects the boundary layer stability is an important factor in reducing the error in these predictions. Wind tunnel experiments and Computational Fluid Dynamics simulations have been conducted for the past two decades looking at the effect of vibrational damping of the CO_2 molecule on second mode instabilities. However, these studies looked at large concentrations of CO_2 in the freestream and were conducted on sharp cones only, neither of which are directly applicable to flight representative conditions. In flight, only small concentrations of CO_2 would be found in the boundary layer as the result of ablation. This study examined the effect of decreasing the total concentration of CO_2 in the boundary layer and determined a minimum concentration of CO_2 where the vibrational damping is effective in reducing amplification of disturbances in a high and low enthalpy flow. A sharp nose and a 12.7 mm nose-radius on a 10 degree half-angle cone was used for the study. For the sharp cone model, a minimum concentration of 10% and 30%, for the high and low enthalpy flow respectively, was required to achieve approximately a 10 cm change in transition location. Due to the differences in the boundary layer temperature profiles between a blunt nosed and sharp cones, in conjunction with the stabilizing effect of the entropy layer, the CO_2 damping on the blunt cone model required significantly lower CO_2 concentrations for the same transition location change.
机译：过渡预测是超音速车辆设计过程的关键部分，不仅要了解热保护系统要求，还要准确地预测空气动力学特性。增加了解化学对边界层稳定性的理解是降低这些预测中误差的重要因素。过去二十年进行了风隧道实验和计算流体动力学模拟，观察了CO_2分子对第二模式稳定性的振动阻尼的影响。然而，这些研究看着FreeStream中的大量CO_2，并且仅在尖锐锥体上进行，这些研究既不是直接适用于飞行代表条件。在飞行中，由于消融的结果，在边界层中只能发现小浓度的CO_2。该研究检测了降低边界层中CO_2的总浓度的效果，并确定了振动阻尼的最小浓度，其中振动阻尼可有效降低高焓流动的扰动。在10度半角锥上使用锋利的鼻子和12.7毫米的鼻半径用于该研究。对于尖锐的锥形模型，分别为高和低焓流动的最小浓度为10％和30％，以在过渡位置达到大约10厘米的变化。由于钝鼻和锋利锥之间的边界层温度分布的差异，与熵层的稳定效果结合，在钝锥模型上的CO_2阻尼需要显着降低同一转变位置的CO_2浓度。

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