We report on the initial validation of the novel code FENNECS, which simulates the spontaneous formation of trapped electron clouds in coaxial geometries with strong externally applied azimuthal flows and in the presence of a residual neutral gas. For this purpose, a realistic gyrotron electron gun geometry is used in the code, and a self-consistent electron cloud build-up is simulated. The predicted electronic current resulting from these clouds that is collected on the gun electrodes is simulated and successfully compared with the previous experimental results for configurations with different externally applied electric and magnetic fields. These different configurations effectively modify the size and depth of the trapping potential wells responsible for the confinement of the electron clouds. This investigation also provides further insight into the link between potential well depth and resulting electronic current.