NUMERICAL ESTIMATION OF WIND AND WAVE FIELDS FOR LAKE
ENVIRONMENT USING MESO-SCALE ATMOSPHERIC MODEL WRF (in Japanese)

Wind and wave fields are computed for the Lake Kasumigaura using numerical atmospheric and wave
models. The meso-scale atmospheric model WRF is employed for predicting the wind field over the
surface, while the temporal and spatial wave fields are computed by the third-generation wave model
SWAN using the output of WRF. The computed results are compared with the observed wind and wave
data. The compared periods are chosen during the passing of the typhoons 200704 and 200709. WRF
reproduces the observed wind speed and direction quite well at two observation points inside the lake
except the peak of typhoon wind. Consequently, the predicted significant wave height and period become
smaller than the observed data near the wind peak. The wind field forecasting is also performed using the
forecasting meteorological data (GFS). The results show the availability of the present model for
forecasting the wind fields over the lake.

 

DEVELOPMENT AND VERIFICATION OF OBJECT-ORIENTED
ENVIRONMENTAL HYDRODYNAMIC MODEL (in Japanese)

An object-oriented numerical model has been developed for analyzing environmental fluid flows. By
introducing object-oriented programming (OOP) style, the code can be easily reusable and extendable for
complex numerical algorithms and conditions. In this paper, first, the concept and advantages of OOP are
explained. Second, the accuracy of the present model is tested using analytical solutions and the field data
measured in the blackish Lake Abashiri. The results obtained by the present model show good agreement
with the analytical solutions. The computational time is greatly reduced without large code modification
by utilizing domain objects and OpenMP. In the comparison with the field data, the model can reproduce
the temporal variation of the vertical distribution of salinity reasonably. We can conclude that the present
model would be a useful and efficient tool for analyzing the environmental fluid flows.

Generalizations of the Wedderburn number: Parameterizing upwelling in stratified lakes

The effect of lake geometry on wind-induced upwelling, in a two-layer stratified lake with a variable bottom
slope and generic planar shape is investigated. (1) The traditional linearized classification parameter for
upwelling, the Wedderburn number, is extended to include finite amplitude effects in a rectangular basin; this
extension is important when the Wedderburn number is < 3. (2) The Wedderburn number is generalized to
incorporate the influence of a bottom slope and a variable-width basin for both the linear (large Wedderburn
number) and nonlinear (small Wedderburn number) assumptions. Laboratory and numerical experiments of a
two-layer stratification were performed and used to validate these extensions. (3) Both extensions are combined to
derive a methodology to estimate the interface displacement in real lakes where both geometry effects may act
together, and this methodology is used to discuss the response of the thermocline to the onset of wind in Lake
Como and Mundaring Weir reservoir. In most cases, symmetry of the basin geometry along the fetch is the key
characteristic that defines tilting of the interface for real lakes. Asymmetries in terms of bottom slopes induce
shortening or stretching of the effective internal fetch, while a larger excursion of the interface results in narrower
upwind areas of the lake due to horizontal redistribution of displaced water volumes. This paper gives the correct
definition of the length scales to define the Wedderburn number for any stratified basin.

AN OBJECT-ORIENTED APPROACH TO ENVIRONMENTAL FLUID MODELING (ISTP21)

An object-oriented hydrodynamic model has been designed to analyze fluid flows in natural water bodies. The model is intended to be used for simulating transport phenomena under a stably strati-fied condition, which is often observed in lakes and coastal waters. In order to facilitate the adaptation of a hydrodynamic model to a wide range of applications, the basic components of the model, such as computational domains, numerical schemes, and independent variables are defined as objects that can be easily created and exchanged during a computation. This design facilitates the flexible and efficient use of different numerical algorithms. We generalize a domain decomposition technique to facilitate arbitrary domain connections such as nesting and block-structured grid techniques. This technique allows proper discretization of complex water bodies and efficient parallelization. Computational domains are discretized based on a generalized curvilinear boundary-fitted coordinate in the horizontal plane and a z-coordinate in the vertical direction with both staggered and collocated variable arrangements. In order to demonstrate the ability of the present model, it is first applied to internal wave generation and breaking phenomena in an experimental tank. The model predicts the temporal evolution of measured internal wave fields quite well. A numerical experiment reveals the detailed breaking process of the internal wave on a slope with the help of a nesting technique. Lastly, the model is applied to a field-scale phenomenon, i.e., the wind-induced response of the halocline in the blackish lake, Lake Abashiri. The results obtained using the present model show reasonable agreement with the data measured in the field.

