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Numerical results are obtained for the isothermal laminar flow of air between a pair of disks attached to and rotating with a hub in a fixed cylindrical enclosure. The presence of radial clearances or 'gaps' between the rims of the disks and the curved enclosure wall and the finite thickness of the disks, are considered in the calculations. The gaps allow time- and circumferentially-dependent axially-directed air flow exchanges between the contiguous inter-disk spaces. As a consequence, axisymmetric calculations of the flow, whether using boundary conditions in the gaps or extended to include the entire flow domain, fail to faithfully reproduce the experimentally measured radial variations of the mean and rms circumferential velocity components in the inter-disk space. Likewise, three-dimensional calculations using the symmetry-plane boundary condition in the gaps also fail to reproduce these variations. In contrast, computationally intensive three-dimensional calculations of the entire flow domain, including the gaps, yield results in very good agreement with the measured mean and rms velocities. These three-dimensional calculations reveal large velocity fluctuations in the gap regions accompanied by corresponding large fluctuations of the inter-disk flow, reflecting a destabilization of the structure and dynamics of the latter by the former. The axisymmetric calculations as well as those using the symmetry-plane condition in the gap are included in this study principally to elucidate their shortcomings in simulating the three-dimensional flows considered; they are not the main goal of the study. Notwithstanding, the physically approximate, full domain axisymmetric calculations yield useful qualitative results. They show that increasing gap size decreases disk surface shear and the associated disk torque coefficient, but at the cost of destabilizing the inter-disk flow. This observation is in agreement with earlier findings and is better understood as the result of the present study. [DOI: 10.1115/1.1487355]

The eigenvectors obtained from proper orthogonal decomposition (POD) are used as the selection criteria to classify the individual events contained in data files of two-component velocity signals recorded in the non-homogeneous (vertical) and homogeneous (horizontal) planes of a fully-developed turbulent cylinder wake. This procedure uses the dominant eigenvectors from POD as initial templates to perform a pattern recognition (PR) analysis of the signals so that the individual coherent events appearing randomly in the signals can be educed more objectively. It is found that the prototype or ensemble average of the group of events classified as the large-scale structure in the vertical plane has a circulatory motion with strong negative streamwise and outward lateral velocity fluctuations. In the horizontal plane, the average structure is a double roller with negative streamwise velocity fluctuations in its centerplane. The class of instantaneous events selected contribute significantly to the variance in the outer intermittent region, but much less in the fully turbulent core. While the first eigenvector from POD captures some of the features shown by this prototype, it is not enough to depict by itself all the characteristics shown by the ensemble average of the repetitive three-dimensional instantaneous events that occur in the fully-developed turbulent wake.