Abstract
A large scale pulsation that oscillates in a narrow gap that connects two rectangular channels is observed. The phenomenon is similar to the mixing of cooling fluid in rod bundles of power plants from one sub-channel to another through the gaps between the rods. The fluid mixing in rod bundles has been studied from 1960s to the present time to predict the temperature distributions of the rods and the cooling fluid in a nuclear reactor that were used when a power plant was designed for its safety assessment. The nature of the fluid mixing has been accounted for by turbulence and secondary flows, which also occur in the turbulent flow field. Recently, hot wire measurement revealed that the fluid mixing was periodic and coupled with Reynolds number and the gap geometry. In addition, the visualization of the flow in a rectangular channel containing a cylindrical rod showed the existence of a vortex in the narrow gap between the rod and the channel and it also accounted for the fluid mixing. The amplitude of the visualized vortex seems to become larger as it flows downstream, however the cause of the amplitude increase has not discussed yet. In this study, a numerical analysis has been performed in a composed channel consisting of two rectangular channels which are connected by a narrow gap near a wall. An algebraic Reynolds stress model is used to predict Reynolds stresses in the channel precisely. Calculated results for axial mean velocity, streamwise and gap-parallel turbulence intensities are compared to the experimental data to validate the proposed numerical model. The proposed model can predict characteristic isocontours of axial mean velocity, streamwise turbulence intensity and gap-parallel turbulent shear stress, although quantitative differences are observed in several predictions from experimental data. Calculated planar turbulent shear stress and secondary flows near the gap are shown to compare with the result by large eddy simulation. In the gap region, velocity vectors show the large scale pulsation that is also observed in the experiment. Streamwise variations of axial mean velocity and axial velocity fluctuation, turbulence intensity and turbulent shear stresses of the gap center are discussed as an indication of the leading point of the large scale pulsation.
