Yoshinobu Kubo

Effects of end plates and blockage of structural members on drag forces

Abstract The aerostatic and aerodynamic characteristics of structural members were investigated using two-dimensional wind tunnel tests. In these tests problems occur owing to the effects of end plates and blockage. End plates are used to produce two-dimensional flow around models. There is an optimum size for end plates, because the degree of the two-dimensional characteristics of the air flow depends on the size of the end plates. The required condition for the end plates was determined by measurement of the wind velocity distributions and base pressures around the testing body in relation to the formation of the wake. It is concluded that the required dimensions of the end plates can be determined on the basis of Karmans theory of vortex streets. In practice, the diameter of the end plate should be larger than 8.5 times the model depth. On changing the blockage ratio with end plates of a constant size, the aerostatic forces change significantly. This is the result of the combined effects of end plates and blockage.

On the suppression of aerodynamic instabilities through the moving surface boundary-layer control

Abstract Several methods have been reported for suppression of the aerodynamic vibrations of structures under wind action. Most of them are passive in character as against the proposed active approach. Results show that galloping and vortex-excited vibrations of a square prism can be suppressed by the injection of momentum into the boundary-layer.

SUPPRESSION OF WIND-INDUCED VIBRATIONS OF TALL STRUCTURES THROUGH MOVING SURFACE BOUNDARY - LAYER CONTROL

Abstract The suppression of aerodynamic response of square-section tall structures, like pylons of long-span bridges and very high buildings, is studied. The method used is the moving surface boundary-layer control achieved through rotating cylinders at the leading edge of the structure. Effectiveness of the approach in suppressing the aerodynamic instabilities of a square prism was confirmed in earlier studies by the authors. The method also proves successful in suppressing torsional vibrations. Effective and economical arrangements of rotors are investigated with reference to prototype tall structures.

Mechanism of aerodynamic vibrations of shallow bridge girder sections

Abstract Aerodynamic vibrations induced on shallow bridge girder sections are classified by various names such as heaving and torsional vortex-excited vibrations, galloping, and torsional flutter. The present paper investigates the mechanism of the aerodynamic vibrations of shallow bridge sections by measurement of their surface pressures and flow visualization around the sections. It concludes that flow patterns on the surface closely relate to the various aerodynamic vibrations and these vibrations are regarded as phenomenon based on the flow patterns on the surface which is transformed according to the variety of approaching wind velocity.

Active control of super-tall structure vibrations under wind action by a boundary-layer control method

Abstract The authors have been developing active control methods for suppressing the aerodynamic vibrations of super-tall structures, like pylons of super-long bridges and super-tall buildings, which will be constructed in the next century. Boundary-layer control by rotating cylinders at the leading edge is treated as an active control method for suppressing aerodynamic vibrations. Previous wind tunnel tests conducted by using a two-dimensional square prism gave good results for suppression of the aerodynamic response due to vortex-excited vibration and galloping. On the basis of the two-dimensional wind tunnel test results, three dimensional wind tunnel tests were conducted. The experimental results had a relatively good agreement between two and three dimensional tests.

Aerodynamic characteristics of closely and rigidly connected cables for cable-stayed bridges

The aeroelastic instability of cables for cable-stayed bridges has become a serious problem with extension of span length. From the viewpoint of reduction of cost for construction and maintenance of cables, multiple cables have been adopted for cable-stayed bridges. The multiple cables, however, have aeroelastic instability of wake-galloping. Therefore, it is required to develop a method for suppressing wake-galloping of multiple cables. The authors proposed the solution for this problem on the basis not of structural method with damper system, but of aeroelastic method based on the mechanism of wake-galloping. The proposed method is to connect two cables with each other and to place two cables in close and rigid arrangement. As a result, the cables are protected from a generation of wake-galloping induced by the wake of upstream cable.

Improvement of aerodynamic instability of cable-stayed bridge deck by separated flows mutual interference method

Abstract The present research was started to find an aerodynamically stable bridge deck shape for cable-stayed bridges with concrete deck. The deck of cable-stayed bridge needs a kind of fairing to improve aerodynamic instability of the bridge deck designed from the view point of structural strenght. Due to conservative concept to the fairings, the cross section of deck has a streamlined shape. Applying this concept to the cable-stayed bridge with concrete deck, the portion of fairings adds comparatively large dead load to structural cross sectional area. Therefore, the streamlined shape fairings are not suitable for the concrete deck of the cable-stayed bridge. The present paper deals with method to improve the aerodynamic instability of the concrete deck girder of the cable-stayed bridge, by using no-streamlined shape fairings with cross sectional area as small as possible.

Improvement of aeroelastic instability of shallow π section

Abstract From an economical point of view, a two-plate-girder bridge section has been examined as a bridge section for a long span suspension bridge. The two-plate-girder bridge section of H or π section inherently does not have good aerodynamic performances, as shown by the accident of the Tacoma Narrows Bridge with H section-type bridge girder. On the contrary, in the present paper, there has been an attempt to find the possibility of the aerodynamic performance improvement of the two-plate-girder bridge section of the π section by changing the intervals between the two plate girders. With respect to wind tunnel test results, in the vortex-excited vibration in heaving vibration mode, the on-set wind velocity and the vibration amplitude decrease with interval between two plate girders, and the flutter on-set wind velocity increases. These results show the possibility of aerodynamic performance improvement of the two-plate-girder bridge section.

Aerodynamic behavior of multiple elastic circular cylinders with vicinity arrangement

The span length of cable-stayed bridges has become longer with the increment of years, which requires a larger, longer and heavier cable. A heavier construction machine is required to lift the cables up to cable anchoring points on the pylon and large capacity jacking facilities are required to put high tensile force into the cables. From the view point of construction cost, the larger diameter and longer, heavier cables increase the construction cost. In order to reduce the construction cost or keep ordinary construction cost, smaller diameter and lighter cables should be used in substitution of large diameter and heavier cables. Therefore, it is required to investigate the aerodynamic instabilities of multiple cables by changing the number of cables and the space between cables. The present paper shows the experimental results concerning aerodynamic instabilities of two, three and four cables with various arrangements.

Aerodynamic performance of improved shallow π shape bridge deck

Abstract Aerodynamic stability of π section, which is structurally economical, was studied. By locating the main girders at inward location, so that the ratio of the floor deck overhang C to the girder height D′, C/D′, to be 2.0, and also setting solid barriers at inward location, the wind-induced responses were much suppressed even under various angles of attack. Although, some wind-induced responses were still observed with the improved section, the magnitude of the negative aerodynamic damping that caused the vibration was found to be small, thus the section could be thought as a probable choice for long-span bridges. The stability due to the solid barrier location was accomplished by the separated flows mutual interference effect; the unsteady pressure on the upper surface of the deck decreased significantly for the torsional motion.