New techniques have been developed to generate the turbidity currents in the laboratory with high accuracy and investigate the behaviors of turbidity currents, the speed of their front, the flow structure of their head, and the effects of the angle of the slope and the range of size of the sand particles included in the currents. There is a considerable scatter in data often observed in this sort of experiments of turbidity currents. Some difficulties are found in keeping the same conditions during the experiment by controlling the density of turbid water and triggering to generate the currents artificially. We use an air bubble generator for distributing particles in water uniformly without causing the initial disturbance to the current and a plate for falling sand particles in the limited ranges of size into water tank at the same spot where sand particles fall. In this way, we establish the initial process to bring about the turbidity current to keep the same condition during the experiment so as to achieve the high repeatability. The results indicate that the front speed depends on the range of particle size. Even if the initial density of the turbid water is the same, the current including different range of particle size shows the variation in speed. This is because coarser particles are deposited faster than finer particles, which reduce the density of turbidity current, hence the front speed as well. The front speed also depends on the angle of slope, the effect of which are stronger at smaller angles. Moreover, we obtain quantitative data of the distribution of sand particles by a new method of estimating the concentration of sand particles based on the digital image processing. The method uses the luminance of the images which is able to directly correlated with the concentration of the sand particles suspended in the currents. The results show that the concentration of coarser particle in the head decreases considerably even in the early stage of constant speed while the strong diffusion of finer particles is dominant behind the head accompanied by the entrainment

The structure of the head of gravity currents spreading out in all directions is investigated in the experiment and in the computation by extending the method developed in the previous paper for the two-dimensional gravity currents. A finite volume of fluid contained in a rectangular region was released instantaneously in another fluid of slightly different density in a larger region. Box models of fluid dispersion were described to elucidate the mechanisms of energy conversion in different stages of development of spreading gravity currents.Visualization experiments were made with dye to trace the head of the spreading gravity currents. By opening the lock gate equipped obliquely in the corner of a water tank of square section, a finite volume of salt water of slightly different density in the corner of the tank was released instantaneously so as to form the gravity currents spreading over the whole area. The computation was also made in the same conditions as the experiments to study the flow structure of the head of spreading currents. The incompressible Navier-Stokes equation for an inhomogeneous fluid together with the transport equation for solute was solved by the finite volume method.Both the experiment and the computation produce the formation and development of the gravity currents spreading in an axisymmetric manner. The results indicate that there exist different stages of development of this kind of spreading gravity currents. After the released fluid spreads at a constant speed, it slows down in the self-similar stage, and then it decelerates further in the viscous stage. It is found that the structure of the head changes according to the stages of development, which affects the mixing of fluid by spreading gravity current and that the head extending in a circle or a circular arc is subject to the three-dimensional instability to form the mountain-valley structure.

In this study, we visualize the transition of state and the sand distribution of the transported sediment in an oscillatory flow. The visualized images of sediment transport in an oscillatory flow in a small tank are taken with a digital video camera. The variation of brightness from the initial image is measured by the personal computer on every pixel in the flame. It is transformed to the concentration of sand by using the calibration relationship between the brightness and the distribution of sand. The result shows that distribution of transported sand is dependent on the velocity or flow like eddy of external fluid.

The sand transport that causes the movement and deformation of a sand hill in a periodically oscillating flow is investigated in the laboratory. An experimental technique is developed to assess the variation of the state of concentration of sand particles in the process of the interaction of the particles with the fluid in motion around the sand hill as well as with the particles themselves. The method of measuring the concentration of sand particles is based on the digital image processing. It is confirmed that under the well-controlled experimental conditions of homogeneous distribution of sand particles, the decrease in brightness of the digital images of an area is determined by the concentration of the particles in the area. Taking a space average on the images with an appropriate scale to the size of sand particles to fade out the unfavorable effect of each particle makes it possible to predict inhomogeneous and continuous distribution of the concentration of sand particles from the brightness of the digital images with the limited resolution.We make a visualization experiment of the two-dimensional flow field around a sand hill on sand bed in a flow sinusoidally oscillating in a straight channel with square section. The digital images of the unsteady flow field with suspended sand particles around the sand hill is analysed to estimate the concentration of sand particles. The results show that in the periodically oscillating flow the sand particles repeat a basic motion every half period such as sand particles running up the slope of the hill by accelerating flow, running off the hill by flow separation at the top of the hill, running down the hill as suspended in a turbidity current, and running into the roll by the rotational motion of shed vortices. It is found that these periodic motions of sand particles have particular distinguishing features of the change in the state of concentration in the process of fluidization, movement, transport, sedimentation, and accumulation.

