Strouhal Number

Not Reviewed
Equation / Last modified by KurtHeckman on 2018/04/27 13:40
MichaelBartmess.Strouhal Number

The Strouhal number calculator compute the Strouhal number based on the frequency, characteristic length and flow velocity.

INSTRUCTIONS: Choose units and enter the following:

  • (f) This is the frequency
  • (L)  This is the characteristic length
  • (U)  This the flow velocity

Strouhal number (St): The calculator returns the Strouhal number.

Related Calculators:

The Math / Science

In dimensional analysis, the Strouhal number (St) is a dimensionless number describing oscillating flow mechanisms. The parameter is named after Vincenc Strouhal, a Czech physicist who experimented in 1878 with wires experiencing vortex shedding and singing in the wind. The Strouhal number is an integral part of the fundamentals of fluid mechanics.

The formula for the Strouhal number is:

    St = (f • L) / U


  • St is the Strouhal number
  • f is the frequency of vortex shedding, 
  • L is the characteristic length (e.g. hydraulic diameter) and 
  • U is the flow velocity. 

The Strouhal number in this particular case relates the fluid dynamics properties of vortex shedding with a characteristics length of this fluid dynamic (typically the  and velocity of the fluid.  This number is also used to describe wing tip motion characteristics for swimming and flying animals.

Vortex shedding1 is the oscillating flow of a fluid, such that vortices are created at the back of an object in a fluid stream and these vortices detach (shed) periodically from either side of the body in the stream.

For large Strouhal numbers (order of 1), viscosity dominates fluid flow, resulting in a collective oscillating movement of the fluid "plug". For low Strouhal numbers (order of 10−4 and below), the high-speed, quasi steady state portion of the movement dominates the oscillation. Oscillation at intermediate Strouhal numbers is characterized by the buildup and rapidly subsequent shedding of vortices.2