The Strouhal number ($St$) is a dimensionless quantity used to characterize unsteady fluid flows, particularly in the context of vortex shedding behind bluff bodies. It relates the frequency of oscillation of a fluid flow to the velocity and characteristic length scale of the flow. The Strouhal number relates the frequency of vortex shedding or oscillation in a fluid flow to the velocity and characteristic length scale of the flow.

Where,

• $St$ is the Strouhal number,
• $f$ is the frequency of oscillation of the fluid flow,
• $L$ is a characteristic length scale of the flow,
• $v$ is the velocity of the fluid flow.