Tornado-strength winds interacting with a highway overpass

Analysis of tornado strength winds interacting with a highway overpass structure is presented with emphasis on air flow patterns above and under the bridge. Experiments were performed in a wind tunnel with the scaled geometry of an overpass. Velocity and dynamic pressure measurements were obtained independently at four locations as the overpass was rotated about its vertical axis between air flow angles of approach between 0° and 90°, at 10° increments. Lift and drag forces on the overpass geometry were also measured. To compare various highway overpass locations with the surroundings, the measured dynamic pressure and velocity, drag and lift forces, and drag coefficients at each of the locations and approach angles were examined. It was found that at all locations, the measured velocities never exceeded the freestream velocity of 190.2 ft/s (58 m/s; 130 mph), with the maximum Re occurring above the overpass and between the I-beams. A theoretical maximum pressure drop for the tornado center was calculated to be 0.5 psi for an Enhanced Fujita 2 scale tornado and compared with the highest pressure drop of 0.278 psi, determined from the experiments. Calculated pressure coefficients C_p were mostly <0 and some close to one dynamic head less than ambient. The drag coefficients C_d remain primarily in the laminar region with later transition to turbulence. Using experimental data from the literature, drag forces on an average size man in crouching and laying positions between the overpass I-beams section were determined to be a maximum of 31 lb_f.