The behaviour of the two-dimensional gravity current in an oscillatory flow is investigated by computation when homogeneous fluid and another fluid of slightly different density were released in a sinusoidally oscillating flow. The incompressible Navier-Stokes equation for an inhomogeneous fluid and the transport equation for solute were solved by the finite volume method developed in the previous paper and extended by incorporating the oscillatory ambient flow with the implementation of the unsteady inflow and outflow boundary conditions.The results indicate that since the tail wind pushes the head forward further than the head wind brings it backward, the mean speed of the current front increases in oscillatory ambient flows. The change of the ambient flow direction makes the front distorted remarkably and enforces the instability of the density interface at some KC numbers. This instability causes the roll up of the interface and the vortex shedding from the head, both of which increase the entrainment rate in low Froude numbers. It is found that the unsteady boundary layer along the wall affects the inner structure of the current head according to oscillatory flows.

A various kind of studies regarding ship noise are being carried out today. Nippon Kaiji Kyokai started studies more than fifteen years ago, and published the results in the form of guidance. Moreover, to deal with rationalization of ship noise, the authors have developed a computer program (hereinafter "KNOISE") to estimation noise levels in the accommodation spaces and engine room of a ship. KNOISE has an excellent user interface offering ease of operation, reduced data input through the use of standard tables, and an interactive data input system. Accordingly, in order to examine the validly of KNOISE, the authors have carried out to make a comparison between measuring data and results of calculation. This paper briefly outlines KNOISE and its development until now. The efficacy of "KNOISE" is inspected through a detail analysis of the measured data and numerical investigations.

The three-dimensional instability of the head of gravity currents is investigated in the experiment and in the computation of lock-exchange flow. Visualization experiments were made with dye to observe the three-dimensional features of the head advancing along an open channel with rectangular section. The computational method for gravity currents developed in the previous paper was extended to the three-dimensional cases and applied to the lock-exchange flow problem. It is based on the finite volume solution of the incompressible Navier-Stokes equation for an inhomogeneous fluid and the transport equation for solute. Agreement between the experimental and computational results is good. The head subject to the three-dimensional instability forms a wavy frontline which consists of masses of heavy fluid going ahead as a mountain- valley structure. Each of the masses falls down laterally as well as forward, and therefore it spreads out in every direction to form an arc front. Some of them become large by running over adjacent masses. The enlarged masses suffer from the instability again, and then some smaller-scale mountains appear in that. As a result, the frontline of the current consists of boundaries of these masses of different scale, often superimposed on each other. This kind of three-dimensional instability also appears even in a non-dissipative case of free-slip boundary, which indicates that it is independent of the instability of the wall boundary layer. It is shown from the comparison with the two-dimensional computation that the three-dimensional flow structure possibly makes a significant contribution to the mixing process across the density interface behind the head and at the front, which may affect the speed of the head.

Precipitation of dissolved materials during the mixing by a gravity current is investigated in the laboratory. The experiment includes the physical process as well as the chemical process of two miscible fluids of slightly different density. Ammonium carbonate and calcium chloride solutions are used for these fluids, and sodium chloride is also dissolved into the former solution to control the density difference between these fluids accurately. When these solutions are mixed during the physical process of the advancement of a gravity current generated by lock-exchange flows, the chemical reaction takes place to make calcium carbonate which hardly dissolves in water to be visible as white substance. The behavior of this visible suspended substance is analyzed by digital image processing.The results show that the white substance made by the mixing of the two fluids appears most clearly in the upper region of the current head. The substance remains stationary and elongated downstream along the density interface. After a while it collects in masses of clouds in a three-dimensional way, and eventually sinks to the bottom. It is found that this chemical process depends on the dynamics of the gravity current, and that the chemical reaction is suppressed with increasing density difference.

This paper reports the study on monitoring the cylinder lubricating condition of two stroke marine diesel engine by ferrograph analysis.When the number of large/severe or cutting ferrous wear particles is under 300 per 1 ml cylinder drain oil sample, the cylinder lubricating condition might be very good and be satisfied.

