Exploring the inaccessible
Researchers seeded the water in the MI flume tank with micro-glass spheres coated in a reflective surface. When a laser is flashed in the water it can show the flow of water around the AUV. Photo by Peter King
By Kelly Foss
Memorial researchers are working to develop new technologies that will allow autonomous underwater vehicles (AUVs) to be used for resource exploration in difficult areas.
Local underwater acoustic imaging technology development company PanGeo Subsea has partnered with the university to determine if their instrument, a sub-bottom imager (SBI), can be integrated onto Memorial's Explorer AUV. When attached to a vehicle, the SBI is pulled along the bottom of the ocean just a few metres above the ground. It emits sound pulses down into the earth to generate a 3-D picture of what's underneath – useful information for those interested in oil and gas development in Newfoundland and Labrador.
Previously, the SBI has been used with larger remotely operated underwater vehicles (ROVs). But those vehicles must be tethered to a ship. AUVs are untethered, unmanned submarines capable of executing a pre-programmed mission autonomously -- the vehicle controls itself.
"The idea is to attach the instrument to our AUV so we can do exploration in places where you can't send a support ship," explained Dr. James Munroe, one of the researchers on the project. "For example, AUVs can go a few hundred kilometres under an ice sheet and come back with information on what's under it."
But considering the AUV and the SBI are both very expensive, researchers couldn't just put them together, throw them into the ocean, and hope it would work. They needed to be certain.
"The vehicle is over four metres long and the instrument is almost as wide as the AUV is long," said the physics professor. "When you throw a wing containing the instrument onto the AUV, there's a potential to throw the hydrodynamics completely off. So, it wasn't clear if this thing would still fly through the water or whether the control fins in the back would be impacted by the wake of the wing."
In addition, the AUV has limited battery power. So, if towing the SBI required more energy because of the drag, it would greatly reduce the distance it could travel. The wing needed to be designed so that it had little or no drag. Finally, they needed to be sure the AUV and the SBI could actually work in harmony.
"One of the challenges is that the AUV has a minimum speed," said Dr. Munroe. "Just like in a plane, you can't go zero kilometres an hour. But the SBI also has a range of operation. If you go too fast, you can't "ping" the sound down and get information back up in time. We needed to know if the maximum operating range of the SBI was within the lowest operational range of the AUV."
The team of researchers carefully designed the wing and how it was attached to the vehicle. Then computational fluid dynamic simulations were carried out to understand what would likely happen when the vehicles were put in water. In December, they put those theories to the test.
"The flume tank at the Marine Institute allowed us to immerse the vehicle and wing in water and conduct a series of experiments where we changed the rate of flow over the vehicle and adjusted its pitch up and down," said Dr. Munroe. "Because the water in the tank is moving and not the vehicle, we seeded the water with micro-glass spheres which were coated in a reflective substance. We then flashed a laser light on the water and took a picture at the same time. Then a split second later we took another picture, and then another. That allowed us to figure out the flow field around the wing."
Analysis of the flume tank tests is still ongoing, but so far it seems to have confirmed the computational fluid dynamic simulations. That means the researchers may soon begin making plans for field trials which will see the AUV, with it's wing attachment, deployed in the waters of Holyrood.
Supported by the Atlantic Canada Opportunities Agency's Atlantic Innovation Fund, this work is part of the REALM project, led by Dr. Andrew Vardy. Further information may be obtained from the project manager, Ron Lewis.