Present research aims on the construction of force balances and the surface roughness effect on bluff bodies The first part is to build external force balances in the Flow Sensing System Laboratory National Cheng Kung University for wind tunnel testing to improve the capability of experimental investigation into fluid flow around bluff bodies We developed a two-dimension balance It was designed for measuring drag force and side force which consists of four load cells In order to find out the relation between the output voltage and the applied load we did calibration several times to get the measurement uncertainty for accuracy and a calibration matrix A circular cylinder model was tested by the balance in the wind tunnel and it revealed a great consistency compared to the result from the literature Afterwards it was applied to the wind-tunnel experiments The second part is to investigate the phenomenon of flow around a circular cylinder at Reynolds numbers between 7×104 and 4×105 The pressure taps signals were used to determine the pre-critical regime one-bubble regime two-bubble regime and the supercritical regime The experiment used a circular cylinder fitted by textile with different relative roughness The force signals obtained were analyzed with fast Fourier transform to get the shedding frequency This study shows in the case of different surface relative roughness each flow state still exists and occurs at lower Reynolds number That surface roughness can contribute great drag reduction to bluff bodies Drag and lift signals were analyzed to look for the intermittency effect Then we compared relative roughness from each textile to their critical Reynolds number drag coefficient and pressure coefficient on ±90o It revealed a relation between them and can be expressed by a regression curve

The design of 2D force balance and the effect of textile roughness on the aerodynamic characteristic of a circular cylinder

宗修, 蔡. (Author). 2017 8月 15

學生論文: Master's Thesis