# Wright State University Lake Campus/2018-1/Ping pong air drag A w:vortex street around a cylinder. This can occur around cylinders and spheres, for any fluid, cylinder size and fluid speed, provided that the flow has a Reynolds number between roughly 40 and 1000.

The Reynolds number is defined as

$\mathrm {Re} ={\frac {\rho uL}{\mu }}={\frac {uL}{\nu }}$ where:

• $\rho$ is the w:density of the fluid (SI units: kg/m3)
• $u$ is the velocity of the fluid with respect to the object (m/s)
• $L$ is a characteristic linear dimension (m). For a sphere L=2R is the diameter.
• $\mu$ is the dynamic viscosity (Pa·s or N·s/m2 or kg/m·s)
• $\nu$ is the kinematic viscosity (m2/s).

The viscosity of air depends mostly on the temperature. At 15 °C, the viscosity of air is 1.81x10−5 kg/(m·s) 18.1 μPa·s or 1.81x10−5 Pa·s. The kinematic viscosity at 15 °C is 1.4810x10−5m2/s or 14.8 cSt. At 25 °C, the viscosity is 18.6 μPa·s and the kinematic viscosity 15.7 cSt.

Here, 1 cSt = 1 mm2·s−1 = 10−6 m2·s−1.

$F_{D}\,=\,{\tfrac {1}{2}}\,\rho \,v^{2}\,C_{D}\,A$ 