Axial and azimuthal turbulence intensities in the rod-gap region are shown, for developed turbulent flow through parallel rod arrays, to increase strongly with decreasing rod spacing. Two array geometries are reported: one was constructed from a rectangular cross-section duct containing four rods and spaced at five pitch-to-diameter or wall-to-diameter ratios; the second was a test section containing six rods set in a regular square-pitch array to represent the interior flow region of a large array.

Measurements were made of the mean axial velocity, wall-shear-stress variation, axial-pressure distribution and Reynolds stresses. Techniques for resolving secondary-flow velocities to within ±1% of the local axial velocity failed to detect significant non-zero mean secondary-flow components. Analysis of the turbulent flow structure showed an energetic azimuthal turbulent-velocity component in the open rod gap for both geometries. The axial turbulent velocity has a coupled large-scale semiperiodic structure, with an antiphase relationship across the rod-gap centre or subchannel boundary. This structure is considered to be generated by an incompressible-flow parallel-channel instability, and, for closely spaced rod arrays, is the dominant process for intersubchannel mass, heat or momentum exchange.