Open up Google Earth and search the area between 135°W and 155°W and between 35°N and 42°N. What do you see? Water. Get closer. You still see only water? Well, even if you don’t see it, there is something else: rubbish, tons of rubbish. In fact, in this location there is over a million tons of debris in an area covering 1.4 million km2. It’s called the Great Pacific Garbage Patch.
And this polluted spot is not unique, there are five such regions, where converging currents form a gyre, dragging and catching millions of tons of debris.
These ‘garbage patches’ are not made of large objects, such as bottles and containers, but small floating particles, made of plastic or other lightweight debris, which are gathered by the currents and accumulate in specific areas, slowly destroying the local marine ecosystems and also affecting any nearby coastal regions.
The question is obvious: why not clean it up? But the answer is not so simple. Being made up microparticles, the physical characteristics of the debris make it impossible to use standard cleaning vessels and nets – never mind the fact that such procedures would take thousands of years and billions of euros.
Dutch inventor and entrepreneur, Boyan Slat proposed a solution in 2012: to take advantage of the oceanic currents to collect the rubbish in a passive retention barrier and then to remove more easily. In other words, not to go chasing after the rubbish, but to wait for it to come of its own accord.
The difficulty was in proving whether this was indeed possible, let alone cost-effective. Even assuming that adequate facilities were available, the cost of simulating the oceanic currents with their complex characteristics, the climatic variables, the particle concentrations at different depths, etc. was simply unaffordable.
Here is where computational fluid dynamics has a role to play. To start with, it was necessary to carry out dozens of physical tests at the Maritime Research Institute Netherlands at a scale of 1:18. This allowed the numerical models to be optimised, accounting for the different behaviours and conditions relating to the action of the waves, currents and winds, enabling scientists to evaluate the performance of the passive retention barrier for different sizes of particle.
This information allowed the construction of virtual models to realistically reproduce practically all conditions expected in the North Pacific and then to conduct thousands of tests to predict the behaviour of the barrier in those conditions, focusing on the necessary length, depth, buoyancy, and retention capabilities.
To validate the computational models, the engineers of the Ocean Cleanup project made use of SIMULIA’s XFlow program. As a result, they improved the design of the retention barrier, ensuring that it remained operational even in extreme storm conditions, while minimising the expense of the tests performed.
The system is now in its pilot phase, with a 100-metre boom deployed in the North Sea about 25 km from the Dutch coast. The final project will involve a retention system from California to Hawaii to collect the Pacific debris, prevent its further fragmentation as it drifts North, and allow its safe removal.