As a result of these nonlinear effects, the Navier–Stokes equations in this case become difficult to solve, and approximations or numerical methods must be used to find the velocity and pressure fields in the flow. Consider the case of a two-dimensional fluid flow in a rectangular domain, with a velocity field v ( x , t ) and a pressure field p ( x , t ) . We can use a finite element method to solve the Navier–Stokes equation for the velocity field: