Mechanical Engineering

Yuichi Murai

Revolutionizing Ship Designs through Studies on Bubbles!

Yuichi Murai , Professor

Faculty of Engineering, Graduate School of Engineering (Mechanical Systems Course, Department of Mechanical and Intelligent System Engineering, School of Engineering)

High school : National Institute of Technology, Ishikawa College

Academic background : Doctorate from the University of Tokyo

Research areas
Research keywords
bubbles, visualization, turbulent flow, flow control, energy saving, reduction in greenhouse gases

My aim is to embody the pioneer spirit

It was in 1991, just before the collapse of the Japanese economic bubble, that I decided to continue my studies at the graduate level to become a researcher. My motive for continuing to study rather than securing a job, at a time with a job opening-to-application ratio of over 20, was the science of bubbles. Currently, my bubble studies are providing the shipbuilding industry with surprising results in energy savings and greenhouse gas reduction. Some might even argue that my studies sparked a revolution in marine engineering. Come to think of it, before entering university, I had assumed that mechanical engineering was a field in which students learned how to manufacture things. The content thereof, however, was actually more interesting - it is a field for discovering the physical laws of nature and utilizing them for industry. After I became a researcher, one of my findings at the laboratory received attention, setting in motion an industry and economy I did not know much about. I have repeatedly witnessed these developments, an example of which is the new developments in marine engineering. I am having so much fun that I have no time to sleep. I do not recommend that those in junior high school and high school enter business schools just to learn how to circulate money. Economic cycles and revitalization are driven by sciences and technologies invented by people specializing in the sciences and engineering. Our country is sustained by this driving force. In addition, there are many inventions and discoveries in the world awaiting your creation, numbers of which are equal to the amount of stardust. Let’s learn together at Hokkaido University, in which the pioneer spirit is nurtured, with the aim of making a giant leap forward in the Department of Mechanical and Intelligent Systems, School of Engineering, which is renowned for exciting yet practical learning.


Fig. 1 The falling speed of the iron ball varies with bubbles.

Fig. 2 The turbulent flow on the surface of the plate changes characteristics in the presence of bubbles.


This kind of equipment is used to carry out bubble experiments

Bubbles in liquid change the falling speed of an iron ball falling therein. As shown in Fig. 1, a small amount of bubbles slows down the ball while a large amount speeds it up. Think about why this is so. I will not simply give you the answer. If you cannot suppress your desire to know the answer, then aim to be a scientist or engineer, because knowledge, as well as the thirst for knowledge, is important. Furthermore, this is not a magic trick developed by humans, but by the physics of nature. Having deciphered the characteristics of bubbles obtained in Fig. 1 at an accuracy of 100%, the experiment shown in Fig. 2 was then carried out to visualize the turbulence of flow when water is poured onto a plate of approximately 10 cm using a laser. It was found that larger bubbles remove turbulence on the spot while smaller bubbles restrain overall turbulence. It was this discovery that attracted major attention regarding energy saving technologies for ships. Turbulence created in the water, or turbulent flow, forms the majority of the friction that ships receive from seawater. By simultaneously deciphering the laws of physics that restrain turbulence, I was able to propose new technologies.


Fast-paced ships with bubble generators

A bubble generator at the bow of a vessel covers the downstream portion of the vessel entirely with bubbles. Eliminating turbulence in seawater according to the principle of Fig. 2 reduces friction resistance from seawater by 20% to 30%. This effect is enormous since large ships consume three quarters of their fuel solely to overcome the friction from seawater. In the case of a large ship with a total length of about 300 m, which emits CO2 in an amount corresponding to that of 500 passenger cars, the generation of bubbles is equivalent to turning off 100 passenger cars. Since the number of trips made by large ships throughout the world is about 10,000, with most ships being ocean-going and operating without interruption, bubbles clearly contribute to the reduction in global warming. Currently, we are carrying out exclusive research at Hokkaido University on how to generate a large amount of small bubbles with less energy using hydrofoils, with ships that use this approach already starting operation in Japan and Europe.

Fig. 3 Classification of mechanisms for reducing friction with seawater

Fig. 4 Test involving a large ship with a bubble generator



My next goal is a breakthrough in wind power generation

I received an award from the Minister of Education, Culture, Sports, Science and Technology in 2007 for a series of studies on bubbles. Regarding my next goal, I am going to utilize the technologies accumulated from the hydrodynamics of bubbles in windmill design. While these study subjects may seem totally different, my strategies are clear: in both cases, the flow of energy is greatly changed by ingenuities in flow control. For example, the latest findings revealed that two similar windmills placed in a specific arrangement produce twice as much output or more. This will lead to the formation of windmills just like the V-shaped flight formation of migratory birds. Inventions and discoveries are destinations for observing minds with a solid foundation of knowledge. Let’s study at Hokkaido University and be pioneers in the field of engineering that contributes to the real world!