| Summary: | 碩士 === 淡江大學 === 航空太空工程學系碩士班 === 96 === The research experimentally investigates the effects of different backward-facing step inlet condition on the flow characteristics in three-dimensional separated and reattached flows. A wind tunnel system is used to generate a uniform flow into the inlet of the test section duct with smoke. Test section duct is made by transparent acrylics and the step height of the backward-facing step duct is 1.75 cm, aspect ratio is 4, and expansion ratio is 1.33. Investigate six-cylinder turbulator made by six transparent and cylindrical acrylics placed inside the duct with the same distance and no turbulator at the inlet of the test section duct with different duct length condition to generate the effect of the flows. The Reynolds number are between 500 and 17000, which cover the laminar, transitional and turbulent flows. The velocity measurements were conducted using laser Doppler velocimetry include the profile of the backward-facing step inlet, the reattachment length (the size of the recirculation region), the spanwise distribution of the reattachment length, the different flow structures of the three-dimensional separated and reattached, and the recirculation region of the flat wall.
Results of this study show that the reattachment length increases with Reynolds number in the laminar flow region, decreases with Reynolds number in the transitional flow region, and almost remains constant with Reynolds number in the turbulent flow region. The reattachment length increases in the laminar and transitional flow region and decreases in the turbulent flow region when duct length increases. The transitional and turbulent flow are also delayed to generate, and the Reynolds number region of the laminar and transitional flow are also increased when duct length increases. The turbulator much reduces the reattachment length, and makes the transitional and turbulent flow to be generated early. The turbulator also can make the Reynolds number region of the laminar and transitional flow to be decrease, and the Reynolds number region of the transitional flow even can decrease to 50%.
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