Oil flow visualization is widely used to investigate the three-dimensional surface flow topology. By conducting this technique, we captured a swirling pattern known as 'Owl eyes' represented by two counter-rotating vortices. This structure provides evidence of flow separation on the downstream side of the Gaussian bump and highlights critical points. Focus points reflect outward swirling flow, and saddle points show where streaklines converge in one direction and diverge in the other.
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