propellers

Every time a boat crosses the seas, it is accompanied by a continuous noise underwater: that of the propellers that propel it. The noise problem of propellers in marine ecosystems is identified academically since 2004, but its reason for being is even older: the first time they analyzed its cause It was in 1893. What there is no solution to that disturbing low-frequency sound that spreads for kilometers, disturbing fish, cetaceans and other marine living beings. And its reason for being is even older: the first time cavitation was analyzed was in 1893. A team from the Kiel University of Applied Sciences has set out to remedy it with its project MinKav. Brief notes on cavitation. To understand the problem, we must first see what happens to the blades of a propeller when they rotate at high speed. With their movement, the blades generate a pressure difference between their faces. Thus, on the back side the pressure drops so much that the water changes state, going from liquid to gas. More specifically, thousands of small vapor bubbles. The problem is when these bubbles leave that low pressure zone: they then implode violently, returning to the liquid state, which causes pressure waves that are transmitted at high speed through the water. If the waves collide with a surface, they can deteriorate it considerably. The phenomenon of cavitation is accompanied by vibration and noise, as if it were gravel falling on a machine. This sound is broadband, with low frequency components capable of traveling long distances. Why is it important. Of all possible aquatic pollution, human-caused acoustics are the least mainstream, but their effects are documented. A couple of concrete examples of the importance of sound for aquatic species: whales They use sound to communicate, orient themselves and huntthe fish for such essential tasks how to detect predators or spawning and crustaceans are sensitive to vibration in the background. To get an idea of ​​the magnitude of the problem, according to the International Chamber of Navigation There are approximately 50,000 merchant ships operating continuously around the planet and they all emit that sound. It is not something specific. And the research team adds a twist: a propeller with less cavitation is not only less noisy, it can also potentially be more efficient (cavitation is wasted mechanical energy). Less noise and fewer emissions. The discovery. The HAW Kiel team has identified when the problem originates: the sound peak does not occur when the bubble forms, but right at the end of the collapse. And its intensity depends directly on the speed at which this collapse occurs. The faster you go, the stronger the blow. Illustration of human, marine animal and environmental sound sources in the marine environment, with proportional sound waves. National Office of Oceanic and Atmospheric Administration How are they doing it. The experiments are being carried out at the Naval Hydrodynamics Laboratory of the German university, in a kind of aquarium with a miniature propeller, so that they can reproduce the flow conditions around the propeller. Equipped with underwater microphones and high-speed cameras, they have determined where and when that noise peak occurs. The next step is computer simulations to experiment with designing different propeller geometries to reduce noise without sacrificing performance, efficiency or durability. The most obvious solution, lowering the rpm, is not an option: a commercial boat cannot afford to go slower. Pending subjects. However, MinKav started in January of this year, will last three years and have a budget of 390,000 euros, modest for a problem of global scale. Even if MinKav were to come to fruition, it would have to go from the laboratory to scale-up on a commercial ship. In Xataka | A Spaniard has patented a mast that transforms wind and waves into electricity: his invention challenges diesel in boats In Xataka | A “roomba” to clean rivers: the ship that the Three Gorges Dam has launched in China Cover | Pexels