Teshikaga is located at the outlet of valley where the mountain range is divided by the Kushiro River in the east of Hokkaido. In Teshikaga, the strong northwesterly winds are often observed in winter and the damages caused by snow storms are frequently reported. It is important to analyze the topographical factors of formation for the strong winds in order to take appropriate countermeasures to reduce the damages. For this reason, we conducted a mesoscale meteorological simulation with the weather research and forecasting model (WRF) for a strong wind event in winter and investigated the mechanisms of formation of the strong wind in Teshikaga. It was found that the downdrafts and strong wind near the ground surface appeared at the outlet of valley where Teshikaga is located. The region with strong wind appeared at the outlet of the valley when the wind directions at the inlet and outlet of the valley were along the midline of the valley. Furthermore, based on the normalized valley width and the Froude number at the inlet of the valley, the flow field was classified as an unsteady state with a strong wind near the outlet of the valley. These results indicate that the strong wind in Teshikaga are formed due to the effect of the gap wind.

View full abstract

We numerically examined surface-wind characteristics in Boso peninsula with T1915, especially paying attention to the interaction between the activation of strong winds near the maximum wind velocity radius of typhoon and the locally wind enhancement due to complex terrains. Time-series of surface winds obtained by a numerical weather prediction and a typhoon models showed that there existed strong wind area just outside the core of typhoon along the west coast in Boso peninsula; the wind speed and direction in the upper atmospheric boundary layer were about 60 m/s and south, respectively. The trajectory analysis on spatial distributions of time-averaged wind fields obtained by a wind-engineering model (CFD model) with a RANS model for the west coast in Boso peninsula indicated that the strong wind spots over the complex terrain were generated by an upslope and also upward undulations. Large-eddy-simulations for the strong wind spots gave the large turbulence intensities and peak factors compared with those observed above flat terrains.

View full abstract

Long term observation of wind condition at high altitude in central part of Tokyo has been conducted using doppler lidar. The characteristics of the measured data were organized, and the influence of thermal stratification, surrounding high-rise buildings and the strength of the wind speed were examined. Variations in data acquisition rate, i.e. decrease at the altitude and in winter were shown, and it is difficult to grasp statistical characteristics of wind conditions only by observation data. The influence of buildings about 200 m height was shown around 300 m in altitude, and the wind conditions were greatly different depending on the wind direction due to the influence of surrounding buildings condition using the reliable data.

View full abstract

By using a wind tunnel, we have carried out a series of experiments about thermally unstable boundary layers. Their mean velocity and velocity fluctuations are studied in terms of a model of a random superposition of eddies that are attached to the surface of the tunnel floor.

View full abstract

A drag-force model is essential in practical predictions of flows around vegetation/urban canopies, since it is too prohibitive to resolve details of the canopy elements and associated fluid motions, especially in geophysical applications. The model, however, has a serious difficulty inherent in its formulation: how one can specify the representative velocity? In ideal situations such as flows past an isolated obstacle, it is the velocity of the inflow and is easily available. In flows past multiple obstacles, on the other hand, a straightforward extension of the idea behind the ideal situations leads to introduce undisturbed flow, which is the flow that would exist at an obstacle location in the absence of that obstacle but with all other obstacles present. Here the undisturbed flow has been evaluated directly in fully resolved computations of a two-dimensional flow past circular cylinders and the fundamental properties of the flow are discussed.

View full abstract

POTEKA (POint TEnki KAnsoku in Japanese) is a compact weather station that can observe multiple meteorological variables such as temperature, pressure, wind direction / speed. A high density ground surface observation network (POTEKA network) with the resolution of approximately 1~2 km has been composed by approximately 150 POTEKAs on the plain of Gunma and Saitama prefectures in Japan. The POTEKA network has observed several gust events such as downbursts and gust fronts. These observation results revealed that the two events of JEF1 downburst on July 14, 2016 and F1 downburst on June 15, 2015 had the similar characteristics while downburst occurrence, for example, the transition time relation between wind speed and temperature / pressure, the cumulonimbus advancing situation, the localized wind change and the horizontal scale of a downburst. The comprehension of these similar characteristics may be able to predict the downburst gust on the plain of Gunma and Saitama prefectures.

