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Theory

The high aspect ratio fin creates a pattern of leading and trailing edge vortices as it oscillates, which provides the DOL-Fin a unique ability to form a reverse Kármán vortex street, as depicted by the gray vortex arrows in the figure below.  This cannot be accomplished with traditional low aspect ratio dive fins.  This vortex pattern induces a powerful flow of water through the area affected by the fin and is the most efficient means for producing hydro-dynamic thrust found in nature.

Reverse Karman Vortex Street

Where the Strouhal number = frequency * (Total Amplitude) / (Average Velocity of Fin); the DOL-Fin typically operates in the range of 0.2 to 0.4, which is characterized by the formation of a clear reverse Kármán vortex street of just two vortices per cycle, which has been shown in independent experiments to produce optimal efficiency for the generation of thrust.  Learn more about the Strouhal number and oscillating fluidic thrust generation on the Links page.

The DOL-Fin has an aspect ratio of approximately 12:1. The average swim fin has an aspect ratio of about 1:2, or, at best, 1:1. One advantage of the high aspect ratio DOL-Fin is that it has significant drag reduction compared to the average swim fin.  This is because the drag induced by the vortexes shedding from a lifting surface is roughly proportional to  the inverse of the aspect ratio of the fin.
                                 

                 Vortex generation by a pair of dive fins

                                 

                Vortex generation by a high aspect ratio dive fin

The high aspect ratio of the DOL-Fin also provides a long span while keeping the area of the fin low.  The low area reduces parasite drag of the fin itself when a swimmer is gliding through the water without stroking the fin and reduces body pitching when the swimmer is under propulsion.  Compared to a shorter span fin, the long span of the DOL-Fin allows its fin to affect a larger volume of water for a given stroke amplitude.  This is a major contributor to the DOL-Fin's ability to maintain high levels of thrust efficiency even at moderate and low swimming speeds, where a diver is likely to be operating most of the time.  Under these conditions, less power is required to produce a given amount of thrust if the fin pumps a lot of water a little, rather than pumping a little water a lot. 

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