swirling

Swirling is the motion of a material medium in which portions of a fluid or gas rotate around an axis, creating circular or spiral trajectories. Swirl can occur in liquids, gases, or granular media and is usually accompanied by angular momentum and, in many cases, by a low-pressure core or a visible spiral pattern known as a vortex.

In fluid dynamics, swirl arises from boundary conditions, lateral forcing, or the rotation of components such as impellers, injectors, or nozzles. It can also develop when a flow has tangential components or experiences shear. Vorticity is the local measure of rotation; when swirl is sustained, a coherent vortex or swirl-dominated flow may form. Swirl can enhance mixing, heat and mass transfer, and in some cases stabilize or destabilize flows, depending on geometry and Reynolds number.

Common examples include the whirling motion observed in a stirred coffee or a draining sink, natural phenomena such as whirlwinds, tornadoes, and ocean eddies, and engineered systems like combustion chambers, gas turbines, and chemical mixers where swirl improves fuel-air or reactant contact.

Researchers visualize and quantify swirl using flow visualization, tracer methods, and measurement techniques such as particle image velocimetry (PIV). Vorticity fields and dimensionless swirl descriptors help compare flows across scales and guide design. Swirl is related to, but not identical to, a vortex; a vortex is a region of rotational motion often featuring a concentrated core, while swirl describes the broader rotational motion that can accompany it.

Related concepts include vortex, eddy, and spiraling motion.