Wind induced ovalling vibrations in silo groups

With a modern day tendency in architecture to build slender structures with a low eigenfrequency, wind loading often constitutes the critical design load. During a storm in October 2002, wind induced ovalling vibrations were observed on several empty silos of a closely spaced group (pitch-to-diameter ratio of 1.05) consisting of 8 by 5 thin-walled silos in the port of Antwerp (Belgium) [1]. While realistic information on dynamic wind loads is increasingly important to improve the design of such flexible structures, it is difficult to obtain reliable design pressures. Design codes typically provide only basic quasi-static wind pressures for a restricted class of structures with a simple geometry. To clarify the cause of the wind induced ovalling vibrations in the silo group, CFD (computational fluid dynamics) simulations are an interesting alternative to more expensive wind tunnel tests or in situ measurements to determine the transient wind loads on the silo structures. Since the pressure distribution around the cylinders of the group is highly dependent on the orientation of the group, the complex flow regime around and within the 8 by 5 silo group has been simulated for 7 angles of incidence between 0 and 90, leaving other influencing parameters unchanged (e.g. spacing ratio, Reynolds number,...). First, the numerical procedure is validated for the case of a single silo in cross flow under identical circumstances. The 2D URANS (unsteady Reynolds-averaged Navier-Stokes) simulations of the entire group consequently reveal similarities of the flow regime around the silo group with the flow within tube arrays (e.g. heat exchangers) [2] and the flow around rectangular cylinders [3]. Analysis of the fluctuating wind pressures on the silo surfaces shows that ovalling oscillations of the third and fourth eigenmodes will be induced at the lee side of the silo group. These correspond with the lowest structural eigenfrequencies of the silos and the pattern of the visually detected vibrations during the 2002 storm.