The Background Oriented Schlieren (BOS) method has been used to qualitatively identify the (variation of) density gradients in the helical structure of a propeller slipstream. The helical structures are identified for two sideslip angles. In contrast to standard BOS correlations between exposures and a reference image, two exposures at a given time interval were cross-correlated. This revealed a more clear description of the propeller slipstream as it determines the variation of the density gradient during this interval. It enhances the visualization of the helical structure of the propeller slipstream. Based on the visualizations of the blade tip vortex trajectories the propeller slipstream contraction can be estimated.

中文翻译：

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背景导向纹影螺旋桨滑流的定性研究

背景定向纹影 (BOS) 方法已用于定性识别螺旋桨滑流螺旋结构中的密度梯度（变化）。确定了两个侧滑角的螺旋结构。与曝光和参考图像之间的标准 BOS 相关性相比，给定时间间隔的两次曝光是互相关的。这揭示了对螺旋桨滑流的更清晰描述，因为它决定了该间隔期间密度梯度的变化。它增强了螺旋桨滑流螺旋结构的可视化。基于叶片尖端涡轨迹的可视化，可以估计螺旋桨滑流收缩。