Experimental study on the unsteady characteristics of flow reattachment at different boundary conditions using MEMS sensors

• Main Authors: Shih-yuan Su, 蘇詩媛
• Other Authors: J. J. Miau
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/66318108708734863378

碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 96 === The purpose of the research is to investigate the unsteady, three-dimensional behaviors of flow reattachment over a surface-mounted rectangular block under different boundary conditions. The aspect ratio of the rectangular block model, i. e., its width versus height (H), was 4. The model was located at 10H and 16H downstream from the inlet of the test section, respectively. Experiments were made for the Reynolds numbers at 2.54×104 and 5.09×104, with and without the incoming boundary layer roughened by a screen installed immediately upstream of the model. The region of flow reattachment was clearly identified by the self-made thermal tuft MEMS sensors applied on the surface of the model. In addition, the phenomenon of flow separation and reattachment around the model was successfully simulated by a CFD method. Results show that the reattachment length at the centerline of the model is around 3H at the two different Reynolds numbers studied, irrespective of the locations of the model from the inlet. Similar results were obtained by the two-dimensional CFD analysis. On the other hand, it is noted that the large-scale 3-D flow at the junctions of the sidewalls and the model caused the reattachment lengths near the two sidewalls substantially shorter than that at the centerline of the model. In the case of the incoming boundary layer roughened by a screen upstream, the reattachment length was found significantly shortened, in a way that the reattachment length at the centerline of the model appeared to be shorter than those near the two sidewalls. Cross-correlation analysis of the signals of the MEMS sensors and X-type hot-wire was performed to study the three-dimensional flow structures embedded in the flow reattachment region. As found, the characteristic spanwise lengths of the three-dimensional flow structures are about 0.5H and 0.3H, respectively, for the incoming boundary layers without and with roughened by a screen upstream of the model. It is further noted that the unsteady behaviors of flow reattachment were featured with the intermittent events remarkably noted in the streamwise (U) and vertical (V) velocity traces. Namely, at the moments when U gets decreased appreciably, V very likely appears positive and at high value. These events are attributed to the presence of unsteady, three-dimensional flow structures, which conceivably induce strong momentum mixing in the flow reattachment region and cause the reattachment length shortened substantially.