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CHILDREN'S SCIENTIFIC LABORATORY
The second discovery of cavitation. Children's Science Lab
Directory / Children's Science Lab At the end of the XNUMXth century, the English navy had to replenish two ships that were perfect for that time. "Dering" and "Turbinia" had to pass the last test - for speed, which, by the way, was put forward by the designers as their main advantage. Alas, the estimated speed was not achieved. A detailed study of the possible causes of failure showed that propellers at high speed wear out very intensively, becoming covered with potholes, caverns, and the numerous steam-air bubbles that appear on the blades are to blame. Under such circumstances, technology was first introduced to cavitation. It's the technique. Because science has known this phenomenon for twenty years. It was theoretically predicted by the English physicist O. Reynolds. And if the designers were more attentive to the fundamental research of their compatriot, perhaps there would be no embarrassment. Yes, the theorist could have warned engineers against excessive hopes. But no more. If he had been asked: how to build a truly ultra-fast ship, bypassing cavitation in some way, the scientist would hardly have found an answer. And to this day, after more than a century since cavitation was discovered, the science that studies this phenomenon is indebted to technology. It is not always possible to even make an accurate calculation of the threshold beyond which cavitation, which is destructive for a machine or structure, occurs. It still crumbles, she reveals the metal of propellers, the blades of pumps and turbines, the concrete bodies of dams, canals, locks. Even more difficult - and tempting thoughts about this were not born yesterday - to turn the destructive forces of cavitation and make them allies. Why does mighty modern science give in to the most important secrets of cavitation? First, let's recall what she knows about this phenomenon quite definitely. Cavitation bubbles appear in a liquid if a reduced pressure is created in it. This happens, for example, when flowing around a solid body at high speed or, which is essentially the same, when the body itself is moving rapidly in a liquid. Sound and ultrasonic waves, passing through the liquid, also create areas of low pressure, causing cavitation. Cavitation bubbles live for a very short time. With great speed, in a tiny fraction of a second, they collapse. This collapse, like an explosion, generates a shock wave. Let it be just microexplosions. In brief moments there are hundreds, thousands of them. They overlap each other, multiplying their strength. At different points of the liquid, the temperature instantly jumps up to thousands of degrees, the pressure - up to many tens of atmospheres. The bubbles can have the thinnest sting rays that act on a hard surface like a cumulative projectile that destroys armor! This is where the incredible powers of weightless bubbles come from. Most often, unfortunately, these forces are destructive. Only in a few cases do they begin to work usefully today - for example, they clean the surface of parts, help reveal the natural pattern of finishing stones, mix "incompatible" liquids like gasoline and water. To better fight harmful, destructive cavitation and use it more fully for good, there is only one way - to penetrate deeper into its secrets. What is the difference between a cavitation bubble and a normal one? What's going on inside? What laws govern the transformation of energy in it? If today scientists knew the answers to these questions, you see, tomorrow ultra-fast ships would become real. But so far there are only numerous, arguing hypotheses. And, therefore, the engineer is not able to calculate with the required accuracy a new structure or machine in which he would like to harness the forces of cavitation. How insufficient is the knowledge of this phenomenon so far, such an example shows. Almost half a century ago, sonoluminescence was discovered - the glow of liquids under the action of ultrasound, as well as sonochemical reactions that occur only when reagents are irradiated with sound. Both of these phenomena are very energy intensive, and only cavitation can cause them. The effects have become a kind of test for cavitation. However, their mechanism and nature is still a mystery. Why is cavitation so unapproachable? What obstacles stand in the way of its secrets? In order to more clearly imagine the transformations taking place C with a cavitation bubble, one must first of all carefully follow how it is born, moves, disappears, in a word, over all the stages of its life. The cavitation bubble has become one of the main characters of science films. In dozens of laboratories around the world, it has been filmed on countless meters of film. But alas, even ultra-high-speed filming does not keep up with the moments of his life. Our movie hero lives