Upwelling and the internal wave field in a threelayer stratified lake (ISSF)

Wind-induced motions of a three-layer strati ed lake were investigated using a laboratory wind fume and a nonhydrostatic hydrodynamic model. We have examined the unsteadiness of the upwelling and successive motions of the density interfaces in a three-layer stratied water body. Laboratory experiments showed that upwelling begins with vertical mode 1 followed by vertical mode 2, accompanied by the accumulation of intermediate water on the upwind side. The numerical model successfully reproduced the measured upwelling processes, and it was used to explain the flow dynamics in the fume. The generation of vertical mode 2 high-frequency internal solitary waves was observed on the interfaces after cessation of winds for both measured and computed results. The present results show that vertical mode 2 motions including high-frequency waves may play an important role in the redistribution of chemical and biological substances in stratied lakes.

A SIMPLE WETTING AND DRYING SCHEME FOR ENVIRONMENTAL FLUID FLOW (JSCE)

The study proposes a simple wetting and drying scheme for analyzing environmental fluid flows using a fixed grid on z-coordinate. The wetting and drying process is especially important in the low-lying coastal areas with large amplitude tides and reservoirs with severe flooding and discharging because it drastically changes the horizontal extent of the water body. The basic idea of the present scheme is a partial-cell (cut-cell) representation of the structured grid based on a finite-volume method to reconstruct the computational domain in response to a surface evolution. The scheme was implemented to an object-oriented numerical model, Fantom3D, and tested against analytical solutions and applied to field scale phenomena. The computed results agree reasonably well with the analytical solutions. We showed that the proposed scheme is a useful technique to compute the flow in partially dry domain for the hydrodynamic models using z-coordinate.

NUMERICAL STUDY ON SALINE INTRUSION IN ABASHIRI RIVER ESTUARY (in Japanese)

We numerically studied saline intrusion and the related river flows in the Abashiri River Estuary that
connecting the Ohotsuku Sea and the Lake Abashiri in northeast Hokkaido. A three-dimensional objectoriented hydrodynamic model, Fantom3D, was used to investigate the stratified flow dynamics in this estuary. In the computation, the estuary was decomposed into 15 domain objects having different sizes to properly express the narrow and highly bending river channel. The one-way nesting technique was also used to resolve depth-scale eddies in the channel. The computed results were compared to observed data obtained in Sept 2006. The water level, velocity and salinity measured at four locations along the estuary were in reasonable agreement with the computed results. The secondary flow at a bending section and the variation of mixing type along the river were also discussed using the computed results. We showed that these computed results are qualitatively agreed well with the previous investigations.

HYDRODYNAMIC SIMULATOR BASED ON BIOLOGIOCAL-CELL STRUCTURE CONCEPT (in Japanese)

We have designed a hydrodynamic simulator with an idea inspired from biological-cell structure. The simulator updates status of cell and membrane objects by nerve-like objects, rather than updates values stored in index-based numeric matrices. The water body may be composed of a single cell (unicellular), or cells (multicellular) interconnected through membranes. The membrane acts as a flux-controller between cells on both sides, i.e., computes advection and diffusion fluxes. Dependent variables and source terms are all objects and embedded in a cell like organelles of a biological cell. Each model object is implemented in C++ to obtain reasonable computational speed, and used from a script language Python to increase user flexibility and extensibility. We explained the advantages of the present design against to the previous ones, and confirmed the accuracy of the present model using analytical solutions.

AN UNSTRUCTURED-CARTESIAN HYDRODYNAMIC SIMULATOR WITH LOCAL MESH REFINEMENT TECHNIQUE (in Japanese)

An unstructured-Cartesian hydrodynamic simulator is developed to realize a straight-forward treatment for local mesh refinement. The present simulator discretizes a bathymetry into a low-resolution 2D structured array of container. Then, to realize horizontal local mesh refinement, each container belongs to water area is independently packed with columns of computational cells in a structured manner. Each cell in the columns is connected vertically and horizontally with available neighbor cells in an unstructured manner. All these structures are fully implemented with object-oriented design and hydrodynamic processes are described as the interactions between these objects. In this paper, we first explain the basic idea of the simulator, hydrodynamic objects and their interactions. Second, numerical implementations and techniques are briefly introduced. Lastly, the accuracy and efficiency of the present simulator are confirmed for wind-induced response of a circular basin, wet-dry seiche motion and non-hydrostatic stratified flow field.

A FINITE-VOLUME HYDRODYNAMIC SIMULATOR BASED ON CENTROIDAL VORONOI TESSELLATION (in Japanese)

A finite-volume hydrodynamic simulator based on centroidal Voronoi tessellation (CVT) is proposed to realize smoothly varying local mesh refinement. The fundamental properties and generation techniques of CVT, weighted CVT and boundary-fitted CVT are explained with a suitable scalar advection scheme. Hy- brid parallelization of the shared and distributed memory models has been implemented to achieve efficient long-term high resolution predictions. The simulator has been applied to the brackish lake Abashiri and the salinity fluctuations in the halocline were compared with the measured data. The measured salinity fluctuations due to the internal Kelvin wave are well simulated both in phase and magnitude.