whirls

Whirls are rapid, circular motions of fluids or gases around a central axis or point, often forming a vortex or eddy. In everyday language, the term describes the swirling motion seen in water flowing down a drain, smoke curling in air, or wind rotating around a small core. Whirls can occur on scales from a few millimeters to hundreds of kilometers in the atmosphere or ocean.

Whirls arise when angular momentum is conserved as a flow is constricted or sheared, or when instabilities generate rotational motion. They are characterized by a rotating core, lowered pressure in the center, and nonzero vorticity. Their strength depends on density, viscosity, velocity, and geometry of the surrounding flow. Whirls can be transient or sustained by ongoing forcing.

Examples include whirlpools in rivers or coastal waters, eddies behind obstacles in currents, and atmospheric phenomena such as tornadoes, whirlwinds, dust devils, and other rotating weather systems. In engineering and the physical sciences, rotating flows are studied in mixers, turbines, pumps, and reactors, where induced or unavoidable whirls influence mixing, transport, and wear. Whirls can transport momentum and heat differently than nonrotating flows.

Measurement and study employ fluid-dynamics tools such as particle image velocimetry, laser Doppler velocimetry, and Doppler radar, as well as theoretical approaches based on vorticity and circulation. While “whirl” is a descriptive term, it forms part of a broader vocabulary that includes vortices, eddies, and circulations used to classify rotating flows.