Density field in a gravity current is measured from digital images. The digital images of a gravity current is taken with a digital video camera on the side of tank. The gravity current is formed salt water colored by red dye.. The light is absorbed when it passes in colored water and the amount of absorbed light increases with increasing dye concentration. The brightness of the digital images is measured by the personal computer on every pixel in the frames. It is transformed to the concentration of dye and to the density using the equation of calibration from the brightness into the density.The result shows density field of a gravity current and changing density field by collision with the wall.

The structure of the head of gravity currents is investigated in the experiment and the computation of lock-exchange flow. A finite volume of fluid was released instantaneously in another fluid of slightly different density in an open rectangular channel when a lock gate was opened. A box model was introduced to describe different regimes of a developing gravity current on the basis of energy argument.Visualization experiments were made by the shadowgraph method and with dye to trace the head of gravity currents, the results of which validate the box model. The initial depth and volume of the released fluid affect the front speed. At the first stage the released fluid spreads at a constant speed, and then it slows down so that the frontspeed decreases with time to the power of -1/3 in self-similar regime. The current that has begun to be dominated by viscosity slows down further in the final stage during which the front speed decreases with time to the power of -4/5.The computation was also made in the same condition as the experiments to study the structure of the head of currents in different regimes. The incompressible Navier-Stokes equation for an inhomogeneous fluid together with the transport equation for solute was solved by the finite volume method developed in the previous paper. The computation produces the transition between the development regimes so that the front speed obtained from the computed density field is in good agreement with the experimental result. The computational results indicate that the transition to the next regime is accompanied by remarkable changes in the geometry and the inner structure of the head. The head has maintained a sharp density interface even at low Reynolds number and the relatively large-scale entrainment of ambient fluid across the upper interface causes strong mixing behind the head. The models with the inviscid boundaries failed to catch the transition to the second stage, which suggests that the transition may require some stimulus associated with viscous motions. The result for the case of short lock-length shows the sudden decrease in the volume and in the density of the head brought about by the vortex shedding along the upper interface, which may be one of the mechanism to account for the strong mixing in the first and the second regimes.

The bearing metal of a marine diesel engine is basically disassembled every five years as a part of a special survey. However, this disassembling and assembling works require a large amount of manpower and time. Moreover, it is also dangerous. The classification society to which the authors belong has dedicated considerable time and efforts to examining whether used oil analysis can be applied as an alternative to the aforementioned periodic survey.This paper reports on the correlation between the used oil analysis results by ferrography and the inspection results of 2 stroke diesel bearing metal obtained at special surveys.As a result, the authors believe that the continuous engine system oil ferrograph analysis can be effective as one form of a condition monitoring based survey.

A stern tube shaft and bearing metals are basically surveyed every five years by a surveyor. But, it is a very dangerous work and also requires so much time to a ship owner. In our experiences, regarding oil lubricated propeller shaft, over 80 percent of vessels were in the good lubricating conditions which the shafts and bearing metals were placed back to the original positions without any repairing work.Our research found that there was a strong relationship between the stern tube oil analysis results (especially on the wear metal debris) and the shaft/bearing metal survey results. When we could set up a stern tube oil condemning limits, International Association of Classification Society (IACS) announced the acceptance of NDI for the stern tube by an oil analysis. This paper reports that the Time-based Survey can be replaced the Condition Monitoring-based Survey on the oil lubricated propeller shaft and bearing.

Sound power level evaluation methods for marine engines are investigated. Sound pressure based methods are discussed in this paper. In the highly reverbrant environment such as at engine room in a ship, precision of the sound pressure based methods tends to get worse.Through minute investigation on 10 test results, the authors introduce a new approach for the sound power evaluation in which mean level of PI index is used for environment correction factor. This approach, named the hybrid method, has both precision and practicality.

Nippon Kaiji Kyokai is now trying to detect the lubricating conditionas on marine machinery by applying ferrography.After studying the relationships between the inspection results by surveyors and the used oil analysis results, we reached the conclusion that, if the direct reading ferrograph (DR) shows to be under a certain limit on each machinery that system is in a good lubricating condition at high reliability.This paper reports the mean DR values of 2 cycle diesel engine, steam turbine and oil lubricated stern tube where good lubricating conditions are observed.