Abstract
The purpose of this study is to analyze the drag coefficient of three dimensional objects in accelerated motions. The drag coefficient of the fundamental objects are usually observed under steady flow. Many researchers have expressed the effect of unsteadiness by an added mass or impulse force, however the unsteady drag coefficient has not been known yet. The free-fall sphere was observed by using a high-speed camera and the time-varying of its speed was analyzed by a motion capture method. The range of Reynolds number of falling sphere was from zero up to 105. The vortex generation and shedding were observed by means of the dye visualization of wake structure. The change in drag of falling sphere was obtained from the change in acceleration. The unsteady drag coefficients were found 108 times higher than the steady drag coefficients in this experiment. Furthermore, the unsteady drag coefficients were tend to close to 0.44 which is the steady coefficient of sphere on corresponded Reynolds number based on the terminal velocity.
