Tank Testing Technical Group
Main Research Theme
When a ship tries to move, it receives resistance mainly from water (seawater). "Propulsion Performance" means the strength of the force required to overcome this and propel the ship. When a ship needs smaller propulsion force (for example, a ship of smaller water resistance) than the same size ship at the same speed and with the same cargo, it can be said that it has better propulsion performance.
Traditionally, we have developed ships with good propulsion performance in the absence of waves, tidal currents, and wind (flat water). But in recent years we are conducting research to develop hull form of the ship with less resistance in waves and less horsepower or fuel consumption. Furthermore, ships are evaluated by not only propulsion performance but also comprehensive performance including operation cost and ship price. Therfore, it is necessary to consider such propulsion performance during hull form development.
In order to drive the hull, a propulsion device (propulsor), which is the tire of a car, is required, and a well-known one is a screw propeller (usually called a propeller. In addition to the propeller, outer ring propeller, sail, etc. are also propulsor.).
In addition to the usual type propellers that are often seen, there are various types of propellers: variable pitch propellers, duct propellers, contra-rotating propellers, and potted propellers being currently talked about. They are chosen considering their respective advantages and characteristics.
In order to produce efficiently propulsion (thrust) for ship's advance, the pressure on the back (advancing) side of the propeller blades drops to near vacuum. In such a low pressure place, water boils even at room temperature (15℃), and bubbles are formed. This phenomenon is called cavitation. In the cavitation, when the bubbles collapse, high pressure occurs and damages the propeller and rudderas. Also, noise is generated at this time. This noise becomes a problem for submarines and ships with acoustic sonar. Among problems caused by cavitation, the most important one is vibration of the stern. This problem is focused in commercial ships from the viewpoints of comfort and safety, and the improvement of this occurrence prediction technology is most expected.
The propulsion performance of a ship with a hull shape is still basically evaluated in plain water. Therefore, the performance of the actual ship is estimated through the performance measurement by pulling the model ship in a very large pool called a towing tank. This model test method was developed over 100 years ago, and data has been accumulated and the test method has been improved to this day. Our institute has two towing tanks. One is "Mitaka No. 2 Ship Model Experiment Tank" (commonly called "large tank" or "400m towing tank"). This tank is one of the world’s largest towing tanks and is 400m in length, 18m in width, and 8m in depth. The other is "Mitaka No. 3 Ship Model Experiment Tank" (commonly called "150m towing tank" This tank is 150m in length, 7.5m in width, and 3.5m in depth) and can also change the water depth. In addition, both can generate various waves. This feature enable us to conduct research on performance in waves assuming actual sea areas.
In addition to these tanks, we have a large cavitation tunnel to evaluate propulsion performance centering on the propulsor. Unlike the towing tank, this is a circulating tank in which water circulates inside the tank, and the inside of the tank can be decompressed to near vacuum so that cavitation can be generated. This facility is one of the largest cavitation (circulating) water tank in Japan with a length of 16m and a height of 10m.
Our group is working on the development, improvement of accuracy and efficiency of the tank test method to obtain more accurately resistance and propulsion performance of a ship, which are the basis of the hull form development. Regarding the tank test method, the International Towing Tank Conference (ITTC, an NGO) establishes a technical committee on the most advanced rational test methods in the world to conduct research, investigation, discussion, and recommendation activities. Our institute (NMRI) participates in the ITTC as a member, and participates in its Advisory Council that manages the conference. And a few researchers are working in the council as technical committee members in each term to provide international cooperation and contribution.
Left Fig. Fabrication of KCS model ship and dimension inspection, Right Fig. Tank test of KCS model ship
Conventional hull form development has been conducted by empirical manufacture of many model ships to find a good hull form in series tests by using a database that accumulates past test results, wave-making resistance theory, propeller theory, etc. On the other hand, many numerical calculation methods (NFD, including Computational Fluid Dynamics, CFD) have been developed in response to the development of high-performance computers in recent years. Currently, the new method using NFD has been adopted. In this method, the optimum ship shape is searched by NFD before the tank test, and finally confirmed by the tank test. In order to provide verification data for the reliability confirmation of CFD programs, our group conducts repeated tests for uncertainty analysis and transmits the obtained verification data to the world. Through these efforts, we plan to propose a hull form with excellent propulsive performance based on a new concept.
Overview of Research
"What are micro bubbles?"
Due to the impact on the global environment, international regulations on ships are tightened. At NMRI, we are working on the research of eco-friendly vessels. In this research, the propulsive performance evaluation of vessels plays an important role. We would like to introduce a study called "Micro Bubble PIV" that makes the propulsion performance evaluation more accurate.
Everyone would have a desire to see the invisible world. For example, we can feel the flow of water and air outdoors, but we cannot see where the flow is coming from or going to.
As a record of those who challenged this question, a sketch of the vortex flow around a flat plate placed in the stream (1513: below) drawn by Leonardo da Vinci is known as an old record. Another famous painting that depicts the flow of water in detail is "Madonna with St. Dominico, St. Peter and St. Christopher". This fresco is displayed in the museum of the church of St. Dominico in Bologna, Italy. Prof. Mikio Hino pointed out in his book "Fluid Mechanics" that the vortex flow around the feet is drawn in detail.
Since ancient times, our ancestors have been aware that fallen leaves in a stream flow along a stream, and that movement can tell the direction and speed of the stream. An object that serves as a mark such as a fallen leaf flowing in a stream is called a tracer. Nowadays, we put substances such as fallen leaves in the flow in order to see the movement of the invisible flow. This method is called the tracer injection method.
