H. Kawai

Effect of corner modifications on aeroelastic instabilities of tall buildings

Abstract Effects of corner cut, recession and roundness on aeroelastic instabilities such as vortex induced excitation and galloping oscillation were investigated by wind tunnel tests for square and rectangular prisms. According to the tests, the following results were obtained. Small corner cut and recession are very effective to prevent aeroelastic instability for a square prism by increasing the aerodynamic damping, but large corner cut and recession promote the instability at low velocity to reduce the onset velocity of the instability when the damping is small enough. Corner roundness is also effective to increase the aerodynamic damping to suppress the instability. Low-speed galloping type instability occurs for a shallow depth rectangular prism. The corner modifications promote the instability at low speed, because the vortex shedding is prevented by the modifications. The motion induced vibration occurs for a deep depth rectangular prism, and it is little affected by the corner modifications.

Characteristics of fluctuating suction and conical vortices on a flat roof in oblique flow

Abstract The characteristics of suction fluctuation on a flat roof in smooth and turbulent flows are investigated by a wind tunnel test. The large suction fluctuation is induced near the windward corner by conical vortices when flow attacks oblique to a roof. This suction fluctuation is induced by two different causes in the low and high frequency ranges. The low frequency part of the fluctuation is owing to the slow in-phase sway of a pair of conical vortices separated from the edges of the roof. The high frequency part of the fluctuation is brought by the rotation of spiral axes of the conical counter-rotating vortices. The lateral component of approaching turbulence amplifies the sway of the vortices and then induce much larger suction fluctuation.

Pressure fluctuations on square prisms - applicability of strip and quasi-steady theories

Abstract The applicability of the strip and the quasi-steady theories were checked in the light of experimental results of pressures on square prisms in a turbulent boundary layer. These theories can be applied to estimate mean pressure and fluctuating pressure in the low frequency range on a windward face. A power spectrum of fluctuating pressure on a windward face is greatly attenuated in in the high frequency range by the distortion of oncoming turbulence. Mean and fluctuating pressures on side and leeward faces are greatly affected except in the very low frequency range by vortex shedding and wake turbulence so that the applicability of these theories is fairly doubtful. On the very low frequency range, these theories may be applicable even in the case of side and leeward faces. Probability densities of fluctuating pressure were fairly skew and these agreed well with those of the quasi-steady theory which included the nonlinear terms, even in the case of side and leeward faces.

Vortex induced vibration of tall buildings

For a tall building of which aspect ratio is larger than 8, the resonant velocity of vortex induced vibration or the onset velocity of galloping can be within the design wind velocity. In this paper, the vortex induced vibration and the galloping of tall buildings with aspect ratio, H/√BD=10 with side ratios, D/B=12 to 3 are investigated in smooth flow and turbulent boundary layer flows over open terrains and urban areas. n nExperimental results show that the vortex induced vibration still arises in the turbulent flow for tall buildings with the shallow section, and surprisingly the vibration strengthens as the turbulent intensity increases.

Structure of conical vortices related with suction fluctuation on a flat roof in oblique smooth and turbulent flows

Abstract A structure of conical vortices on a flat roof in oblique flow is investigated by hot-wire velocity measurements in smooth and turbulent flows. The vortices are stronger in smooth flow than in turbulent flow. The centerlines of the vortices go along the line inclined 13° in the smooth flow and 10° in the turbulent flow relative to the eaves. These lines match the ridge lines of the maximum suction. The lateral component of upstream fluctuating velocity correlates well with the fluctuating suction on the roof both in the smooth and the turbulent flows. The large correlation zones spread toward the downstream along the lines inclined 23–25° to the eaves which are located near the edge of the conical vortices. The distinct feature of the correlation is caused by the alternative growth and decay of each vortex.

