Theoretical modeling on land, sand, harbors, and sea.

In shallow water such as inland lakes, contaminants often bind to very fine, muddy particles in the sediments. Predicting the movement of the sediments is very important for knowing how the contaminants are transported. These fine particles originally sink to the bottom, but they can be stirred up by waves, currents, dredging, and other disturbances.

"Since waves are created by the action of wind, it is important to study the effect of wind on water and how waves are generated. In turn, we want to know how waves churn up the sea bottom and mobilize the fine particles. Once they're mobilized into suspension, how do these fine particles move about in the water body? Do they stay mostly on the bottom, or do they float everywhere?" asks Mei, who is doing some fundamental studies on this subject with grad student Zhenghua Huang.

When the wind blows on the surface of lakes or rivers, very regular streaks of leaves or debris form on the water surface, called Langmuir circulation. (Irving Langmuir [1881-1957] realized that the formation of streaks indicated some sort of vortices which tend to cause flow to converge along certain lines in one direction, and then diverge below the sea surface.)

"We are trying to understand this phenomenon more theoretically, particularly in shallow water," specifies Mei. "In addition to this mechanism on the surface which Langmuir observed, how would the circulation affect the bottom? In what way would the circulation also mobilize very fine particles and transport particles across the water? This is a fundamental fluid dynamic question with the ultimate aim of understanding how fine particles are transported in very shallow water, inland lakes, and possibly rivers. The problem is of interest to engineers, environmentalists, and oceanographers."

Harbor engineering

Many harbors such as Long Beach, CA, and inland harbors on the Great Lakes are plagued by very long period wave oscillations. Harbors have always been designed to protect ships from waves due to wind coming from outside, often with breakwaters to deflect waves back to the sea. "Even though sea waves have short periods of about 10 seconds, very often you can observe long period oscillations of about 100 seconds inside the harbor," notes Mei. "The mooring lines which keep the ships in a fixed position also have a natural frequency very close to that 100 seconds." If the ships oscillate at these long periods, it puts a severe strain on the mooring system and can snap these thick cables.

Mei was on the technical review committee for Long Beach harbor about five years ago, where Pier J, which was rented to the Danish container company Mersk, was big enough to dock six container ships. "The company complained that their ships had experienced long period oscillations, and rocked so strongly that it slowed down the loading and unloading operations. Extra time costs extra money. In addition, sometimes as many as one big steel mooring cable per day would snap, causing workers in these loading operations to drop down or fall off the boats." Mersk asked the harbor to fix this problem, or else both it and Long Beach would lose money if it was forced to find another harbor and use a different shipping route.

"One of the engineering and technical challenges is to know why long period oscillations exist in the harbor where all the sea waves coming in from great distances are short period. This is not easy to understand," says Mei.

Mengyi Chen from Taiwan is developing a nonlinear theory with Mei about how to predict long-term oscillations in the harbor even though the incoming wind waves are short. When Mei was on the Long Beach committee, "there was no good way to predict how long waves can be generated by short waves. Therefore the port authority asked the Corps of Engineers' Coastal Engineers Research Center to do laboratory simulations. They built a harbor in the big wave basin in Vicksburg, and send in very long waves to excite oscillations of equal period in harbors. This is a linear mechanism. But waves coming from the Pacific Ocean are known to have little energy in these long periods. However, that's the usual way that many researchers run this type of test."

New breakwaters costing $25 million have been built to protect Pier J based on this type of study. Whether they are effective may take years to find out. Mei speculates, "It may work, or it may work for the wrong reason, or it may not work all the time. But nobody knows. I was never satisfied with that kind of situation. We think that when we complete our theoretical model of how long period waves in harbors are generated by short waves, it will have far reaching applications for many harbors in the world."

Rearranging fluid mechanics

CEE is involved in the I-Campus project to streamline the first year graduate course on fluid mechanics. "Fluid mechanics is a basic subject for almost every kind of engineering discipline- mechanical, nuclear, civil, ocean, aeronautical- and in quite a few science departments, such as Earth and Planetary Science," states Mei. "Over the years, almost every department has established a course on graduate fluid mechanics. Not only is there duplication, but because each department teaches its own fluid mechanics course, it reduces the intellectual interaction between departments. Most of us think this is not desirable."

As a result, professors from five departments-CEE (Heidi Nepf), Mechanical Engineering (Ain Sonin and Triantapyllos Akylas), Ocean Engineering (Dick Yue), Aero/Astro (Marck Drela), and Mathematics (John Bush) are collaborating on a revised course. "With support from the Provost of the School of Engineering and the Microsoft iCampus project, we want to see how information technology can help us design a modular course so that students from across the Institute can take some modules dealing with common and basic principles, and other modules showing various applications of fluid mechanics," says Mei.

"The aim is to reduce redundancy in teaching and create more exciting interactions between different departments, resulting in more effective teaching for all students. The increasing use of the Internet will make visualizations, numerical simulations, etc., more effective and interesting to the students. We hope to change the way that fluid mechanics is taught at MIT. E hope to start an experiment which will not only affect the teaching of fluid mechanics, but may have implications on the teaching of similar basic topics such as solid mechanics, thermodynamics, and control theory."