View full abstract

Typhoon simulation is one of the methods to obtain recurrence wind speed. The existing wind speed model used in the simulation requires roughness length for each wind direction. However, when the roughness length is estimated from the observed wind speed, there is a problem that the value differs depending on typhoons. In this research, we investigated the characteristics of the recurrence wind speed ratio (RWR) of Friction-Free Wind and the surface wind. And a model for estimating the recurrence wind speed was proposed. As a result, it was shown that RWR does not depend on the return period, and the proposed method can be applied at the point where observed values exist. In addition, it was shown that RWR is affected by height above the ground and terrain.

View full abstract

In this study, we analyzed the wind condition and operating characteristics of the Tottori ranch wind farm based on the operation results and meteorological data. As a result, the annual average wind speed was predicted to be 5.6 m/s in the preliminary wind condition survey, but it was actually 4.5 m/s, and there was a deviation of about 20%. Similarly, the past average was 4650 MWh for the annual target power generation of 5372 MWh. From these results, it is possible that the wind condition survey was not properly conducted. On the other hand, since the shape of the wind speed distribution was significantly different from the shape of the general Weibull distribution, the reliability of the wind condition analysis using the Weibull distribution was doubted, suggesting the importance of long-term wind condition survey. The result was In addition, it was revealed that wind turbulence caused a loss of about one month.

View full abstract

A drag type vertical axis wind turbine “rotation flow wind turbine” developed by Hirosaki University is being investigated as a power source of directly-driven seawater pumping in seagrass seedling production. In order to put the pumping system into practical use, we first selected a commercially used vane pump and measured its characteristics as well as the aerodynamic properties of the wind turbine in the wind tunnel before designing the pumping system. The designed system was installed at the Tairadate fishing port (Ishizaki area) in Sotogahama, Aomori Prefecture, the wind speed, rotation speed, and torque were measured when the wind turbine was pumping water in natural wind. As the results, similar performance was recognized with both the aerodynamic properties in the wind tunnel and the designed parameter of the pumping system. However a small decrease in the pumping amount was observed due to the inflow of air into the pump originated from worm of cover during long-term operation. Therefore, it was considered necessary to take measures to prevent the inflow of air, such as submerging the pump in seawater.

View full abstract

In the previous paper, the authors conducted wind observations by the ferry in the Tsugaru Straits, and it was shown that there are abundant wind resources from the frequency distribution of wind speed that influence the power generation. When offshore wind power generation is planned in the coastal areas of northern Japan in the future, consideration of extreme wind speeds is also important for safety, then long-term data from the Japan Coast Guard of lighthouses installed along the coast are being considered. The data of lighthouse that is open to the public is investigated to estimate the extreme wind speed. In addition, using wind data observed on land wind turbines installed around the lighthouse, the correlation with the strong wind characteristics of the above lighthouse for a short period of time is analyzed, and examined the wind direction, wind speed, and turbulence intensity during strong winds. The detailed characteristics regarding Finally, the method of calculating the design wind speed of a large wind turbine is examined, comparing these correlations and the results of wind condition simulations.

View full abstract

Wind tunnel tests are conducted to investigate aerodynamic characteristics of a parafoil kite used for the Autonomous Kite-Sailing Power Generation. Then a flight simulator of the kite is developed to estimate the energy harvesting performance of the Autonomous Kite-Sailing Power Generation. An example simulation based on the wind tunnel tests revealed the differences of the kite performance depending on the altitudes of flight trajectories.

View full abstract

Typhoon Jebi caused tremendous damages to buildings in Kinki area, Japan. In particular, it has been observed that more than 10,000 houses were damaged in Izumisano City, Osaka Prefecture. Previous study has showed that some houses were damaged by wind borne debris. According to the data in the study, approximately 15% of all surveyed houses were damaged by debris. It indicates that secondary damage because of debris shouldn’t be made light of. Therefore, our study was focused on to identify the features of houses damaged by debris. In addition, the relationships between damaged houses and surrounding environments were investigated. According to our research, the greater number of damaged houses there were in the area to the wind direction of peak gust, the more houses were likely to be damaged by debris. Additionally, it was found that buildings density could affect to the likeness of damage caused by debris. In order to prevent the expansion of typhoon damage, it is important to maintain old buildings that are more likely to be damaged by strong wind or to get construction for wind-resistant reinforcements on those buildings.

