Progress Update

You’ve seen waves crashing on the shore, engulfing swimmers and boogie boarders alike as they wash foam onto the sand. Now imagine those waves, only hundreds of feet higher, approaching the beach on a timescale of hours instead of seconds! Does this sound surreal? Meet internal waves: the ocean’s even bigger waves. Luckily for the ships that sail the seven seas, these waves go unnoticed by most passersby as they form well below the ocean’s surface– secretly perturbing heat, salt, and other nutrients around the ocean.

Flow

Internal waves are generated by the interaction of currents and surface waves with an irregular seafloor, mixing together seawater with different temperatures and salinities. These waves break at locations where the undersea topography varies in elevation, specifically where the seafloor slopes upward, and are believed to contribute a considerable amount of energy to ocean mixing processes (Encyclopaedia Britannica, 2019). This energy transfer is an important factor in climate models, but how do scientists really account for it? Right now, there is no perfect solution. MIT Professor Raffaele Ferrari noted that despite progress in understanding how internal waves are produced and travel, researchers do not yet understand how they break and therefore how energy contributes to ocean mixing. “We can probably account for 20 percent [of energy dissipation], but we can’t account for the other 80 percent. It’s a missing link,” says Ferrari (Chandler, 2009).

And yet, these waves can tower at skyscraper heights, particularly in areas with drastic drops or sharp rises in undersea topography (Aggeliki, 2019). Some of the largest and most powerful internal waves pass unnoticed to the naked eye across the Luzon strait between Taiwan and the Philippines (Alford, 2015).

Flow past the Luzon Strait’s West Ridge– which sits 2000 meters below the ocean’s surface– creates enormous internal waves, yet is unnoticeable at sea level, as shown in the simulation below (Kelly, 2015).

Simulations of wave velocity and energy dissipation, depicting the unique two ridge topography of the Luzon Strait that greatly amplify the waves’ power. Image from: Kelly, 2015. 

MIT researchers teamed up with the Office of Naval Research (ONR) to travel to the Luzon Strait and get to the bottom of how the most powerful waves in the world are formed, and what their potential implications are for our climate. Sensors can track slight variations in salinity and temperature of ocean water at different depths, which can be coupled with satellite data to understand the influence of these waves. Through these satellite images and complex sensors, they found internal waves in the area were generated over the entire ridge present in the straight, not from a specific point, at heights of 500 meters, and generated turbulence up to 10,000 times that found in the open ocean. Though the researchers could match their new data to simulations they had built, questions still remain about how the energy is dissipated (Chandler, 2009).

Researchers’ diagram illustrating the internal tide generation observed across the Luzon Strait. Image from: Alfred et. al, 2015.

The team also determined another vital function that these waves serve: maintaining coral reef ecosystems in need of nutrients from deeper waters hundreds of miles away. As global ocean temperatures continue to rise, the role of internal waves across fragile ecosystems, like these reefs, will require further study to better understand how environmental scientists can best protect these sites.

Part of the reason why so little is known about internal waves is the difficulty in observing them. Though research groups like the MIT-ONR team have access to complex satellite data and sensors, data collection in ocean field sites is expensive and time consuming. As Gene Fieldman, NASA oceanographer, has observed, “we have better maps of the surface of Mars and the moon than we do the bottom of the ocean” (Dunbar, 2009). 

Design

One of the most difficult aspects of studying internal waves is simply observing them. To mimic this naturally occurring phenomenon, we set out to design a scaled-down prototype for use in a small lab setting called Tanky. The design consists of four major components that are key in creating and observing internal waves. 

The first component is the tank that will hold the various testing materials. As a prototype, we plan to use a large (approximately 56 Qt.) plastic bin filled with fresh water and pool salt to represent the two layers of the ocean which vary in density, salinity, and nutrient content. The goal is to replace the plastic bin with a self-constructed acrylic tank for the final iteration of the design. The proportions of the tank are selected to idealize internal waves in just two dimensions.

Diagram of final tank dimensions.

This simplification will allow users to more clearly observe the phenomenon. The following simulation shows that 3D (top) and 2D (bottom) internal waves behave similarly, so the simplification will not significantly affect the flow.

Simulations of 3D internal waves (top) and 2D internal waves (bottom). Image from: Arthur, 2013.

The second subsystem consists of submerged objects and inclined obstacles which represent the seamounts, shorelines, and other common seafloor topographies. The design of these obstacles presents many interesting tradeoffs and constraints. The goal is to create a repeatable, lightweight, consistent, but inexpensive system. To keep the topography from moving with the flow, we considered designing a locking mechanism like quick release buckles or a jigsaw shape that would match the bottom of our topography. These ideas proved unnecessarily complex, and we decided velcro provided plenty of force to keep the topography in place. We are considering 3D printing the topographies, but realized it would be more accessible and faster, without sacrificing quality, to cut the topographies out of materials such as aluminum or wood. The material selection will be finalized during our prototyping phase.

Potential materials to be used as topographical elements, found in the BDW.

The submerged topographies used in our first design iteration will be composed of excess materials found in the Brown Design Workshop (BDW). As seen in the above image, these initial topographies will vary in shape, density, material and size. The benefit to using heavier materials is the minimized need for an efficient locking mechanism.

The third aspect of our design is the mechanical wave generator. We explored various types of sea walls to off-the-shelf wave generators to determine what could generate consistent waves at minimal cost. After deliberations, we settled on two Servo motors and an Arduino Uno to generate waves of various frequencies within the tank; we hope that utilizing these materials will make this subsystem affordable and easily replicable. A barricade will be secured to the motors by rigid bars which will mechanically oscillate from 0-45º in order to initiate disturbances in the water. 

The final component of the design is a 960 fps high speed camera, which can be used to record data and monitor changes on a frame-by-frame basis. However, in order to adhere to the ideals of constructing an accessible setup, we will also aim to collect data using a standard smartphone camera.

Diagram of final experimental system

Using this design, our team hopes to create a system that will be able to simulate internal waves in various topographical regimes. Some day, these types of lab simulations may be used to learn more about how the ocean works, including the mapping of undersea topography and its effect on climate modeling. For the time being, this hands-on three dimensional wave simulation can catalyze student’s interest in further understanding internal waves.
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Citations and Further Reading:

General References & Background

Aggeliki, K. “Topography of the ocean floor.” Bright Hub Engineering, 2019.

Britannica, The Editors of Encyclopædia. “Internal Wave.” Encyclopædia Britannica, 21 Nov. 2016.

Chandler, David L.“Hidden waves pack a big punch,” MIT Mechanical Engineering, 2009.

Chandler, David L. “The ocean’s hidden waves show their power.” MIT Mechanical Engineering, 2014.

Dunbar, Brian. “Oceans: The Great Unknown.” NASA, Institute for Global Environmental Strategies, 8 Oct. 2009.

Flow Visualization

Kelly, Morgan. “Dissecting the ocean’s unseen waves to learn where the heat, energy and nutrients go.” Princeton>Research, 2015.

Alford, Matthew H. The Formation and Fate of Internal Waves in the South China Sea. Macmillan Publishers Limited, 2015.

Related Simulations

Kelly, Morgan. “Dissecting the ocean’s unseen waves to learn where the heat, energy and nutrients go.” Princeton>Research, 2015.

Arthur, Bobby. “Breaking Internal Wave – 2D vs. 3D” YouTube, 2013.