The Strouhal number is related to frequency as follows: Strouhal number = f*L/U L and U must be set appropriately in the Reference Values (below Dynamic Mesh in the Tree to the left of the viewer window). The reference length and velocity have the default values of 1m and 1m/s respectively, which is why the Strouhal number is initially identical to frequency. Unfortunately, there is no automatic recipe to estimate optimal values for all cases. In many cases, the inlet velocity can be used as the reference velocity. The reference length for the external flows is typically the size of the body (e.g. the chord length for an airfoil), and for the internal flows it may be a pipe diameter and so on. These are typically the values that you would use to calculate the Reynolds number. In more complex cases, it may be necessary to carry out some preliminary estimations. For example, in a natural convection flow without any velocity inlets, one may have to compute the reference convective velocity out of the known temperature difference and the thermal expansion coefficient.