Ambient energy harvesting from the airflow environment is defined through an additional mechanical structure exposed to wind force. Using a simplified vortex shedding model we model the cylinder like a bluff body interaction with flowing air. The mechanical resonator is composed of the cylinder attached to the beam with a piezo patch. The vortex shedding on the cylinder is modelled as Van der Pol oscillator. In particular, we analyses the effect of cylinder diameter on the output voltage. We report the grow of the wind speed interval the top voltage output as a results of the increasing diameter. The uncertainty the power output caused by the uncertain bluff body diameter increased considerably at the upper edge of Vortex Induced Vibration mechanism.