SUMMARYThis paper presents a simple and practical scheme for capturing moving interfaces or free boundaries in multi-uid simulations. The scheme, which is called THINC (tangent of hyperbola for interface capturing), makes use of the hyperbolic tangent function to compute the numerical ux for the uid fraction function, and gives a conservative, oscillation-less and smearing-less solution to the uid fraction function even for the extremely distorted interfaces of arbitrary complexity. The numerical results from the THINC scheme possess adequate quality for practical applications, which make the extra geometric reconstruction, such as those in most of the volume of uid (VOF) methods unnecessary. Thus the scheme is quite simple. The numerical tests show that the THINC scheme has competitive accuracy compared to most exiting methods.

ABSTRACT:Recently, countermeasures against the urban heat island effect have become increasingly important in Tokyo. Such countermeasures include reduction of anthropogenic heat release and enhancement of urban ventilation. Evaluations of urban ventilation require construction of a high-resolution computational fluid dynamics (CFD) model, which takes into account complex urban morphology. The morphological complexity arises from multi-scale geometry consisting of buildings, forests, and rivers, which is superimposed on varying topography. Given this background, airflow and temperature fields over the 23 wards of Tokyo were simulated with a CFD technique using a total of approximately 5 billion computational grid cells with a horizontal grid spacing of 5 m. The root mean square (RMS) error of the air temperature between the simulation and observation results at 127 points was 1.1°C. Using the developed model, an urban redevelopment plan for two districts in metropolitan Tokyo was examined from the viewpoint of air temperature mitigation. Numerical results showed that a reduction by 1 ha in the area covered by buildings increases the area with temperatures below 30°C by 12 ha.

Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified assumptions made by Macdonald (Boundary-Layer Meteorol 97:25-45, 2000) to develop a model for the profile of the mean wind velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following plan area densities: λ p = 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind velocity and Macdonald's five-point average of wind velocity can be large in both types of arrays for large λ p . It is also confirmed that Prandtl's mixing length varies significantly with height within the canopy, contrary to Macdonald's assumption for both types of arrays and for both small and large λ p . On the other hand, in accordance with Macdonald's assumption, the sectional drag coefficient is found to be almost constant with height except in the case of staggered arrays with high λ p .

This paper describes the overspeed control of a vertical-axis wind turbine (VAWT) without an electric control system or mechanical brakes. To improve its performance with straight blades, a VAWT with a variable-pitch mechanism consisting of a four-bar linkage was designed. This wind turbine exhibits directivity to the wind, which is able to make use of rotational speed control in a VAWT. This wind turbine can achieve passive yaw control by means of tail vanes, which are installed on the eccentric link lever. The yaw angle characteristics of the tail vanes were measured in an open-circuit wind tunnel and in field tests. The yaw angle variations from the numerical analysis are in good agreement with the experimental data from the wind tunnel tests. It was found that overspeed prevention by means of tail vanes is useful in protecting a VAWT from strong winds without using mechanical brakes.