Taylor Made: Swirls

If you are ever looking for visual examples of fluid mechanics in action, rivers are a good place to start. Natural riverbeds, which would be classified under open channels in TAM 335, are typically heterogenous and can cause various forms of turbulence.

In laminar flow, fluid particles follow pathlines that never cross; in turbulent flow, these lines interfere. Turbulence indicates that the amount of kinetic energy in the fluid has overcome its natural damping ability—in other words, the energy level in the fluid particles has exceeded that which is able to be regulated by the fluid body’s characteristics. Turbulence is often described as irregular, violent, or unstable, but I will argue that its characteristics produce a beautiful motion, most easily showcased in sizeable bodies of water. 

A bluff is a body that separates flow over a significant part of its surface (as opposed to a streamlined body, which is designed to lower the drag force occurring between itself and the fluid it passes through). When fluid flows over a bluff, such as a boulder, it creates vortices, or flow that circles around an axis. See NASA’s drag coefficients for general shapes here.

The conglomeration of bluff bodies in riverbeds in the form of rocks, logs, and other natural debris is such that river flow experiences random separation and interference. Flow separation can lead to vortex shedding, in which each section of flow creates a vortex pattern after passing over the body. In a constrained flow channel (like a river), the flow separation in one area of current can easily interfere with that of another, creating unique interference patterns. 

This past month, the Rio Grande in Colorado was raging significantly higher than it did a year ago, due to a much larger accumulated snowpack. The aggressive volumetric flow rate caused greater turbulence, resulting in an active body of water that crashed and oscillated in discernable patterns dictated by the random arrangement of river debris.

As a side note, river rafters are cautioned against passing near bridge pylons in strong currents. The pylons disrupt the flow, creating a net change in velocity from the boundary layer to flow farther away from the pylon. In aggressive waters, the gradient can easily be extreme enough to flip a raft without warning.

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