T-junction pipes are used to distribute one flow into two flows or join two flows into one flow. Separated vortex flow regions near the corners of junctions are caused in these types of flows. They reduce the effective cross-sectional area of the pipe flows and then create large flow resistance or drag. The corners of junctions are generally rounded to avoid flow separation and reduce flow resistance. We tried to reduce the flow resistance of counter-flow T-junction pipes in which two flows in the opposite direction entered the junction, mixed, and then vertically flowed out together by using a simple method. We mounted two small weir-shaped obstacles on the walls of the two upstream pipes by the side around the junction corners, which is a new way we propose of controlling flow separation. The pressure distribution along the pipes was measured and the drag of the T-junction pipe was estimated. Additionally, the effects of the Reynolds number on the flow resistance and its rate of reduction by mounting small obstacles were clarified. The two major results we obtained were: (1) the flow resistance of T-junctions could be reduced by about a maximum of 30% by mounting small obstacles at heights of 0.30 D and 0.47 D (D: pipe diameter) from the upstream of the corner. We also found (2) the rate of reduction in flow resistance increased with decreasing Reynolds numbers between 5 - 10 x 104, but this decreased rapidly between 2.5 - 5 ～ 10~4.
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