I am studying the problem of flow over square cylinder in Reynolds no. Range 50-250

I am performing transient simulation with laminar model using non iterative time advancement, fractional step scheme, second order implicit tansient formulation. Timestep taken is 0.1. I am using fft of lift data for calculating strouhal number. I am finding the peak strouhal number by importing the fft data to ms excel. However this value keeps decreasing as I increase reynolds number, whereas according to literature available, it should increase. I am wondering if this method of calculation is correct? Also should changing the timestep affect the fft obtained?