Models are presented for the chemical evolution of the galactic disc with radial inflows of the gas at a velocity constant with time, but that may vary with the galactocentric radius. The models include the infall of metal-free gas from outside the disc at a rate decaying exponentially with time, a star formation rate that varies as the surface density of the gas layer to the power of m, with m = 1 or 2, and in some cases an outer edge to the stellar disc at the star formation cutoff radius. The models are compared with the observed age-metallicity relation and metallicity distribution for stars in the solar neighborhood and with the observed radial variations of the metallicity, the star formation rate, and the surface densities of the gas and stars. Inflow velocities of approximately -(0.5-1.0) km/s are expected.
Lacey, C., & Fall, S. (1985). Chemical evolution of the galactic disk with radial gas flows. Astrophysical Journal, 290, 154-170. https://doi.org/10.1086/162970