Dolphin Kick

When I first heard someone mention the term dolphin kick, I thought it was a reference to the 1980s Patrick Duffy show, Man from Atlantis. The BBC says the dolphin kick “replaces a standard underwater leg kick with a whipping motion that minimizes water resistance.”[1] It’s a little easier to make sense of this if you watch the video. Suffice to say, if it’s done correctly, as has been used by Michael Phelps[3,5], it confers an advantage to the swimmer.

Example of meshing[6]. There is a
cut plane through the face showing
pressure contours.

Researchers[2] at George Washington University’s Fluid Simulations and Analysis Group (FSAG) have worked to understand the underlying physics of the underwater segment. According to Dr. Mittal, the goal of their project “is to understand what makes swimmers like Phelps and Coughlin such great dolphin kickers, both of them get a significant advantage during the dolphin kick phase,” says Mittal. “They usually come out of the water about half a body length or more ahead of the competition. We’re trying to understand the fluid dynamics behind this.”[6]

In one of their earlier publications[4], they mentioned they combined laser scans of Gabrielle Rose and Lenny Krayzelburg as input for simulations. In these, they come up with a mesh representation of the swimmer, develop a computational fluid dynamics (CFD) model of the water flow.

To create animations, the meshes were matched “frame-by-frame” to videos of Natalie Coughlin and Michael Phelps, respectively, doing dolphin kicks. Each dolphin kick was divided into 32 segments[6], each segment had 2000 interpolated frames. Static and dynamic simulations were run on FSAG’s linux clusters.

An example frame is represented by the mesh on the top left. Each mesh is composed of approximately 30,000 triangles. In areas where there is a lot of movement, smaller triangles (“refined mesh”) are used.

Post-processing is done on the simulation results to visualize the analysis. For example, the topmost picture shows a frame from Gabrielle Rose. The colored contours represent vorticity magnitude (red is high). Here is an animated example of the vortices. There are other interesting examples on the ICS site [4].

Model of Gabrielle Rose; contours indicate relative vorticity magnitude[6]