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Flows in the T-junction of a counter-flow pipe are run counter to each other and they usually flow out vertically together. A flow separated from the junction corner forms separated vortex regions and they reduce the effective cross-sectional areas of the pipe, and this increases flow resistance, i.e., drag (pressure loss). The corner of the junction is generally rounded to prevent the flow from separating and to reduce drag. This method can reduce drag by 30% with a rounded radius of 0.1D (D: pipe inner diameter), but some process is needed to remove the corners. We propose a simple method of reducing drag in the flows of T-junction pipes by mounting two small weir-shaped obstacles on the upstream of the walls of the two pipes beside the junction corners. This method is a simple way of reducing drag without having to use a removal process. The pressure distribution along the pipes was measured and the drag in a T-junction pipe was derived. The flow pattern was visualized with a tracer method and this was evaluated to confirm the separation of flow from the corners. As a result, we clarified that drag in a T-junction could be reduced by a maximum of about 30% by mounting small obstacles at heights of 0.30D and 0.47D from the upstream of the corners.

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