On the use of an active turbulence grid in wind tunnel testing of bridge decks

A new experimental set-up with an active turbulence grid for wind tunnel testing of bridge decks in homogeneous freestream turbulence is presented. Unlike other active turbulence generators applied in bridge aerodynamics, the active turbulence grid generates stochastic turbulence and not quasi-harmonic velocity fluctuations that are correlated along the section model. A twin deck and a single deck section model are tested for aerodynamic properties using different incoming turbulence. The results show that the static coefficients can be highly sen-sitive to the properties of the incoming turbulence and that the sensitivity varies between the tested cross-sections. Significant changes in aerodynamic derivatives were observed with changing turbulence properties. The turbulence-dependent changes in the aerodynamic derivatives are less clear compared to the static co-efficients since the presence of turbulence and buffeting forces makes it more difficult to identify the aero-dynamic derivatives. It is also clear that the turbulence creates large variations in the instantaneous angles of attack, questioning the validity of linear models for prediction of self-excited forces. It is concluded that the active turbulence grid is a valuable asset in bridge aerodynamics research as it allows for novel studies of bridge aerodynamic properties in conditions relevant to long span bridge design.