View full abstract

On September 9, 2019, Typhoon FAXAI left huge damage in eastern Japan, mainly in Chiba Prefecture. One of them was the collapse of two power transmission towers. In this paper, in order to estimate the wind speed when this damage occurs, the influence of the topography on the flow field around the collapse site of the transmission tower was investigated by numerical fluid analysis. Furthermore, the static structural analysis of the transmission tower was carried out by applying wind load based on the obtained flow field. As a result, at the wind direction angle of 146.25 degrees, local wind speed increased due to the topography, and the No. 79 tower began to collapse at a wind speed of about 40 m / s.

View full abstract

T1915(Faxai) crossed over Tokyo Bay while including the southern Kanto region in the strong wind and made landfall on Chiba City with strong power around 5:00JST on September 9th, 2019. In Boso Peninsula in Chiba Prefecture, the storm caused serious strong wind disaster, such as the destruction of homes, the fallen trees, and the blackouts due to the collapse of transmission and distribution lines. Regarding the damage to the transmission line, two steel towers of a 6-circuit transmission line with 66kV installed in Kimitsu City were destroyed. In this paper, the situation of transmission line damage and the estimation results of the towers strength by the equivalent static wind load using the local wind speed based on the weather and CFD analysis are showed. Causes of the collapse of the towers were that the transmission line with east-west route was located in the strong wind area on the right side of the typhoon track and that the local wind increased due to the effect of special complex topography.

View full abstract

On 23 September 1903, a tornado attacked three elementary schools in the western suburbs of Tokyo. At the Yodobashi Elementary School, a schoolhouse collapsed and six pupils were killed. This paper describes details of the disaster by collecting meteorological data, research documents, and newspaper articles of the time. The tornado occurred about 200km northeast of a typhoon center, as a mesofront developed near Tokyo between warm southeasterly wind and cool inland air. A field survey made by the next day of the disaster revealed that the tornado lasted about 17 minutes as it ran northward for 12.8km. The tornado damage was severest near Shinjuku, where several tens of houses, including a schoolhouse of the Yodobashi School were destroyed. According to newspaper articles, the collapsed schoolhouse had five classrooms with 130-150 pupils, and before attacked by the tornado, teachers and some parents were alarmed by stormy weather and were going to let the pupils take refuge. There were no casualties in other two schools, although a schoolhouse was largely damaged in one of them.

View full abstract

Since the wind direction and speed of natural winds fluctuate, the cross-ventilation air volume also fluctuates, and there is a possibility of excessive indoor airflow and rain, sand, and fallen leaves. Therefore, it is necessary to confirm the actual cross-ventilation volume under the natural wind where the wind direction and wind speed fluctuate. In this research, the optimum window shape and opening pattern are clarified by measuring the cross-ventilation air volume by changing the window opening and closing pattern and the window shape, and analyzing it for each wind direction in the existing apartment building.

View full abstract

This study aims to measure the gust factor for the pedestrian wind environment by the small thermistor anemometer in the wind tunnel experiment. Firstly, the non-directionality and bias of the probe of the small thermistor anemometer were investigated. Then, the measurement performance by the small thermistor anemometer for capturing the peak wind speed at the pedestrian-level around an isolated building was tested. It was found that the thermistor anemometer can accurately capture the turbulence fluctuations up to approximately 5 Hz. In strong wind regions near the side wall of the building, the peak factor and gust factor measured by the thermistor anemometer were 3.0 and 1.5, respectively. The measured factors were in good agreement with that obtained by large-eddy simulation. Moreover, the measured gust factor in this study was found to qualitatively agree with the empirical equation of the gust factor in the full field measurement. In future study, the relation between the peak gust and the 3s moving average period, and the underestimation of the gust factor in low wind region will be further investigated.

