| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 97 === This study investigates the near-wake flow structures behind a flat-plate wing at different angles of attack. The flat-plate wing model with AR = 1 is operated at freestream velocity of 10m/s and the corresponding Reynolds number based on the chord length is 1×105. The force balance is used to measure the nonlinear lift curve slope and the stall angle of attack (AOA) of the wing model. For velocity measurements, the wing model was tested at AOA = 10°, 20°, 30° and 40° in the study. In the mean time, the velocity distributions were measured at different chord lengths (C) location downstream behind the trailing edge from 0.05C to 2C in these experiments. In order to analyze the properties of three-dimenstional (3-D) effect, the 3-D flow properties were obtained not only by streamwise and spanwise mean and fluctuation velocities by hot-wire anemometric measurement, but also by streamwise and transverse mean and fluctuation velocity measurements.
The force experimental results show that the lift curve slope of the wing with AR = 1 was very different with that of AR = 2 and 3 shown in previously researches. At AR = 1, the lift curve is characterized by high values of CL,max, CL.max and non-constant lift-curve slope, the stall angle is at AOA = 37, but the stall angle of AR = 2 and AR = 3 is about AOA = 20.
More detailed flow properties when AR = 1 were also investigated and the results indicate that the size of vortex and, the distance between the vortex core and trailing edge will increase with the increase of AOA. For AOA = 10, 20, and 30, the flow field near the root on upper surface of the wing will be affected by tip vortices, which produced the vortex lift to make high stall angle of attack and the nonlinear lift curve. And the flow property is very different for AOA = 40 where the flow will separate from the leading edge and change into stall.
This study also found that the motions of the vortex center in the near wake are primarily due to the continuing rollup of the shear layer arriving from the inboard regions. This rollup process causes more and more of the spanwise vorticity to be rotated into the axial direction and added to the outer layers of the tip vortex. As for the vortex trajectory in the near field, all vortices move inboard and downward along the downstream except for AOA=100, which does not change the core location at this experiment. In addition, the vortex size will keep the same in the near field at AOA=10, 20 and 300, while it changes at AOA=40 along downstream.
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