Study on Miter-Bend Flow : Part 2-Pressure Loss caused by Miter-Bend

The pressure loss due to flow bending is one of the most important elements showing flow characteristics in a miter bend. But hydraulic or hydrodynamic investigations have not been made sufficiently on the mechanisms of the pressure loss. So in this report its mechanisms are studied by making experiments and considerations on bending flows with different bending angles and crosssectional area ratios. First, the authors observed the flow patterns in bending open channels in laminar flow. Its results showed that the flow in a bending open channel is very complicated with secondary flow, and separation and re-attachment at the convex corner and at the position ahead of the concave corner. Besides, the authors compared the experimental results with the calculated results of streamlines in two-dimensional potential flow, which were reported in the first report, and showed they coincide fairly well. And based on boundary layer theory they calculated the separation point ahead of the concave corner, and showed that its results and the experimental results coincide qualitatively. Next, the authors measured pressure distributions in turbulent flow in miter bend, used for experiments, of rectangular crosssection, which are joined upstream and downstream with long, staight conduits of the same crosssection. And they compared the calculated results in the previous report and the experimental results corrected with pressure loss for friction, and showed that they coincide compartively well when the bending angle is small, but that they differ a little when it is large. Now, they clarified that in actual flow the pressue downstream of the vena contracta after bending increases due to re-attachment and that the stagnation point moves downsteam on the outer wall from the concave corner. Finally, they considered the mechanisms of pressure loss based on the observations of the flow patterns in bending open channels and the experimental results of pressure distributions. Consequently, referring to the experimental results by other researchers, they clarified the pressure loss due to flow bending is the sum of the loss due to sudden contraction and that due to sudden enlargement independently of bending angles, crosssectional area ratios and the shape of the crosssection of a conduit, and so on. The equations for pressure loss calculated thus are correct enough to be used as the empirical formulae for pressure loss in bends.