Numerical solutions of the compressible, multiphase Navier-Stokes equations are applied to conditions appropriate for the development of pyroclastic flows from collapsing Plinian eruption columns. Temporal and spatial variations in horizontal velocity, dynamic pressure, solid volume fraction, and temperature are analyzed for three experiments that model variations in behavior due to differing eruption conditions. Pyroclastic flows display a complex evolution with time and runout distance from the vent. Evolution of flow parameters is most generally manifested by alternating periods of erosion and deposition. Velocities of flow heads decrease and solid concentrations in the flow heads increase with distance from vent. Application of numerical modeling to such phenomena can help in assessing volcanic hazards and pyroclastic transport processes.