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Flukeprints are the giant circular patterns that whales leave on the surface of the ocean as they disappear beneath the waves. Researchers have long known that they are caused by turbulence from the whale’s tail or fluke.

But exactly how  this turbulence creates these beautiful patterns has never been properly understood.

Now Germain Rousseaux at the Universite de Nice-Sophia Antipolis in France says he’s solved the mystery using the results from a unique set of experiments in which he uses an artificial fluke in a tank of water to recreate the patterns. 

He then videoed the way these patterns form and disperse using a laser imaging technique called particle imaging velicometry. This shines a laser beam off tiny particles in the water to see how they move. The result is a 3D video image of the resultant flow.

Flukeprints tend to be circular or ovoid and contain a central region that is smooth compared the surrounding water. It’s almost as if something is flattening the water in this region, preventing the nearby surface waves from penetrating.

The explanation turns out to be remarkably simple and fascinating. The key is to think about the pattern of water that forms in a kitchen sink when you leave the tap on. The steady stream of water hits the sink surface and spreads out in a circular pattern surrounded by a rim called a hydraulic jump, like the one below.  

Rousseaux says flukeprints are essentially hydraulic jumps caused by jets of water from beneath the surface. 

As the whale submerges, its flukes generate powerful vortices in the water. In the centre of the vortices, water is forced upwards hitting the surface from below, like a jet. This creates the jump, which is the circular rim around the flukeprint. 

One of the curious properties of hydraulic jumps is that they are ‘white holes’, the time reversed equivalent of black holes.  

White holes are special because they can emit waves and particles but do not allow them to enter, as we discussed in this post a couple of years ago. It’s exactly this process that creates the region of perfect calm at the centre of the flukeprint.

That’s a fascinating explanation. It means that the flukeprints whales leave behind are white holes on the surface of the ocean. Amazing!

Ref: arxiv.org/abs/1206.3893: The Flow Induced By A Mock Whale  Origin Of Flukeprint Formation

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