The technique that makes the flow visible to the human eye in this way is called "flow visualization technique."
Flow visualization technique has been used for a long time in the research dealing with various flows: flow around a ship hull, flow around a wing, flow in a pipe, etc. By using this technique, various new fluid phenomena such as transition from ordered flow (laminar flow) like comb teeth to turbulent flow like intertwined threads can be found. In addition, researchers concerned with fluid flow are not satisfied with visualization techniques that only look at flows, and have developed methods for calculating numerical values such as velocity and direction from images of flow visualization.
A typical example of these methods is PIV. PIV is an abbreviation for Particle Image Velocimetry. This is an application of the principle of judging the scale and direction of a stream from the motion of fallen leaves in a stream and smoke from a chimney in the wind. PIV was developed as a quantitative visualization technology. In this method, the computer is used instead of the human eyes to obtain the flow direction and its velocity reasonably and efficiently with less error.
Here, we would like to explain how the procedure is actually taken in PIV. First, we take images of the flow field continuously. Then, choose two images taken at different times (Fig.1). Comparing the first (Right) and second (Left) images, you can see that the green object is moving to the right on the second image. You can only know the direction and amount of movement from two images, but if you know the time interval between two images, you can find the speed.
Fig.1 Principle of PIV Measurement
By the way, in recent years, from the viewpoint of global environment protection, the reqirement to tighten international environmental regulations for ships has been increasing. To reduce carbon dioxide emissions from ships, ships should decrease its fuel consumption as possible. To achieve this, many efforts are being made all over the world to improve fuel efficiency by improving the propulsion performance of ships that push water away. To this end, it is important to investigate the flow of water around the ship that impedes the ship's forward movement and to design the efficient ship as possible.
NMRI is also working on research to reduce carbon dioxide emissions from ships. Our Group is developing technology to evaluate the propulsive performance of a ship by measuring the flow around the ship in detail using PIV, which is one of the quantitative visualization techniques described above.
In general, a fine powder is often used as a tracer for PIV measurement in experimental water tanks. They are small particles which has the specific gravity close to water's, such as silver-coated hollow glass beads (Fig. 2) and nylon particles.
Fig.2 Silver-coated Hollow Glass Beads
However, it is practically difficult to collect the powder once sprinkled in the aquarium for visualization, and as a result it will contaminate the water in the experimental aquarium. This limited us to use PIV easily.
"Watching the flow with micro bubbles"
To solve this problem, we are working on the development of a method by using very fine air bubbles (micro bubbles) as tracers that do not remain in the aquarium unlike the powder tracer.
Micro bubbles used here are bubbles with a diameter of 50 μm (0.05 mm) or less and smaller than the diameter of hair. Normally, the bubbles you see in ciders and baths will float quickly, but microbubbles are so small that they do not float immediately. Also, when you look at the photo in which the generated bubbles are placed in the beaker, it looks white and cloudy as shown in Figure 3 (left). If you expand it, you can see small bubbles as shown in Figure 3 (right).
Fig.3 Microbubbles (left: Turbid microbubbles. right: Enlarged photo. White particles are microbubbles)
Here, we show an example of a measurement result using a micro bubble as a tracer. The flow field in sinusoidal waves generated in the oscilation tank is shown in Fig. 4.
Fig.4 Oscilation tank
Below movie (Fig.5) is an actual image of a sinusoidal waves generated in the oscilation tank. White dots are microbubbles.
Fig.5 Image of sine wave flow field
From this movie, you can see that the microbubbles draw a circular motion in water. When the velocity was calculated from this image and compared with the theoretical value, it was found that there was a good agreement as shown in Fig.6. In the future, we plan to improve the measuring device to measure the flow around the model ship.
Fig.6 Measurement result and theoretical profile
Red: Theory, Blue: Measurement (Good agreement can be found)
In order to increase the propulsive energy efficiency of ships, attempts are being made to improve the flow field around the ships by attaching additional objects such as protrusions and cylinders to the hull and to decrease fuel consumption. In fact, some recent ships have ducts or fins attached to the hull to improve performance. These attached objects are called energy-saving device.
Our department is conducting tests to confirm the effects of these devices. To evaluate their effect precisely through tank tests, they must be manufactured to exact dimensions. Furthermore, they must be mounted at the precise position. Therefore, we sometimes needed to make special jigs for alignment and it takes a lot of time. To conduct evaluation test efficiently, a new tool to shorten the preparation of the tests has been reqired. In this situation, the 3D printer attracted our attention.
In recent years, 3D printers have become more accurate in manufacturing and have become cheaper. In addition to the attached objects, jigs and reference fittings used for alignment can be manufactured with sufficient accuracy by 3D printers. Now, we can conduct a series of tank tests more efficiently than ever by using the 3D printer.
Here, we show you some objects made by 3-D printer.
Fig.7 Objects manufactured by the 3-D printer (Left photos) and an example of a CAD drawing for manufacturing with a 3D printer (Right)
We have many experiences of manufacturing ducts, other and jigs. In addition, we have made rudders. But we have not yet manufactured a screw propeller. Furthermore, we also manufacture the 3D printer parts themselves by the 3-D printer. In Fig. 7, we show an example of a CAD drawing for manufacturing with a 3D printer. Manufacturing process by 3-D printer is as follows.
- Read 3D CAD data.
- Convert it to a file format for a 3D printer.
- Transfers it to the 3D printer.