Flow-Induced Vibrations of Prismatic Bodies and Grids of Prisms

Abstract

Flow-induced transverse, torsional, streamwise, and plunging vibrations of prismatic bodies and grids composed of prisms are reviewed for a wide range of cross-sectional shapes and angles of incidence. For flow at zero incidence, rectangular prisms are susceptible to three kinds of vortex-induced excitation, in addition to galloping and wake breathing, depending on their chord-to-thickness ratio. These include leading-edge vortex shedding (LEVS), impinging leading-edge vortices (ILEV), and trailing-edge vortex shedding (TEVS). A prism with elongated cross-section typical of elements of a trashrack or a headlight screen, which is free to vibrate in the transverse direction, may be excited by different harmonics of ILEV for incidence angles up to 13° and by alternate-edge vortex shedding (AEVS) for larger angles. Excitation by ILEV diminishes drastically with increases of the degree of turbulence in the approach flow. If a rectangular prism of elongated section has a degree of freedom in the longitudinal direction, on the other hand, it may undergo violent plunging vibrations, excited by AEVS, for incidence angles of about 13° and larger. Rounding the leading and trailing edges amplifies this excitation. As demonstrated by a practical example, serious vibrations can be avoided if the grid or trashrack is stiff enough, so that the maximum reduced velocity stays below the critical values marking the onsets of all the possible sources of excitation.