Strouhal Number
 

After obtaining the strouhal number and making sure its about 0.21, what use is it for us? What does the Strouhal number exactly means other then relating f, d, and v. Why is it important that the value must be 0.21.

Please help

The Strouhal number is a sort of adimensionalized frequency of you body.
So it depend on the aerodynamic of your body and the reynolds number.

If you take a cylinder,or other body,you have different Strouhal number for different reynolds number.
In what case you have 0.21 of strouhal number?

The relationship between Strouhal number, vortex shedding frequency, diameter, and velocity is exactly why the Strouhal number is important. Once the Strouhal number is known for a cross-sectional shape, it is useful for calculating the vortex shedding frequency at given flow conditions for that member. If the shedding frequency is close to a natural frequency of a structural member with little or no damping, the vortex shedding can induce vibrations that could cause catastrophic stresses and deflections. If there appears to be such a potential problem, the engineer can change the natural frequency by strengthening, stiffening, or bracing; increase the damping; or change the aerodynamic characteristis by making the structural member non-symmetrical to the flow (e.g. wrapping a wire around an auto antenna).