Department of Civil Engineering

Abstract Local flow properties and regional weather or climate are strongly affected by land-atmosphere interactions of momentum and scalars within the daytime convective boundary layer (CBL). In this study, we investigate the impact of green space scale on the daytime atmospheric boundary layer (ABL) over a synthetic urban domain using a recently developed large-eddy simulation-land surface model (LES–LSM) framework.

ABSTRACT In this paper, we developed a two-way coupled moving solid model to simulate a floating box oscillating in a water tank. The numerical model is developed based on the FVM (finite volume method) to solve the LES (Large-Eddy Simulation) model. The free-surface is described by the VOF (Volume of Fluid) method with the PLIC (Piecewise Linear Interface Calculation) scheme. The simulation is executed on a fixed mesh. The velocity at the solid boundary is specified as the boundary face velocity of the fluid cells.

Wind characteristics and wind turbine characteristics in Taiwan have been thoughtfully analyzed based on a long-term measured data source (1961–1999) of hourly mean wind speed at 25 meteorological stations across Taiwan. A two-stage procedure for estimating wind resource is proposed. The yearly wind speed distribution and wind power density for the entire Taiwan is firstly evaluated to provide annually spatial mean information of wind energy potential.

Abstract. Sensible heat flux measurements were carried out to examine the characteristics related to frequency and duration of extreme flux events above a desert surface. First, the flux bursts were identified by a modified hyperbolic hole" quadrant analysis" in which the threshold criterion depends on the mean and standard deviation of the flux time statistics. Second, the frequency distribution of the extreme flux burst length was examined.

Orthonormal wavelet expansions are applied to atmospheric surface layer velocity and temperature measurements above a uniform bare soil surface that exhibit a long inertial subrange energy spectrum. In order to investigate intermittency effects on Kolmogorov's theory, a direct relation between the nth‐order structure function and the wavelet coefficients is derived. This relation is used to examine deviations from the classical Kolmogorov theory for velocity and temperature in the inertial subrange.

The conditional sampling formulation typically used in eddy accumulation flux measurements was tested at two sites using velocity, temperature, and specific humidity time series measurements. The first site was at Owen's Lake in southern California, and the second site was an irrigated bare soil field at the Campbell Tract facility in Davis, California. The constant β relating the turbulent flux to the accumulated concentration difference between updrafts and downdrafts was related to the statistics of the vertical velocity.

Abstract Transient responses of sap flow to step changes in wind speed were experimentally investigated in a wind tunnel. A Granier-type sap flow sensor was calibrated and tested in a cylindrical tube for analysis of its transient time response. Then the sensor was used to measure the transient response of a well-watered Pachira macrocarpa plant to wind speed variations. The transient response of sap flow was described using the resistance–capacitance model.

This study proposes a two-dimensional Lagrangian stochastic dispersion model forestimating spatial and temporal variation of scalar sources, sinks, and fluxes withina forest canopy. Carbon dioxide and heat dispersion experiments were conducted forfield testing the model. These experiments also provided data for field testing a newlydeveloped one-dimensional Lagrangian analytical dispersion model. It was found that these two models produce similar scalar source-sinkand flux distribution patterns.

Abstract. The structure of atmospheric surface layer turbulence at low wavenumbers was analyzed using 56 Hz triaxial velocity and temperature measurements above a uniform dry lake bed. A key feature of this experiment was the small roughness length of the surface that resulted in a small roughness Reynolds number.

Abstract. Similarity models in the inner region of the unstable atmospheric boundary layer (ABL) are generally based on four dimensional parameters: buoyancy, friction velocity, surface heat flux, and the height above the land surface. In the free convection limit the friction velocity can be neglected, thus reducing the measurement needs in practical applications.

Abstract. The probability density functions (pdf's) for the longitudinal and vertical velocities, temperature, their derivatives, and momentum and sensible heat ffuxes were measured in the atmospheric surface layer for a wide range of atmospheric stability conditions. The measured pdf's for both the velocity and the temperature ffuctuations are near-Gaussian and consistent with corresponding laboratory measurements for nearneutral and stable stability conditions.

Abstract Reaeration in combined wind/stream driven flows was studied experimentally in a laboratory wind-water tunnel. Wind velocities, stream flow properties and oxygen transfer rates were measured under a variety of co-and counter-current flows. For pure open channel flows and pure wind-driven flows, the results of oxygen transfer experiments show good agreement with other laboratory studies. For wind/stream combined flows, the transfer velocity depends on the stream flow condition and wind speed.

Longitudinal velocity and temperature measurements above a uniform dry lakebed were used to investigate sources of eddy-motion anisotropy within the inertial subrange. Rather than simply test the adequacy of locally isotropic relations, we investigated directly the sources of anisotropy. These sources, in a daytime desert-like climate, include:(1) direct interaction between the large-scale and small-scale eddy motion, and (2) thermal effects on the small-scale eddy motion.

ABSTRACT In this paper, we present a two-way coupled moving solid algorithm with discrete element method (DEM) to simulate dynamic fluid-solid interaction. This model allows obstacles moving in the multi-phase fluids to generate realistic motion for both the fluids and obstacles. We discretize the Navier-Stokes equation using finite volume method (FVM) for the fluid part. The volume-of-fluid (VOF) method is used to track the fluid-fluid and fluid-solid interfaces.

ABSTRACT: In this research, a mathematical model is proposed for predicting wind-induced response of a building with a tuned mass damper (TMD) when soil-structure interaction (SSI) is considered. It may reasonably reflect soil characteristics. Furthermore, TMDs are effective devices for reducing structural vibration. This study may help researchers to more accurately evaluate wind-induced oscillation of high-rise buildings with SSI as well as select appropriate TMDs in controlling structural response.

This study used wind tunnel experiments to determine the loss factors of building openings for wind-driven ventilation. It is found that the loss factor is a function of internal porosity, but independent of Reynolds number. The loss factor of a partially open door increased as the door angle decreased. In addition, the influence of thickness ratio on the loss factor is much smaller than the internal porosity. The present study also derived the relationship between the loss factor and the discharge coefficient used in the orifice equation.

When the wind direction is parallel to the opening façade, the wind shear near the building opening generates turbulence and entrains air across the opening. This kind of shear-induced ventilation cannot be predicted by the orifice equation because the time-averaged pressure difference across the opening is close to zero. This study uses wind tunnel experiments and the tracer gas decay method to investigate the ventilation rate of shear-induced ventilation.