Your english description is so difficult to understand, can you post a plot or explain the problem more clearly?
If you try to use a segment of a tube in a CFD simulation of the full tube flow, then you need to specify symmetry boundary conditions on the side walls (no swirl) or periodic boundary conditions (swirl). If the flow is rotationally symmetric, a small number of cells in circumferential direction should be sufficient. Some CFD codes even work with a single cell row. Make sure in this case that the segment has a small angle.