Deep Sea Basin at NMRI can create characters and patterns by waves. The basin is shaped circular and has 128 wave generators installed surrounding the basin. The wave generators can be used to generate waves like a grid, logo marks, concentrated waves, which are rising like water jets and turning waves like a vortex.
The technology of the "wave control at will" is essential for a variety of experiments. You can find our wave control technology, various waves, letters and patterns, via the video.
Consecutively you can find "Logo mark of NMRI ", "Concentrated waves", "Mesh waves (checkerboard waves)", "Rotating waves", and "Regular waves".
Consecutively you can find "KEN (Japanese letter)", "YOSHIKO (Japanese letter)", "Heart symbol", "TAKA (Japanese letter)", "&", "SUSHI (Japanese letter)", "Mark of Lion" and "Star". These are introduced by Japanese TV program.
The English abbreviation "NMRI" of National Maritime Research Institute is drawn one by one.
How the characters or patterns can be draw by waves? It is based on the physical phenomena that the propagation speed of short waves is slow and that of long waves is fast. At the experiment in the basin, regular waves or irregular waves with a given spectrum are often used. Besides, in the investigation of a marine accident wave creation with any given time series or triangular waves (single big wave) are required, for example. Although drawing letters and patterns by waves are not a directly use the experiment, the "wave control at will" will be used for the development and demonstration of wave making technology
FIG. 1 Deep Sea Basin
Deep Sea Basin
NMRI has a variety of ship model basins; towing tanks, square tanks, and ice model basin. The experiments are carried out taking advantage of each feature.
Among these basins, Deep Sea Basin which can make various waves is introduced to show our technology of wave making, for example.
As shown in Fig. 1, Deep Sea Basin is a circular tank with a diameter of 13.6 m and has 128 wave generators around it. Each wave generator is a flap-type wave generator with a width of about 33 cm, and each of the flap-type wave generators can be moved independently according to the given signals.
Here using 128 wave generators in Deep Sea Basin, drawing letters and patterns on the water surface are performed.
FIG. 2 is time series of wave height (an example of R=10 m) when a plate (wave generator) is subjected to an impact motion
FIG. 3 : Time-reversed motion given to the plate
FIG. 4 : Concentrated waves are generated on an arc line at a distance R from the plate (wave generator)
FIG. 5 : Spatial distribution of waves at convergence time (wave height on line A-A′ in FIG. 4)
FIG. 6 : A method of generating single-point concentrated waves
FIG. 7 : Spatial distribution of single-point concentration waves at concentration time
In order to draw characters or patterns on the water surface, firstly, it is necessary to "raise the water surface at the designated point". For this purpose, a characteristic of waves is used. That is, the propagation speed of short waves is slow and that of long waves is fast. For example, when you throw a pebble in a park pond, you can see the long waves first spread and thereafter ripple waves spread to follow the long waves.
FIG. 2 shows the time series of the wave height at a certain distance (R) from a plate (wave generator) which is subjected to an impact motion. From the figure, it can be seen that longer waves arrive first and shorter waves, which look squeezed, arrive later.
Now how to use this property?
If short waves (slow waves) are generated first and long waves (fast waves) are generated later, the long waves will catch up with the short waves at a certain point. Waves are resulting and the wave height would be increased. Further, it is imagined, if the time difference from the occurrence of short waves to that of long waves is shortened, the waves are immediately caught up closer to the wave generator. On the contrary, if the time difference is lengthened, the long waves are resulting to the short waves farther. In this way, the location of caught up could also be controlled.
This is a basic idea. Waves obtained by resulting waves from short waves to long waves is called "concentrated waves".
Specific methods for generating concentrated waves are as follows.
As described above, FIG. 2 shows the time series of the wave height measured at a certain distance (R) from a plate which is subjected to an impact motion, but the time-reversed motion as shown in FIG. 3 is given to the plate.
Let's look at FIG. 3, you can find that first shorter waves are generated and later longer waves are generated. This causes concentration just at the distance R. The author found the method in 1981 for the case of a two-dimensional channel1). This time it is applied in the case of a three-dimensional basin. Examples of the results are shown in FIGS. 4 and 5.
Photo 1 Character "R" by waves
Photo 2 Logo mark of NMRI by waves
Draw characters and patterns with by waves
As described above, it has been confirmed that it is possible to generate the intended patterns. Of course, it can't be drawn so fine. The resolution is determined by the width of the plate of the wave generator and the wave length which can be generated. The resolution is about 30 cm for Deep Sea Basin.
Electrical signals to the wave generators are calculated based on the theory, however, according to the theory, it is possible to predict not only the concentration time but also the wave height of each point at each time. In FIG. 8, the predicted wave height in time series at the center of Deep Sea Basin are shown when the letter "R" is drawn. The calculated wave heights are compared with the measured values and it can be seen that these two sets of values agree well with each other.
FIG. 8 : Time series of wave height when "R" is drawn.
Photo 3 Two-dimensional concentrated waves
FIG. 9 : Difference between respiration and no respiration (time series of wave height when "R" is drawn)
As described above, it has been confirmed that it is possible to generate the intended letter. Of course, you can't draw very finely. The resolution is determined by the widths of the wave-generating plates and the wavelengths that can be generated. However, the resolution is about 30 cm for deep sea water tanks.
All wave-producing signals are generated based on theoretical calculations, but according to this theory, it is possible to predict not only the concentration time but also the wave height of each point at each time. In FIG. 8, the predicted values of the time series of wave heights at the centre point of the aquarium when the letter "R" is drawn are given and these values are compared with the measured values; it can be seen that the two sets of values agree well with each other.
At the end
In 2010, Actual Sea Model Basin was constructed at NMRI. The basin is shaped rectangular, and 382 wave generators are installed around it. The wave-making method described here is also effectively used in Actual Sea Model Basin.
In addition, I would like to develop a new wave generation method which meets needs.
1）S．Ohmatsu："A Simple Method to Generate Arbitrary Waves in Model Basin" (in French), Papers of Ship Research Institute, No.65 (1981).
2）S. Ohmatsu: "On the Transient Wave Making and Absorbing Method in Time Domain" (in Japanese), Papers of National Maritime Research Institute, Vol. 9, No. 2 (2009).