A columnar object (vortex generator) is arranged in the airflow to generate a stable and continuous vortex, which is called Carmen vortex (figure 1). The relationship between the number of cycles (frequency) of the vortex and the speed is shown in the formula (1).

The constant St in the above formula is called Strouhal number. The Reynolds number range corresponding to the representative dimension d of the vortex generator is very wide (3× 10 squared to 3×10 quintic), so it can be seen that f has nothing to do with the kind, temperature and pressure of the object. Therefore, it can be said that the velocity of fluid can be obtained by measuring the number of vortex cycles (frequency number) f.