View full abstract

Analyses of flow and pollutant dispersion based on computational fluid dynamics (CFD) have attracted considerable attention. Thus, this study developed a turbulent diffusivity limiter using the travel time for predicting point-source pollutant dispersion based on CFD analysis using the k-ε turbulence model and Eulerian concentration transport equations. In this method, the travel time is numerically predicted employing a virtual tracer called the radioactive tracer. Then, the method was applied to predict point-source pollutant dispersion in a two-dimensional uniform turbulent flow. Results showed that the method could reproduce the theoretically predicted increase in the plume width proportional to the travel time during the early stage of dispersion. Moreover, the turbulent diffusivity and plume width at the early and final stages of dispersion could be controlled by adjusting the combination of three model parameters in the turbulent diffusivity limiter.

View full abstract

The large-eddy simulations were carried out to analyze the effects of boundary layer mesh and cell types (hexahedral and polyhedral cell) on the reproduction of pedestrian-level wind environment around a 1:1:2 isolated building. It was found that the application of boundary layer mesh can reduce the mesh non-orthogonality and skewness close to the flat ground, so the numerical accuracy of the flow field can be improved. However, it is also found that at the local region around the sharp corner of the building, it is difficult to well control the mesh non-orthogonality and skewness when applying the boundary layer mesh. As a result, the boundary layer mesh did not show improvement in the sensitive region such as the shear layer around the building. In terms of effects of cell types, the case with hexahedral cells generally shows slightly better accuracy than the polyhedral cases. However, the cell numbers of the polyhedral cases are over twice less than that of the hexahedral case, which indicates that polyhedral mesh is much more economical for the computational resources.

View full abstract

The performance of the vertical-axis wind turbine with straight blades in a shear flow was investigated by the experiment in an open circuit-type wind tunnel, and two and three-dimensional computational fluid dynamics. The diameter (D) and height of wind turbine were 800mm, respectively. The number of straight blades with a NACA0018 airfoil section was three. Two-dimensional shear flow was generated by using the porous flat plate. The effect of the location of shear flow to the rotor on the performances of the vertical-axis wind turbine were found. The power coefficient of VAWT in a shear flow has become more than that in the uniform flow. The optimum position of the porous plate was at Ψ (Yp/D) = 0.125, where Yp is the position of tip of porous plate. Good agreement was found between the experiments and numerical simulations when the large-eddy simulation was employed. The relation between the increment of torque and the three-dimensional flow structure around a blade has been clarified by the computational fluid dynamics.

View full abstract

In this study, we conducted wind tunnel experiments using two types of wind turbine models (MEL blade: rotor diameter 1000 mm and Zephyr blade: rotor diameter 1160 mm) with different wind turbine blade shapes. In particular, we measured the wake velocity deficit distributions in the near and far wake regions behind a wind turbine at the optimum and stall tip speed ratios. Based on the obtained results, the airflow characteristics at each tip speed ratio were investigated. After that, we focused on the behavior of the wind turbine model of the MEL blade operating at the optimum tip speed ratio and conducted a large-eddy simulation using the actuator line model. The numerical results were compared with the wind tunnel experiment results. At the same time, based on these results, we examined in detail the three-dimensional structure of the wind turbine wake. As a result, it was shown that the generation and collapse of tip vortex strongly influences the airflow characteristics in the wind turbine wake region.

View full abstract

Confirmation of wind resistance stability of long-span bridges is very important to a design of them. It is necessary to set design conditions considering the characteristics of the local airflow whenever we design them. This report describes the verification of the inclination angle due to wind speed fluctuation in the Kobe area where the construction of a long-span bridge is planned. We used computational fluid dynamics and Weather Research and Forecasting to confirm the effects of terrain that affect past wind observations. From this result, we set the range of the inclination angle due to wind speed fluctuation used in the design.

View full abstract

This paper presents an investigation of the applicability of Rational Function Approximation (RFA) extraction directly from wind tunnel tests with a forced vibration test. Wind tunnel experiments on a bluff body section model with B/D= 9.25:1 aspect ratio have been carried out. A free vibration test was also conducted as a reference. The results will be compared with the analytical approach of forced vibration tests and RFA extraction. Pre-set amplitude, excitation frequency and data length effects will be discussed in this paper.

View full abstract

In this study, focusing on the phenomenon that the vortex shedding occurs alternately on the left and right and the periodicity is constant, the wind pressure data obtained in the wind pressure experiment is divided into left and right. We propose a method to analyze the wind characteristics and model the fluctuating lift spectrum from each periodicity.

