Kazuki Hibi

Three-dimensional numerical simulation of air flow around a cubic model by means of large eddy simulation

Abstract The unsteady, turbulent flowfield around a cubic model has been simulated by means of large eddy simulation (LES). The resolvable-scale flowfield has been obtained directly by solving the filtered, three-dimensional, time-dependent Navier-Stokes equations. The subgrid-scale motions were simulated by an SGS eddy viscosity model. The computed mean velocity dstributions and various turbulence statistics (i.e. turbulence intensity, velocity spectrum, integral scale, etc.) were compared to those obtained from wind-tunnel experiments to examine the accuracy of LES from the viewpoint of engineering applications. The correspondence between numerical simulation and wind-tunnel experiments is found to be good.

Proper orthogonal decomposition and reconstruction of multi-channel roof pressure

The proper orthogonal decomposition (POD) and reconstruction of pressure on the roof of a low-rise building is described. The point pressures acquired simultaneously at 494 taps, uniformly distributed on the building surface, were used in the analysis. The pressure space correlation was computed and employed to calculate the eigenvalues and eigenvectors (modes) for two cases: roof pressure field (RP) and roof and walls pressure (RWP). The first two of the RP modes were found to be respectively similar in shape to the pressure mean square and the mean pressure derivative computed with respect to the wind direction. Strong interaction between the wall and roof pressures was identified from the POD analysis. The POD results for the RP case were employed to reconstruct the point and area-averaged roof pressure. Approximately 30% and 2% of the computed modes were needed to represent the largest negative peak of point and area-averaged roof pressure, respectively. Only one mode was sufficient to reconstruct the mean point and area-averaged roof pressure.

Dynamic wind pressures acting on a tall building model — proper orthogonal decomposition

Dynamic wind pressures acting on buildings are complicated functions of both time and space. Earlier studies have applied the proper orthogonal decomposition (POD) technique to such random wind pressure fields to identify hidden deterministic structures. In this study, the POD technique is applied to fluctuating wind pressures acting on a tall building model, and the following facts are found: the along-wind and across-wind forces can be approximated by only a few dominant modes, while the torsional moment requires approximately 10 modes; and the wind-induced response of a tall building has a maximum evaluation error of less than 6% with the reconstruction of the dominant modes.

Wind response of a tower (Typhoon observation at the Nagasaki Huis Ten Bosch Domtoren)

Abstract Observation of typhoons was conducted at a one hundred metre tower. In this paper, characteristics of the wind and the response of the tower during typhoons are investigated and discussed. Extraordinarily large turbulence intensity was observed near the center of the typhoon. The gradient wind of the typhoon was simulated and the existence of the super gradient region was indicated from the comparison between the result and observation data. The fundamental natural period of the tower was increased with the increase of the response amplitude. Furthermore, the damping ratio of the tower during the typhoon was evaluated by the random Decrement technique and variation with the response amplitude and wind speed was shown.

Extreme wind pressure distributions on low-rise building models

Abstract This paper first discusses the conditionally sampled actual wind pressure distributions causing maximum quasi-static wind load effects at the base of low-rise building models with square and rectangular plans. The maximum normal stresses in column members were also examined to discuss the wind load combination of the along-wind, across-wind, uplift, and three moments. Then, it examines the actual wind pressure distributions causing the maximum quasi-static stresses in structural frames. These are compared with Kasperski’s load-response-correlation formula and the quasi-steady pressure distributions.

Proper orthogonal decomposition study of approach wind-building pressure correlation

The wind-induced pressure acting on buildings and structures is greatly influenced by the approaching wind. A detailed analysis of correlation between wind and building pressure is difficult using classical statistical methods. Proper orthogonal decomposition (POD) overcomes this difficulty by allowing us to consider separately the principal coordinates and the associated eigenvectors, characterizing the dominant spatio-temporal structures of the analyzed field. In this paper, the fluctuating wind pressure acting on the building models is analyzed using POD and the correlations between the decomposed pressures and the approaching wind are examined.

An analytical model for simulation of the wind field in a typhoon boundary layer

An analytical model has been developed for calculating the wind field in a moving typhoon boundary layer. The present model has an upper inviscid layer of cyclostrophic balance and a lower friction layer controlled by a surface drag coefficient and eddy viscosity. Velocity in the inviscid region is described first, using the assumption that the wind field moves with the translation velocity of the typhoon. Perturbation analysis is then performed to obtain the tangential and radial boundary layer velocity in the friction region. Wind speeds and directions predicted by the present model agree favorably with observations obtained at a 100 m height tower during three typhoons in 1991. Rapid variations in wind speed due to surface roughness and topography around the tower are also amply demonstrated. In the final section of this paper, a case study is presented to examine the wind field associated with a typhoon, as well as the characteristics of the ratio of surface to gradient wind speeds G(r) and the inflow angle 7. The results indicate that previous observation data obtained at coastal areas during typhoons can be explained satisfactorily by this model.

A numerical study of the wind field in a typhoon boundary layer

The wind field in a typhoon boundary layer (TBL) has been investigated by a numerical model. The results show that vertical profiles of wind speed in the TBL can be satisfactorily stated by conventional power-law expressions. To describe the structure of strong wind in the TBL, two parameters have been suggested: one is a dimensional parameter, fa , approximately representing the absolute vorticity in the wind field, and the other is a non-dimensional parameter, ~, characterizing the heterogeneity of vorticity in the radial direction of a typhoon. Substituting the parameterfx by Coriolis parameter f, the gradient height zg during typhoons can be predicted by the same formula as that used during non-typhoon climates. The ratio of surface to gradient wind speeds G(r) and the inflow angle 7~ in the TBL are also examined using the present numerical results, and the formulae for predicting them are presented.

Numerical simulation of velocity field and diffusion field in an urban area

In this paper, results of numerical simulations for flow and diffusion fields around buildings are presented in order to demonstrate the great potential of the numerical method for predicting the outdoor environment in an urban area. The flow and diffusion fields around a single building, street blocks and a building complex which is actually under construction are simulated here. The k—ϵ two-equation turbulence model and large eddy simulation are used for turbulence modeling. The accuracy of these simulations is examined by comparing the predicted results with wind-tunnel experiments. The animated graphics using various techniques of computer graphics are discussed.

Simultaneous measurements of wind speed profiles at two sites using Doppler sodars

In the wind-resistant design of buildings and structures, it is very important to accurately assess the design wind speed at a particular site, considering the variation in wind speed with terrain roughness. The authors attempt to find a reasonable method for estimating design wind speed for given terrain roughness, through simultaneous wind observations at altitudes up to 420 m at sites with different roughnesses using two sets of Doppler sodars. In this paper, the characteristics of the mean wind speed profiles evaluated for each distinguished wind speed in each wind direction were presented. The longitudinal mean wind speed profiles in the same storms were also compared for two pairs of sites to study the variation in longitudinal mean wind speed as it is affected by inland terrain roughness.