The governing equations shown in Chapter 1 are a coupled sys-tem of nonlinear partial dierential equations, and their numerical solutions depend on boundary and initial conditions. As discussed in Chapter 1, the Navier-Stokes equations are in general elliptic in space and parabolic in time. To solve these equations for a specic ow conguration under consideration, boundary conditions (BC) at all boundaries of the computational domain and initial conditions for all ow variables in the entire eld are required. ere are situations where initial conditions are important, such as predictions of the transition process or fundamental investigations of turbulence. An example is the decay of a homogeneous isotropic turbulent ow (Hinze 1975) oen used for basic investigations in turbulence research. However, in most applications of DNS and LES, the initial conditions play a subsidiary role because the statistically steady-state ow status is of major concern, which should be reached independently from the initial conditions. In many cases, appropriate initial conditions can be chosen to shorten the simulation time until a statistically steady state is achieved.