The flow past a two-dimensional cylinder is one of the most studied of aerodynamics. It is relevant to many engineering applications. The flow pattern and the drag on a cylinder are functions of the Reynolds number Re D = U∞D / n, based on the cylinder diameter D and the undisturbed free-stream velocity U∞.

The concept of air flow over a circular cylinder refers to the study of how air moves and interacts with a cylinder shape. It is a fundamental aspect of fluid mechanics and is used to understand and predict the behavior of air around objects such as airplane wings, submarine hulls, and other cylindrical structures.

This thin layer of molecules will entrain or pull the surrounding flow in the direction that the surface moves. If we put a cylinder that is rotating about the longitudinal axis (a line perpendicular to the circular cross section) into a fluid, it would eventually create a spinning, vortex-like flow around the cylinder.

A vortex street around a cylinder. This can occur around cylinders and spheres, for any fluid, cylinder size, and fluid speed provided that it has a Reynolds number between roughly 40 and 1000.