A systematic flow visualization study on flow structures inside dimples with different relative depths (h/d) and with round or sharp edges is carried out in the first part of this investigation; three flat test plates are made and each one has two dimples with the same print diameter (d) at 40 mm and the same h/d, which is set at 4%, 20% and 50% respectively. Then friction factor is measured on a test channel with two types of test plates. One has dimples array with h/d at 4% and round edge and the second is a flat plate used as reference; surface roughness can be changed for both test plates by painting and covering with water-proof paper. Flow visualization is also done on a dimple located at the center of the plate in the channel. Results show that shallow dimple (h/d=4%) produces non-separated flows at Reynolds number Reδ<1000 and there exists a rather small separation cavity flow at Reynolds number Reδ>1500. Dimple with h/d at 20% produces two symmetric vortices at Reδ<850, single vortex at Reδ ≈ 1000–1600 and symmetric horseshoe-liked vortex at Reδ>1700. Round edged dimple changes its fl pattern at different Reynolds number comparing to its sharp edged counterpart. For the dimple with h/d at 50%, at Reδ <1200 there is only one stable vortex; at Reδ>1800, it is unstable with its rotating direction changing frequently. In this part of preliminary work, dimpled plate with roughness reduces the friction factor by at least 2% in the flow channel comparing to the reference plate at Reynolds number ReDh ≈ 8,500~24,000. The friction factor curve of the channel with roughened dimpled plate approaches a hydraulically smooth channel with the increase of Reynolds number. No increase in friction factor is observed on the channel with dimpled plate having smooth surface in the comparison to the reference channel without dimples.

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