Numerical simulation of the unsteady turbulent flow in a three-dimensional draft tube geometry is performed. The investigation is carried out with a commercial finite volume solver implementing the Reynolds averaged Navier-Stokes equations. The modeling of most practically relevant turbulent flows continues to be based on this equation system. For this reason it is important to evaluate the limitations of this approach. Verification and validation are presented; detailed measurements are compared with computations over a wide range of operating conditions.