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京都大学0048新制・論文博士理学博士乙第3142号論理博第543号新制||理||236(附属図書館)UT51-52-C133(主査)教授 國司 秀明, 教授 奥田 節夫, 教授 山元 龍三郎学位規則第5条第2項該当Kyoto UniversityDFA

STUDY OF SEICHE IN LAKE BIWA-KO [1] : ON THE NUMERICAL CALCULATION BY DEFANT'S METHOD

One-dimensional numerical calculation by Defant's method is performed in order to ascertain the physical mechanism of the longitudinal seiche motions in Lake Biwa-ko. The possible period of seiche of the fundamental mode is found to be 212.0 minutes, and the periods of its higher harmonics are 71.3, 61.0, 39.0, 31.7 minutes and so on. The profiles of the vertical displacements of water surface and the mean horizontal velocity of these seiche motions are shown. It is of interest that the first harmonics and the second have fairly close periods. That these two components really exist is verified from an analysis of the phase shift seen in the observed oscillation with a period of 60 to 70 minutes. These two oscillations of periods of 71.3 and 61.0 minutes as computed are respectively the bi- and tri-nodal seiche in the whole basin

STUDY OF SEICHE IN LAKE BIWA-KO (II) : ON A NUMERICAL CALCULATION BY NONLINEAR TWO-DIMENSIONAL MODEL

A nonlinear two-dimensional numerical experiment has been carried out by using a one-layer model to study a seiche in Lake Biwa-ko. The periods of the longitudinal seiches from this experiment are 255.5, 79.8 and 69.1 minutes, and the oscillations with these periods are the uni-, bi- and tri-nodal seiche in Lake Biwa-ko, respectively. These periods from the two-dimensional experiment are larger than those from the Defant's method in the former paper, but the longitudinal distributions of the amplitude and velocity from both experiments agree well with each other. An amplitude of the bi-nodal seiche is very small in the north basin where the tri-nodal seiche dominates, but in the south basin both seiches have nearly the same value of amplitude regardless of wind direction. Nonlinear effects to the seiche motion are negligible in the deep areas in the north basin, but are not negligible in the shallow areas. The experimental results agree well with the observation, and the comparisons between them seem to show that the spatial distributions of the wind over the lake have an important effect to the seiche

A NOTE ON THE FRICTION COEFFICIENT OBTAINED BY THE WIND PROFILE METHOD FROM OUR WIND FLUME EXPERIMENTS

The effect of the zero-plane displacement of the wind profile over the wind waves on the friction coefficient is presented here by using data from our wind flume experiments. The zero-plane where the wind profile is closest to the logarithmic profile is found at a somewhat higher place than the mean wave crest and at a lower place than the mean wave trough. This fact seems to show that the wind profile over the wind waves is a little different from the logarithmic profile

STUDY OF SEICHE IN LAKE BIWA-KO (III) : SOME RESULTS OF A NUMERICAL EXPERIMENTS BY NONLINEAR TWO-DIMENSIONAL MODEL

The possible periods of the seiche in Lake Biwa-ko are given as 255.5, 79.8, 69.1, 51.1, 38.7, 31.9 minutes and so on from the numerical experiments by nonlinear two-dimensional model described in a former paper of the same title (Imasato [1971]). In this paper, some characteristics of the seiches with the period 51.1, 38.7 and 31.9 minutes are discussed. It is found that the seiche with the period 31.9 minutes consists of the four different component oscillations. These periods from the numerical experiments agree well with the observed periods. The amplitude variations of each seiche with the wind direction are discussed, too. From a numerical experiment by using the model with a distribution of wind direction, it is found that the st"iche with the period 255.5 minutes is controled by the wind field over the south basin, the seiches with the period 69.1, 51.1 and 31.9 minutes by the wind field over the north basin, and the seiches with the period 79.8 and 38.7 minutes by the wind field over both basins

A NOTE ON THE ENERGY TRANSFER FROM WIND TO WAVES

The amount of the energy transfered from wind to waves is estimated from our previous experimental data. This was carried out by assuming that the wind waves dissipate the energy only due to molecular viscosity, and by estimating a/ax (C•E/2) from the distributions of mean wave height in the wind tunnel. It is considered from this estimation that the energy transfer from wind to waves is about 15% of that at the critical layer in the wind field C•t, where t is the total stress of wind, and C the phase velocity of waves. The "wave drag coefficient" rw^2 which relates with the "effective stress" for wave growth is also shown as a function of wind speed

A NOTE ON THE WATER EXCHANGE BETWEEN THE SOUTH AND THE NORTH BASINS OF LAKE BIWA

An estimation on the water exchange between the south and the north basins of Lake Biwa, due to the water movement induced by the wind, was given by a numerical calculation. The model is two dimensional and a wind of 6 m/sec was simulated for 10 hours. Drift currents caused by the wind seem to be the most effective for the water exchange. During 20 hours, 2.93% of the total volume of water of the south basin is transported to the north basin, while 7.31% of the total volume of water of the south basin is transported from the north to the south basin. The mixing area of the waters of these two basins is spread to a width of 6 km

STUDY OF SEICHE IN LAKE BIWA-KO (IV) : OBSERVATION WITH A NEW PORTABLE LONG PERIOD WATER LEVEL GAUGE

A new portable long period water level gauge was designed in order to verify the results of numerical experiments with a computer about the seiche in Lake Biwa-ko, especially about the longitudinal oscillations. Using two water level gauges of this type, an observation was performed for a month in Otsu and Hikone at the same time. The observed periods of seiche agree well with the periods derived from the numerical experiments. The seiche with the period of about four hours is found for the first time in the north basin of Lake Biwa-ko from the frequency analysis of the records in Hikone. The observed amplitude ratio and the observed phase difference between those two observation sites agree with the experimental results about seiche of mode 1, mode 2 and mode 3. But the observed result of seiche of mode 4 disagrees with the experimental result