Reynold's number

Reynold's number is a defined quantity often used to predict whether laminar or turbulent flow will result in a particular situation. Reynolds number Re for a sphere is defined as....

... where r is the fluid density, d is the diameter of the sphere, v is the (terminal) velocity and h. is the coefficient of viscosity.

Paper cups falling in air

The viscosity of air at STP is 1.83x10^-5 Nsm^-2. The density of air is 1.26 kg/m³. The muffin cups have a diameter of ~5 cm and a terminal velocity of ~ 1 m/s. Reynolds number for falling (~spherical) paper cups in air is....

Turbulent flow typically occurs when Re is above about 1000 and laminar flow is usually established when Re is less than 1. The muffin cups fall with turbulent flow, as do autumn leaves.

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