# Problem Specification — Lesson 1 Consider the unsteady state case of a fluid flowing past a cylinder, as illustrated above. For this tutorial we will use a Reynolds Number of 120. In order to simplify the computation, the diameter of the cylinder is set to 1 m, the x component of the velocity is set to 1 m/s and the density of the fluid is set to 1 kg/m3. Thus, the dynamic viscosity must be set to 8.333x10-3 kg/m*s in order to obtain the desired Reynolds number. Compared to the steady case, the unsteady case includes an additional time-derivative term in the Navier-Stokes equations:

The methods implemented by Fluent to solve a time-dependent system are very similar to those used in a steady-state case.  In this case, the domain and boundary conditions will be the same as the steady slow past a cylinder.  However, because this is a transient system, initial conditions at t=0 are required.  To solve the system, we need to input the desired time range and time step into Fluent. The program will then compute a solution for the first time step, iterating until convergence or a limit of iterations is reached, then will proceed to the next time step, "marching" through time until the end time is reached.