A method for determining the flow of a fluid (60) in a gap (64) between a pad (48) and a substrate (12) includes the step of utilizing a hybrid Navier-Stokes/lubrication formulation to calculate the flow of the fluid (60) in the gap (64) at a plurality of time steps. The gap (64) can be divided into a plurality of elements (700). The hybrid Navier-Stokes/lubrication formulation can be used to calculate the fluid flow and the pressure of the fluid (60) at each element (700) at the plurality of time steps. Additionally, a method for tracking and estimating the composition of the fluid (60) at various locations in the gap (64) and a material removal rate model that attempts to account for the effects of the fluid flow in the gap (64), the hydrostatic pressure in the gap (64) and the composition of the fluid (60) in the gap (64) are provided herein.