Recent models and experiments have demonstrated that indirect noise can be generated via acceleration of either entropic or compositional perturbations through a nozzle. These studies found that the acoustic signature depends on the extent of the dispersion effects on the perturbation waves during the convection process. In this work, numerical simulations are undertaken using the URANS formulation to model the mixing of unsteady entropic and compositional waves in an open-ended flow duct in the transitional regime (2500 < Re < 8100). The flow is perturbed by either heat addition or radial injection of an inert gas that is heavier (argon) or lighter (helium) than air. The computed temperature and mass fraction fields are compared to experimental results obtained using both intrusive and non-intrusive techniques. Agreement with the experimental data is good. The signatures of both perturbations are well captured using two-equation turbulence models. However, they lag behind the experimental results at downstream probe locations, suggesting that the centreline velocity is underpredicted. Dispersion effects on the compositional waves are shown to be insignificant for the system studied here. In the heat addition case, heat loss is found to be a key factor in capturing the correct entropic perturbation for long convection lengths.