One of the vital safety mechanisms in the reactor core of most nuclear power plants is the negative feedback from the core temperature on the power output. An increased temperature will reduce the power output of the reactor, 'automatically' cooling the it and preventing the nuclear reaction to spin out of control. For this reason, it is important to have a good understanding of the temperature distribution within the reactor core. The water that flows through the core is the main reason for the transport of heat, hence it is of interest to have a good understanding of the behavior of the flow. On of the aspects that defines the flow properties is the geometry of the reactor, a so called 'rod-bundle geometry'. It is the presumption that a large shear in the fluid, induced by the geometry, causes the formation of largescale coherent structures. These structures affect the mixing of heat in the reactor and are, therefore, of interest.