Bubbly Wake and Submarines
Pablo Carrica tapped a pencil on the desk, impatiently watching his computer screen. He was running a simulation of the flow of oil and gas in an underground reservoir, a task he had completed hundreds, perhaps thousands, of times in his job at a large Spanish oil company.
“The work was pretty repetitive, and not very challenging,” Carrica remembers.
Later, when Carrica began searching for a new job, he was able to capitalize on an acquaintance with researchers at IIHR— Hydroscience & Engineering to find a position in the institute’s ship hydrodynamics program. It wasn’t long before Carrica was hooked. “I realized that this was challenging and fun to do,” he says. “Maybe too challenging sometimes!”
U.S. Naval Research
Today, Carrica is running his own research program at IIHR with two main focus areas in ship hydrodynamics, both using computational fluid dynamics, or CFD. The first focuses on two-phase flow around ships. Carrica and his team want to better understand how bubbles form in the wake of a ship. The goal is to reduce the number of bubbles or design a countermeasure to obscure the bubbly wake, and thus make a ship harder to track. The tinier the bubbles, the longer they take to rise to the surface and dissipate.
Why should anyone care about a few tiny bubbles? The U.S. Navy cares a great deal, because a ship’s bubbly wake can easily be detected through the use of acoustics. “The goal would be no bubbles,” Carrica says, “but that is impossible. More likely is that you’ll be able to devise some kind of maneuver that will cut the wake so that the threat will be thrown off.”
Carrica and his team designed a tiny probe that they use in experiments onboard research vessels. “The probes are like tiny pins,” Carrica explains. The tips of the probes, which are made of sapphire, are only about 25 microns in diameter — smaller than a human hair. The probes are mounted to the research vessel’s hull, where they pierce the bubbles and enable researchers to measure their size and velocity.
In 2009, the team conducted a series of these bubble-flow experiments onboard the U.S. Navy’s research vessel Athena, based in Panama City, Fla. Later, Carrica and his crew conducted similar experiments in fresh water at Coralville Lake, using one of the IIHR research vessels. That boat’s flat bottom offers a well-understood, idealized geometry perfect for researchers who want to evaluate the interaction of bubbles with the boundary layer.
Carrica’s second area of interest focuses on the propulsion and maneuvering of submarines close to the water’s surface. Submarines are primarily designed to move underwater; as they surface, the propulsion system is impacted by the free surface of the water, causing undesirable forces on the submarine that complicate maneuvering. Researchers hope to better understand this interaction so a control mechanism can be developed to counteract the changes in propulsion.
Carrica and his team use CFDShip-Iowa, simulation software developed at IIHR and considered the most advanced CFD code in the world for ship hydrodynamics. The version in use today was introduced around 2003 and uses a completely different methodology than the original version. Carrica says researchers build a CFD model one block at a time, much like a child builds with Lego blocks. Researchers start by building the solver, which solves the fluid flow equations. Next, they can start adding capabilities, one by one. “The next would be motions, so you can simulate what the ship will do,” Carrica says. “Then you have to add the rudders, because they follow the parent object, but they move independently too.” More blocks follow, simulating cables, waves, and more.
CFD developers create grids, which are overset one on top of the other. Each component has one grid or many grids, and they all fit together on the same point in space. A code determines which grid is going to be active at any point in time. “That has the advantage of allowing you to move things arbitrarily,” Carrica says. “You can start moving the rudders and the propellers and the ship itself.
“Like Lego blocks, you just build the whole thing together,” he explains. “The technology has the great benefit of allowing you to move these blocks independently in whichever way you want. You can simulate all the moving parts of these ships.”
Obviously, boredom is no longer a problem for Carrica — his work in ship hydrodynamics continues to fascinate him. “This is definitely what I like,” he says. “We have a lot of fun.”