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Loading and offloading of goods at seaports, fuel-powered marine vehicles slicing through rough waters, hydroelectric power generation, and offshore drilling are few among the many contributors to underwater radiated noises (URN). It is not within our limits to curb all man-made noise radiations but CFD simulations can help control noise emitted by cavitation-infected ship propellers through design optimization techniques to preserve marine life and maintain the underwater ecological balance.
What Is Cavitation and How Does it Impact Marine Vehicles?
It is a natural phenomenon where sudden changes in pressure, i.e., the static pressure of the liquid below its vapor pressure leads to the formation of cavities in a liquid. On being subjected to high pressure, these cavities collapse and generate shockwaves that damage the equipment. These cavities or voids that collapse close to the metal surface experience cyclic stress due to the repetitive implosion. Cavitation, based on its behavior, can be classified into – inertial and non-inertial cavitation. Ship propellers experience inertial cavitation, where collapsing of a void in the liquid produces a shock wave. When intentionally used, cavitation can be desirable and is used in sterilizing surgical instruments, breaking down solid pollutants in water, and homogenizing fluids. Cavitation in ships can have a considerable impact on the propelling efficiency and cause early degradation of blades, vibrations, and noise emissions.
How is Marine Life Affected by the URNs?
Today, oceans are mayhem of engine roar, seismic blasts, and other man-made noises, making it impossible for marine animals to hunt or communicate. If you were to drop a microphone into an ocean with whales, along with the noise generated by human activities, you could hear sounds like taps on metal cans; it's them, the whales echolocating search beams. With container ships blasting at 190 decibels underwater, equivalent to jet take-off clatter, these noises overlap with the sounds that are emitted by marine animals to locate, catch their prey, protect themselves from possible predators, breed, nurse, etc.
Rising water temperatures because of climate change are disrupting the ocean soundscape (an ecosystem of sounds and vibrations) that marine animals use for communication. Since 1901, the surface water temperature has been increasing by 0.14 degrees Fahrenheit every decade. According to a study, warm ocean temperatures accelerate at the speed of sound which can be fatal for marine animals. The two hotspots where this phenomenon could have an impending risk on marine biodiversity are the northwestern Atlantic and the Greenland Sea.
What is the Possible Solution to Cavitation for Optimum Propeller Design?
The mechanical components and drive systems also produce noises, but the noise from ship propellers has the greatest URN strength and propagates longer distances. Hence, it is of great interest to reduce the URN generated from propellers. Very soon, URN will be classified as a source of pollutants, and regulatory agencies are working towards a framework for URN assessment of propeller design. The attributes of the framework are as follows:
As a solution to the URN generated by ship propellers, new patented technology has been developed by Oscar Propulsion and Strathclyde University – the Oscar Pressure pores system. This system bores pressure-relieving holes in the propeller blades helping ships operate with a comparatively silent propeller and without compromising on their efficiency. These holes on the propeller blades help in reducing vortex tip cavitation, i.e., cavitation-induced erosive effects on blades.
The pressure pore system was developed using CFD modeling and cavitation tunnel tests revealing a reduction in URN by 21 dB and cavitation volume by almost 14 percent. CFD modeling helped the team in clearly identifying where to place holes for optimum propeller design. Here at Cadence, our Fidelity CFD is helping industry leaders such as Damen Marine Components and MMG propellers combat challenges in ship propeller design without compromising on robustness, accuracy, and speed (watch the webinar).
Can Electrification Help Reduce Noises from Ships?
Electric container vessels and ships emit reduced noise and generate lower amounts of greenhouse gases (GHG) in comparison to their fuel-powered counterparts, making them the best available alternative. We have Candela release its modern long-range electric water boat C-8, by merging aircraft technology with advanced software and electronics, to sail in absolute silence. The Candela C-POD motors have made this possible, allowing boats to blast through waters at speed of about 30 knots with no URN. Since the boat is electrically powered, there will be no noise from gears, and requires minimal maintenance. Along the lines, we have Yara Birkeland, the world’s first fully-electric container vessel promising to sail with zero emissions, and Balearia electric boat for cargo ferry, which will be tested for the use of hydrogen as fuel.
Computational Fluid Dynamics and electrification are doing miracles in making the world a safer place for all species to reside while also ensuring that sustainable technological innovations are growing at par. Although we do not have legislative laws for marine vehicles to limit the propeller URN, we should strive to keep our marine vehicles at minimum decibels to avoid the extinction of marine species that support ecological balance.