Vortex structure and heat transfer in turbulent flow over a wall-mounted matrix of cubes

Abstract

The paper reports on the turbulent flow structure and the distribution of the local surface heat transfer coefficient of a cube placed in a spatially periodic in-line matrix of cubes mounted on one of the walls of a plane channel. Infrared thermography was applied to measure the surface temperature at the cube walls, from which the distribution of the local heat transfer coefficient was determined. The velocity field and its structure were evaluated from Laser Doppler Anemometry (LDA) measurements and flow visualizations. The spatial periodicity was confirmed from flow field and heat transfer measurements across the entire matrix. The results showed that the flow has a marked vortex structure only in the immediate proximity of the cube, while the flow above the cube and in the streamwise corridors was only mildly distorted, except for a high level of turbulence intensity. Flow separation at the sharp leading top and side edges led to flow recirculations with subsequent flow reattachment at these faces. Reattachment of the top shear layer at the channel floor downstream of the cube produced a two-cell structure in the inter-obstacle space: an arc-type vortex in the wake of the upstream cube and a horseshoe-type vortex in front of the downstream cube. Flow instabilities caused vortex shedding at the side faces of the cube which led to periodic motions in its wake. The measured Strouhal number showed a constant value of St=0.109 over the range of Reynolds numbers considered. The observed local flow structure, in particular flow separation and reattachment, caused marked variation in the distribution of the local heat transfer coefficient, with large gradients detected particularly at the top and side faces of the cube.