only hundred-thousandths or even millionths of a second! We must also take into account: the size of the bubbles is hundredths, thousandths of a millimeter. Finally, cavitation is not one or even a thousand bubbles born in an instant. In one cubic centimeter of the so-called cavitation field, about a billion of them pulsate at once! It is no coincidence that one of the first heroes of holographic cinema, as soon as it appeared in a laboratory, experimental version, again became a cavitation bubble ... And the mysteries did not decrease. Hedgehogs in a test tube In science, it often happens like this: to solve some complex problem, over which the best minds, armed with the most advanced technology, have been struggling for many years, some very simple idea, some elementary, almost school experience, is missing. In the problem of cavitation, this, perhaps, a decisive step was fortunate enough to be taken by scientists from the sector of chemical physics from the All-Union Scientific Research Institute of Organic Synthesis. While some researchers relied on ever more advanced equipment, the latest methods for solving unusually complex systems of differential equations of bubble motion, VNIIOS specialists were looking for a non-frontal, workaround solution. What was their intended maneuver? They argued something like this. Plainly to see the cavitation bubbles prevents their meagerness and extremely short lifetime. It depends on the frequency of oscillations that excite cavitation. If researchers were able to obtain cavitation, say, at frequencies of 10-100 Hz, the bubbles, according to calculations, could live for tenths of a second and measure up to a centimeter. That's when we would see our movie hero really close-up. Has this simple idea never occurred to anyone before? Of course she came. There were many attempts. An article with the results of the last of them, which was undertaken by American researchers, lay on the desk of the head of the sector, M.A. Margulis. And there is nothing comforting about it. Once again, confirmation of the usual point of view has been received: cavitation is a threshold phenomenon, that is, it occurs starting from a certain frequency, and this frequency is calculated, alas, in kilohertz ... And yet, something forced us to reproduce a deliberately unsuccessful experience. This was prompted by good anger at an intractable problem, and exploratory passion, perseverance, and intuition.
It was not difficult for the Americans to do the experiment. Its scheme was simple: an oscillating rod is lowered into a vessel with a liquid, and the spectrometer, if cavitation occurs, must register the glow. They did everything right - nothing like cavitation. They tried to increase the amplitude of the oscillations of the rod, - they say, the excitation will become more intense. The supersensitive spectrometer is "silent". Seething, turbulence in the liquid increases, but there is no stretching. The liquid is, as it were, too elastic; although it swirls, it still manages to flow around a slowly oscillating rod. But it is necessary that she perceive the vibrations of the rod as if it were blows. How to achieve this? It was enough to exclude the flow around the oscillating rod, and low-frequency cavitation was discovered A new experiment was set up with equipment, which, probably, can be found even in a school physics classroom: a test tube, a tripod, a rod carved from plexiglass, a 25-watt speaker, an old tube amplifier ... Its only subtlety - an oscillating rod in the form of a piston was made in such a way that the gap with the walls of the tube was only a tenth of a millimeter. In this case, the liquid could no longer flow around the rod as easily as before. The sound generator is turned on at a frequency of 90 Hz. About what happened next, M. A. Margulis says: We didn't notice anything out of the ordinary for a minute. Then, in a small area near the wall of the test tube filled with liquid, small spherical bubbles appeared under the oscillating piston. Their number grew rapidly. They formed a large clot, outwardly resembling a hedgehog. This hedgehog was visibly pulsating. We gradually increased the frequency. At 200 Hz and above, it was possible to create two or even more extraordinary hedgehogs. They were born in different parts of the test tube. From time to time they rushed to each other, merged and immediately scattered with a crash. It was immediately evident that hedgehogs did not look like conglomerates - accumulations of individual pulsating bubbles, but were large, bizarrely shaped bubbles ... But not everyone had time to grasp the naked eye. Scientists used their usual tool - high-speed filming. They played the footage, but ... they didn’t find any hedgehogs. Prominences, rather thick appendages, intricately curved tentacles that seemed to be shot out of the body of a large bubble, did not at all resemble the needles of a pretty forest dweller. And scientists