While it’s still hot in September, we explain how a motorboat’s propeller creates spiraling air bubbles in the ocean and how this phenomenon is connected to the basic laws of fluid dynamics. This phenomenon, while simply beautiful, is actually related to cavitation, an important engineering problem, and research is being done to solve it.
September is already halfway over, but the weather in South Korea is still as hot as it was in the summer. With temperatures in the low 30s for days on end, office workers and students may be taking solace in reminiscing about their summer vacations. When you think of summer vacations, you probably think of fun in the water, and if you’ve ever seen a motorboat on the ocean, you may have noticed the beautiful spirals that emanate from the motor. This is the shape of the bubbles that the motor spins rapidly in the water. It looks a lot like air blowing out of the blades of a propeller. But propellers don’t have holes to blow air through. So why do we see air bubbles that look like this?
To understand this phenomenon, we first need to look at the nature of fluids. Fluid mechanics in general can seem complicated, but once you understand the basic principles, you can easily explain this phenomenon. For example, the flow of fluids can be predicted in many situations, which allows for a variety of engineering designs. Understanding fluid flow plays an important role in many fields, including marine, aeronautical, and mechanical engineering. However, even small and trivial phenomena that we encounter in our daily lives are closely related to these principles.
First, if an object is moving in a certain fluid, the fluid near it will move as if it were attached to the object. Of course, not all fluids behave like this. Fluids that move in this way are called viscous flow, and the opposite is called inviscid flow. Water is a typical viscous fluid, which means that water near the propeller of a motorboat instantaneously moves at the speed of the propeller, as if it were attached to it. This property of fluids applies not only to movement in water, but also in air, and has important implications for the aerodynamic design of an airplane wing or a car.
Second, fluids are governed by a single equation of pressure, velocity, and height at a constant temperature. A simple way to look at this equation is that the sum of the three components is constant. It is this equation that allows us to know the pressure of a fluid at a point by measuring its velocity and height, which are usually easy to obtain. This is the famous Bernoulli equation, and it’s a very important equation that is the basis of fluid mechanics. It was proposed by the Swiss mathematician Daniel Bernoulli in the 18th century and has been used to solve a variety of fluid problems ever since. In fact, Bernoulli’s equation has applications not only in physics, but also in economics, biology, and many other fields.
Once you understand these two properties, you can get a good idea of how the state of water will change. Water near a propeller will instantaneously have a very high speed of motion along the propeller. Since the height of the water does not change, this will result in an instantaneous decrease in pressure, which is governed by Bernoulli’s equation. As the pressure decreases, the saturation vapor pressure increases, which means that the liquid water is more inclined to turn into a gas. This causes steam to form on the surface of the propeller, and in the water, this steam appears as bubbles. This phenomenon of bubbles forming due to rotational motion in a fluid such as a propeller or turbine is called cavitation.
Cavitation usually occurs at the tip of the propeller, and the reason for this is related to the speed of the fluid. If the propeller maintains a constant rotational speed, the tip farthest from the center axis will have the highest speed. Therefore, the pressure drop will be the greatest at the tip, and the probability of cavitation will be the highest. That’s why the air bubbles coming out of the back of the motorboat are shaped like the helical structure of DNA. These spiral structures are beautiful, like a glimpse into nature’s secrets, and provide an interesting opportunity to observe scientific phenomena in everyday life.
When we look at them from the beach, cavitation can be beautiful. But in real-world engineering, cavitation is a huge headache. In pumps, turbines, and other devices that control the movement of fluids inside a body, the constant presence of cavitation wears down the surface of the device. The bubbles created by cavitation move at high speeds through the fluid, which quickly travels to areas of high pressure where the bubbles are destroyed, causing a loud impact and generating noise. This noise is a critical issue in the design of submarine propellers used for military purposes, as it can increase the risk of exposure to the enemy. The problem becomes more serious if the bubble break occurs near the propeller. The impact of the destruction can damage the propeller. The impact and damage that each bubble causes to the propeller is, of course, very small. However, unlike a motorboat, these structures have to run continuously, which means that cavitation is constantly occurring. Therefore, cavitation interferes with structures that need to maintain a certain level of durability.
Therefore, many researchers have been working on ways to prevent cavitation. Some of the simplest ways are to limit the speed of the propeller, so that the machine can only run at a speed that prevents cavitation from occurring, or to make the blade length of the propeller as short as possible. Others are to lower the position of the pump as far down as possible, since the Bernoulli equation that explains why cavitation occurs has a height component. Still others are to reduce the overall pressure and velocity of the fluid, so that the bubbles are less likely to break. If you can change the type of fluid, you can also use a fluid that is as non-viscous as possible. However, all of these methods can limit the performance of the machine or be economically inefficient. Therefore, recent research has focused on developing models of propellers that suppress cavitation. This could make a significant contribution to the design of advanced aircraft engines, improve the durability of offshore structures, and ultimately play an important role in increasing energy efficiency.
So far, we’ve seen how cavitation works to create beautiful spiral shapes. Hopefully, this article has put to rest your curiosity about how air bubbles form in a propeller from nothing. Understanding the complex principles behind scientific phenomena will enrich our daily experiences and help us to appreciate the small miracles we find in nature.