Vortex induced vibration of circular cylinder

Abstract The flow field around a vortex induced vibrating cylinder is analyzed by the numerical simulation using the discrete vortex model. When the vortex shedding locks in the vibration, the separated shear layers sway in a body like a pendulum. The locked-in is not the necessary condition of the vortex induced vibration, but the vibration develops when the vortex does not shed synchronously with the vibration. At the unsynchronous state, the rolling up of the separated shear layers are prevented and promoted alternatively by the cylinder movement at every few cycles of the vibration. The energy which encourages the movement of the cylinder is supplied when the rolling up of the shear layer is promoted by the movement. The effect of the splitter plate in the wake was also investigated and the result of the simulation compared with the flow patterns experimentally obtained from the smoke wire method. The simulated flow patterns agree well with the one which were experimentally obtained.

A REVIEW OF VELOCITY AND PRESSURE FLUCTUATIONS IN TURBULENT FLOWS AROUND BLUFF BODIES

Abstract This paper is firstly a review of the theory for the changes in the velocity fluctuations in a turbulent floq as it passes around a bluff body with a simple shape such as prism or sphere. Detailed results are given for the ideal cases where the scale of the incident turbulence is much smaller or much larger than the bodys diameter, but it is shown that the results are also applicable to many practical flows, including bluff bodies in the wake of others. This review gives new, simple derivations for the spectra and variances of velocity and pressure fluctuations near the surface of bluff bodies, showing why on different shapes of body and for different scales there are different distributions of pressure variance. A new approach is described for simulating realisations of the random velocity field, using the technique of ‘kinematic simulation’. Experimental results are also briefly reviewed for bluff bodies placed in homogeneous turbulence, and in inhomogeneous shear flows. The outstanding problem remains of understanding how incident turbulence interacts with the turbulence and vortex shedding in the wake of a bluff body, though there has been some progress in identifying and understanding some of the mechanisms.

Numerical prediction of wind loading on buildings and structures : Activities of AIJ cooperative project on CFD

This study presents the activities of the Architectural Institute of Japan (AIJ) concerning the Computational Fluid Dynamics (CFD) for a numerical prediction of wind loading on buildings and structures. In the AIJ project, the flows and the pressures around a low-rise building (breadth : depth : height = 1 : 1 : 0.5) have been computed by 10 members of the working group, who mainly employed the κ-e model or the large eddy simulation for turbulent flows. Here, on the basis of the results of the AIJ project, future subjects for further development of the CFD technique are discussed. Also we present how to find the way to realize the practical use of the CFD technique on prediction of wind loading.

Wind-induced response of a large cantilevered roof

Abstract The subject of this study is to clarify aerodynamic properties of a large cantilevered roof. We conducted free vibration experiments in order to investigate effects of various parameters such as mass, pitch angle, and damping ratio to wind-induced vibration of the roof. In the experiments, we have found that the roof hardly vibrated at its natural frequency until a certain critical wind velocity, then a flutter-like vibration occurred and a sharp peak at frequency lower than the natural frequency appeared in a spectrum of the vibration. In the light of forced oscillation experiments, the frequency of the sharp peak was the natural frequency of the roof lowered by the positive aerodynamic stiffness. The lowered natural frequency synchronized with the frequency of the vortex shedding from the leading edge of the roof, which causes a negative damping effect of unsteady wind force.

Effects of angle of attack on vortex induced vibration and galloping of tall buildings in smooth and turbulent boundary layer flows

Effects of angle of attack and free stream turbulence in occurrence of vortex induced vibration and galloping of tall buildings were investigated by wind tunnel experiments. From the investigation, the following results were obtained: (1) The vortex induced vibration is strengthened by free stream turbulence when the separated shear flow does not reattach to the side face. (2) The onset velocities of vortex induced vibration and the galloping vibration are the smallest and the amplitude of the vibrations are the largest when wind attacks normal to the face. (3) The critical velocity V/n?BD for the level of the normalized rms displacement σ y /H = 1/250 is about 5 and is little affected by the side ratio where H is the height, B is the breadth and D is the depth of the building in the flow direction.