Problems for solving the momentum conservationMomentum conservationequationConservation equation (the Navier-Stokes equation) inNavier-Stokes equation CFD codes are identified. These include the nonlinearNon-linear nature of this equation, dependence of pressure on velocity and coupling of the equations for velocity components. It is pointed out that using a staggered gridStaggered grid for calculation of velocity components allows one to avoid the problem with the so-called ‘checker-boardChecker-board’ pressure fieldPressure field. The SIMPLESemi-Implicit Method for Pressure Linked Equations (SIMPLE) (Semi-Implicit Method for Pressure Linked Equations) algorithmImplicit for the calculation of the flow field is described. This algorithm is based on the presentation of velocity components and pressure as sums of guessed values (or the values inferred from the previous iteration) and velocity/pressure correctionsPressure correction. This presentation enables one to obtain expressions for velocity components as functions of pressure correctionsPressure correction. Substitution of these expressions into a discretised form of theMass conservation mass conservation equationConservation equation leads to algebraic equations for pressure correctionsPressure correction for all the computational cells. The details of the derivation of these equations for a one-dimensional case are presented. These equations are generalised to the three-dimensionalThree-dimensional case. The role of underrelaxationUnderrelaxation in calculating pressure and velocity components is identified. The concepts of errorsErrors, uncertaintiesUncertainties, verificationVerification, and validationValidation are described.