View full abstract

　In this study, LES results for turbulent field around building arrays are compared with wind tunnel experiments. Then, computations for cases with different heterogeneity of roughness are carried out in order to shows the effect of coherent structure in each case on occurrence of peak velocity. As a result, coherent structure with low momentum region appears above building blocks, and its surrounding regions with large velocity cause peak velocity dominantly. In addition, roll-type coherent structure with scale of boundary layer thickness appears in upper heights of turbulent boundary layer, and it is suggested that air parcel with large velocity in upper heights of turbulent boundary layer contributes to occurrence of peak velocity by connecting with the regions with high velocity region above building blocks.

View full abstract

In urban areas, many multi-story parking spaces are constructed, which are attached to airports, large shopping centers, large-scale apartment houses and so on. These buildings usually have large openings at all floors, and these openings are often covered with permeable materials. There are many kinds of combinations of solid and permeable materials used for the four exterior walls of building. At present, there are no specifications of wind force coefficients for designing such walls in the building codes and standards, even in the AIJ Recommendations for Loads on Buildings (2015). The present paper investigates the peak wind force coefficients for designing the permeable cladding used for a low-rise multi-story parking space with a square plan of 50 m by 50 m and a height of 16.38 m, based on the results of a wind tunnel experiment, in which 15 cases of exterior wall conditions (combinations of solid and permeable walls) are tested. Finally, the peak wind force coefficients for designing the solid and permeable cladding are proposed.

View full abstract

“Bamboo Green-House Project ” have been conducted in various areas in Japan. In this project local people construct greenhouses by themselves, which are made with bamboo. These greenhouses have unique cross section, i.e., horseshoe-shaped cross section. A recent typhoon event caused that one of these greenhouses collapsed and turned out its wind-vulnerability. This event necessitates a valid wind-resistant design procedure for the greenhouse. This paper firstly examines wind pressure distribution by a wind tunnel experiment. Subsequently, the wind pressure distributions at which largest load effects are observed are determined by structural analysis. Then, wind force coefficients for design are determined. The comparison of the obtained design wind force coefficients with those for ordinary shaped greenhouses shows discrepancy, which is associated with the difference in separation point.

View full abstract

Mechanically-attached waterproofing system is widely used for flat roofs as an environment-friendly system. However, this system is often subjected to wind damage because of its low wind resistance. Then, we have proposed to install photovoltaic (PV) panels parallel to the roof with small gaps between them, which may decrease the wind loads on the waterproofing system as well as on the PV panels. The present paper verifies the validity of this idea. The wind pressures underneath the PV panels are numerically simulated by using the unsteady Bernoulli equation together with the time history of external pressures acting on the roof of a flat-roofed building. Then, the wind-induced response of the waterproofing system is evaluated by using a non-linear finite element method together with the time history of wind pressures underneath the PV panels.

View full abstract

The wind force acting on a full-scale rectangular net-house of which net has the solidity ratio of 38% was measured during a typhoon. The average wind force coefficient on the wall was 0.6 on the windward side and -0.2 on the leeward side, which were close to the results of the wind tunnel experiment. The average wind force coefficient of the roof was 0.02 in a fine weather condition, however, it remarkably decreased to -0.3 in a heavy rain condition. It is considered that the clogging of the net due to raindrops formed a solid sheet of water affected the wind force coefficients. The other measuring method of wind force by using pressure sensor confirmed the fact that heavy rainfall can effectively affect to the wind force coefficients of a net-house. The windbreak effect of a net was also measured during strong wind.

View full abstract

The possibility of current equivalent static design method to deal with wind fatigue on metal roofing is examined by calculating fatigue damage, D, on the metal roofing fastened with skews. For the calculation of D, wind speed and direction time histories at ground level obtained from probabilistic typhoon model, roof wind pressure coefficients obtained from wind tunnel tests, and S-N curve of load amplitude, S vs Number of cycles to failure, N obtained from repeated loading test on the target metal roofing were employed. From the current analysis which utilized 5016 simulated typhoon information, wind fatigue failure on the target fastener on the metal roof is unlikely to occur if the safety factor is properly considered in the current equivalent static design method.

