Fluctuating forces acting on two square prisms in a tandem arrangement

Abstract

Fluctuating aerodynamic forces acting on two square prisms in a tandem arrangement situated in uniform flow were investigated experimentally. Time-mean pressure and drag, fluctuating drag and lift acting on these prisms and the Strouhal number of vortices shed from the prisms were obtained for various spacing ratios between the prisms s/ w ( s = spacicing, w = width of the prism). Above and below s/ w = 3, different flow patterns were observed and significant changes in the time-mean and fluctuating aerodynamic forces were found. It was also found that the variation of the Strouhal number of the vortex shedding from the two prisms with s/ w can be divided into three regions. Namely, Region I where vortices are shed from the downstream prism alone ( s/ w<3), Region II where the vortices shed from the downstream prism synchronize with the ones from the upstream prism (3< s/ w<27) and Region III where the vortices are shed individually from each prism ( s/ w>27). Moreover, by taking the periodic velocity signal downstream of the upstream prism as a reference signal and ensemble averaging, the behavior of the fluctuating lift and drag acting on the two prisms becomes clear and is discussed. In particular, the phase lag of the fluctuating lift acting on the two prisms is found to be linearly related to the spacing ratio s/ w.