The effects of wall proximity on the turbulent flow field in a square duct structured with detached divergent ribs on one wall

The effect of the wall proximity of detached \({60\,\mathrm{ ^{\circ }}}\) divergent ribs applied on one wall in a square duct on the turbulent flow field was investigated. Laser Doppler anemometry (LDA) measurements were conducted for different clearance-to-rib-height ratios in the range of 0.1–1.0 at a Reynolds number (based on the rib height and mean bulk velocity) of 5000. Mean velocities, Reynolds stresses, triple velocity correlations as well as skewness and kurtosis were determined and yield deep insights into the turbulent flow field. The results showed that a geometry-induced secondary fluid motion occurred above and below the rib. The variations in the highly three-dimensional flow field close to the ribs and the geometry-induced secondary flow motion with the clearance-to-rib-height ratio determined the development of the wall-bounded flows and separating shear layers. Large recirculation regions on the bottom duct wall were prevented by the fluid exiting the gap below the detached ribs and separated shear layers pivoted upward in lateral direction. With decreasing wall proximity, lower mean vertical flow velocities above the ribs and the increasing upward fluid flow originating from the flow in the gap attenuated an intense interaction of the separated shear layers with the wall-bounded flow within the inter-rib spacing. Since turbulent structures originated in the shear layers, distributions of high-order statistic moments depend strongly on the shear layer development. Reynolds stresses and triple velocity correlations increased in the direction of the side walls near the rib due to the lateral flow motion, and their peak regions moved away from the wall with increasing clearance-to-rib-height ratios.