Fumiaki Nagao

Effects of oscillation amplitude on aerodynamic derivatives

In this paper, the effects of oscillation amplitude on the aerodynamic derivatives of the thin rectangular cylinder with B/D = 13 and 150 were investigated. It was clear that the torsional amplitude affected strongly the aerodynamic derivatives H* 2 and A* 2 . This effect reduced the critical velocity of flutter by about 10%. The cause of this effect was investigated by the measurement of the unsteady surface pressure. This investigation made it clear that the variation of A* 2 is caused by the movement of the acting point of the resultant lift force. These results indicate that these effects cannot be disregarded for such a simple thin plate but for a real bridge girder with a more complex cross-section and with delicate aerodynamic control devices.

Aerodynamic efficiency of triangular fairing on box girder bridge

Abstract The effects of the triangular edge fairing on bridge aerodynamic stabilities are investigated by numerical simalations of flow around a box girder bridge using the discrete vortex method and wind tunnel experiments concerned with aerostatic forces, aerodynamic responses and flow visualizations. Because the fairing controls not only the separation from the leading edge but also the streamlines around the bridge deck, the effective fairing shapes for individual bridge sections could be determined by the flow properties around the individual bridge decks.

Effects of handrails on separated shear flow and vortex-induced oscillation

The effects of a simplified handrail on the separated shear flow near leading edge and on the vortex-induced oscillation are investigated by several wind tunnel experiments, by use of flow visualization, to observe aerodynamic behavior and pressure distribution around the deck surface. The handrail is composed of vertical posts and a horizontal bar whose depth and height from deck surface are varied. The flow pattern near the leading edge is greatly influenced by the depth and height of the horizontal bar. Different responses for several handrails are clarified by the flow patterns and unsteady pressure properties.

Basic study on blockage effects in turbulent boundary layer flows

Abstract The blockage effects on aerostatic forces such as the drag coefficient, CD, and the base pressure coefficient, Cpb, were experimentally examined by using two-dimensional rectangular cylinders, Blh = 1.0, placed on the floor normal to the flow direction in turbulent boundary layer flows (TBLF). At the same time, the effects of the wind characteristics of the TBLF on the aerostatic forces were investigated. It was shown that (i) the blockage effect appears clearly in the decrease of the base pressure and the pressure on the roof, (ii) the aerostatic forces were influenced by the wind characteristics of TBLF, in which the gradient of the mean velocity profile at the height of the model mainly controlled the flow properties around the model.

Aerodynamic properties of closely spaced triple circular cylinders

Experimental investigations for the wake galloping of closely spaced triple circular cylinders were carried out. It is clarified that the central cylinder shows almost the same wake galloping behaviors as those of the leeward cylinder of tandem cylinders in close arrangement and the leeward one of triple cylinders also has the resemblant properties in large staggered angles; however, the wake galloping of the leeward cylinder in the small staggered angles was stabilized due to complicated flows around cylinders. The connection of triple cylinders suppresses the wake galloping effectively.

Basic study of blockage effects on bluff bodies

The blockage effects on the aerostatic forces such as the base pressure coefficient, Cpb, and the drag force coefficient, CD, were experimentally examined by using three kinds of bluff bodies, i.e. the cube, the rectangular cylinder, Bd=0.8, and the two rectangular cylinders (Bd=0.8) in tandem arrangement. The CD for these models were corrected by modifying Maskells and awbis methods taking into account new factors contributing to blockage effects as follows; 1) a space factor to allow for the tandem arrangement of bluff bodies, 2) an aspect factor to express the three dimensional effects and 3) a flow factor to reflect the flow properties.

The validity of sectional models on wind tunnel tests for vortex induced oscillation of bridges

Abstract The validity of wind tunnel procedure with sectional models used to investigate the aerodynamic stability of long-span bridges was examined by several wind tunnel tests. The vortex induced bending oscillations of some rectangular cylinders were discussed comparing the bending amplitudes, aerodynamic forces, surrounding flow patterns and unsteady pressure distributions of the sectional models with those of the taut strip models. After that, it becomes clear that there is a significant difference between the responses of 2D and 3D models for the reason of the non-linear dependence of unsteady aerodynamic force on amplitude and the existence of spanwise flow.

Vortex-induced oscillation of a bridge in slowly fluctuating wind

Abstract There are many cases where the vortex-induced oscillations estimated by wind tunnel tests are not observed in full-scale bridges. Many researchers suggest that these are caused by turbulence of natural wind. Certainly, it is true that turbulence controls the vortex-induced oscillation. However, the turbulence scale in wind tunnel test is usually too small to estimate its effects on a full-scale bridge in natural wind. In this study, the vortex-induced oscillation of a bridge model in large-scale turbulence flow was examined by using the slowly fluctuating wind generated with an active-controlled wind tunnel system. As aresult of the tests, it was made clear that the vortex-induced oscillation is controlled with large-scale turbulence flow.

Regression analysis of local wind properties with local topographic factors

Abstract To improve the accuracy of estimation of local strong wind, new local topographic parameters were introduced in the multiple regression analysis. New azimuthal topographic factors were defined independently of particular radius of fan-shaped area for the estimation of usual topographic factors. In addition to them, some micro-scale local terrain factors were defined from scenery pictures of 16-azimuths around the site. The azimuthal strong wind velocity at a construction site was evaluated through the two-step estimation, in which the representative wind speed at a site was estimated by first regression analysis with the usual topographic factors and second regression analysis with newly defined micro-scale factors was followed to estimate the azimuthal wind speed fluctuation. It was shown that the new topographic factors defined here played an important role in all regression analyses.

Estimation of local strong wind around small peninsula-like terrain

Abstract An empirical formula to estimate the properties of the local strong whirlwind behind a peninsula was derived from the results of wind tunnel measurements using some simplified terrain models with trapezoidal cross sections. The terrain parameters discussed in the tests were the height of the peninsula, the upstream and downstream slope angles, the crown width and the end slope angle. As the results indicate, it became clear that the extent of the strong wind behind the peninsula is dependent on the direction of approaching wind, and that the speed up ratio near the ground level is controlled by all terrain parameters discussed in this study.