research: Results
Results and Analysis
The manta ray produced several recognizable flow patterns. Averaging over many flapping cycles, the manta ray ejected two jets of flow in its wake, illustrated in the picture below.
Analysis of the instantaneous flow field using particle image velocimetry and dye-injection revealed that the interaction of vortices in the wake produces the two jets. The image below shows 4 distinct vortices which produce the two jets seen above.
In the image above, the vortices are interacting in such a way that thrust is produced. This phenomenon is dependent on the Strouhal number of the system. In other words, the vortex shedding patterns are dependent on the ratio of the frequency of flapping to the free stream velocity. At other Strouhal numbers, little or no thrust was produced.
The computational analysis replicated the Strouhal-number dependent thrust measurement. The analysis also correctly matched the two jets, as two distinct bursts of thrust are produced in each flapping cycle. Illustrated below are two Strouhal number cases, with Lift and Drag plotted against time. Note that negatice drag represents thrust.
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