View full abstract

Computational fluid dynamics (CFD) has been expected to contribute to investigating the flutter characteristics of long-span bridges instead of wind tunnel tests. This paper deals with applicability of CFD for predicting steady and unsteady aerodynamic forces of long-span bridge decks. The steady and unsteady aerodynamic force coefficients for a rectangular cross section with a side ratio of 1:33.3, four twin-box decks with different opening lengths and two long-span suspension bridge decks were calculated by a numerical simulation based on two-dimensional RANS. The calculated values were compared with wind tunnel test results to evaluate the accuracy of the computation. It was found that they were in good agreement. Then multi-modal flutter analysis was carried out using the calculated values, and the results were compared with ones using the measured values. As a result, the flutter speeds calculated by the CFD data agreed well with those by the experimental data.

View full abstract

The effect of aerodynamic damping on the damping ratio of a four-bundled conductor was examined by comparing the observed damping ratio and theoretical formula of aerodynamic damping. The observed damping ratios of various vibration modes were obtained using the random decrement technique from a tension data conductor observed at various wind speeds. The theoretical formulas of aerodynamic damping in the horizontal, vertical, and torsional directions were derived from a single-degree-of-freedom motion equation and the definition of the quasi-steady aerodynamic force of each direction. The observed damping ratio increased proportionally to the wind speed, as is the case with the theoretical formula. However, the observed damping ratio tended to exceed the theoretical formula. The theoretical formula, in which the constant margin is added, shows good agreement with the observed damping value. The result suggests that total damping comprises constant structural damping and aerodynamic damping, which is proportional to wind speed. It is possible that the structural damping can be identified from the approximation formula of the observed damping ratio.

View full abstract

Dry cable galloping could be observed in long circular cylindrical members such as stay cables. The authors previously conducted a series of wind tunnel tests where wind-induced response at high wind speed range was observed with an inclined circular cylinder model. In this study, more accurate measurements of the pressure distribution during the response were tried to be obtained. As a result, possible pattern of pressure distribution that cause the response was shown. However measured results sometimes differed from trial to trial, and further study is necessary to have clearer understanding.

View full abstract

Metal roofing materials are often damaged during a strong wind event such as a typhoon or a tornado. In order to investigate the failure mechanism and the wind resistance of metal roofing materials, we have conducted a full-scale experiment using Pressure Loading Actuators (PLAs). In the experiment, two types of loading, i.e., dynamic loading simulated by using a wind tunnel test data and ramp loading, are applied to the specimens with metal roofing system assembled with the same way as that used for practical roofs of wooden houses. The experimental results indicate that the failure load of the roof under ramp loading is smaller than that under dynamic loading.

View full abstract

Falling properties of cinder models, non-simple shaped objects, were measured and three-dimensional falling trajectories were obtained by using video movies. Aerodynamic characteristics of cinder models were examined by the trajectory. The falling models imitated the configuration of cinders collected at the site of the experiment, Sakurajima volcano. Two types, one with sharp edges and one without sharp edges, were selected as representatives and a sphere was selected as reference model. The results showed that the drag coefficient varied with the falling velocity becoming smaller as the velocity increased. When the models fell with terminal velocity, the drag coefficients of the cinder models without and with sharp edges were about 0.5 and 0.6, respectively. The drag coefficient of the sphere model was about 0.3.

View full abstract

This paper introduces a newly developed full-scale wind pressure measurement system and presents its performance. The novelty of the system lies in the fact that the system uses absolute pressure sensors, instead of differential pressure sensors. Due to this, users do not need to physically connect one side of each sensor to quite air in order to obtain reference pressure. Consequently, users can easily install the system into target buildings, just prior to the impact of typhoons. The paper first introduces the specification and constituent of the system. Then, it presents the results of testing regarding performances on sensor, electrical circuit and system. Throughout these testing, the developed system demonstrates that pressure can be measured at least at 20 Hz and at a precision of less than 10 Pa, which is sufficient for full-scale pressure measurement under 15 m/s or higher wind speed. Simultaneous measurement of pressures acting on two adjacent locations on a cube model in a wind tunnel with the developed system and a differential pressure measurement system shows a good agreement.

View full abstract