gave this unusual creation a more prosaic name - a large deformed bubble (abbreviated as BDP). It was possible to see on the screen how small transparent bubbles of a spherical shape were torn off from the BJP, and then rushed back. What was it? Cavitation generating thousand-degree temperatures, colossal pressures? Or, perhaps, some new, observed phenomenon for the first time? To check, as we already know, there are special tests, a kind of litmus papers that detect cavitation - sound-chemical reactions and the glow of liquids. Breaking down barriers In the very first test experiment, a low-frequency sound easily started a chain reaction of the transformation of maleic acid into fumaric acid. Doubts still remained - although this reaction is reputed to be complex and capricious among chemists, it requires relatively little energy to initiate. But when ferrous iron in a laboratory test tube turned into trivalent iron, when water molecules began to split in it, like nuts under a hammer blow, there could no longer be two opinions - real cavitation was excited. The researchers themselves at first found it difficult to believe their own results. However, multiple checks confirmed that sonochemical reactions can be carried out already at a sound frequency of 7 Hz, and some solutions began to glow at 30 Hz. We are talking about a discovery that can be called hot. Research on low-frequency cavitation has only just begun. However, from the first days they bring interesting results. For example, as soon as scientists saw the BJP with their own eyes and made sure that they cavitate, one of the most authoritative theories of cavitation collapsed. It was believed that opposite charges arise on the surface of the emerging cavitation bubble. At a certain moment, an electronic breakdown occurs. Hence - a large energy release, luminescence, initiation of the most difficult chemical reactions. The only condition for such a course of things is that the cavitation bubble must be ... an impeccably regular lenticular shape. On the screen, as we know, the researchers saw rather some fantastically shaped plant. "Got" not only electrical, but also another - the thermal theory of cavitation. She said: in the process of rapid compression and collapse of the cavitation bubble, the vapor-gas mixture is heated to temperatures of a thousand degrees. At the same time, it naturally begins to glow like the filament of an ordinary light bulb, and the plasma temperature splits the molecules, initiates the most incredible chemical reactions. However, now, as a result of the most thorough research, it has been established that sonoluminescence is the same cold glow as that of fireflies flickering in the night. Almost every new experiment showed the already familiar cavitation from an unexpected side, revealed its extraordinary abilities. Let's say the destructive power of high-frequency cavitation was well known. In a matter of minutes, it could turn the smooth surface of metals into a rough one, chipping rather large particles. Low-frequency cavitation, on the contrary, turned out to be a thin, delicate instrument. It was not difficult for her to smooth, polish the roughest surface, gouging out only microscopic particles of metal. cavitationLow-frequency cavitation easily and quickly prepared emulsions from liquids immiscible under normal conditions, crushed solid granules immersed in a liquid, launched the most energy-intensive chemical reactions ... Of course, ultrasonic, high-frequency cavitation can do all this. But to create it, as you know, you need special equipment, generators. Now connect the source of oscillations to the network that feeds the home radio, and all the useful abilities of cavitation are at your service. For example, it is necessary to mix substances with the utmost care and speed in a chemical reactor with a capacity of several railway tanks. This task is the most ordinary, common for the chemical, pharmaceutical, microbiological industries. The traditional solution: as a stirrer, they take something like a propeller or screw screw, made from the most expensive, chemically resistant alloys. And you can mount a simple source of oscillations in the reactor, plug it into the outlet of a conventional network - the effect, according to calculations, will be even better. It is unlikely that anyone will be able today to predict the various practical applications of the "second" discovery of cavitation. So far, it only clears the way for a deeper understanding of this most interesting phenomenon, overturns the barriers that have stood in the way of researchers for many decades. Understanding the true mechanism of cavitation, how and where its extraordinary forces arise, is still ahead. And behind it, as always happens in science, there are new opportunities for an engineer, designer, technologist, which are impossible to foresee today. Author: L. Galamaga
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