Several approaches have been used in the past to model the source of a high pressure under-expanded jet such as the computationally expensive resolution of the jet shock structure and the simpler pseudo- source or notional nozzle approaches. In each approach assumptions are made introducing inaccuracies in the CFD calculations. This work assesses the effect of different source modeling approaches on the accuracy of CFD calculations by comparing simulation results to experimental data of the axial distribution of the flow velocity and H2 concentration.