Translated material
Spreading stream
All day yesterday I remembered:
Problem 9. "Spreading stream." A stream of water falls on a horizontal plane and spreads radially. At some distance from the center, the thickness of the water layer increases sharply. Explain the phenomenon.
This is one of the tasks of the 1997 Young Physicists Tournament . The meaning there is this: teams receive about 20 vaguely formulated research problems for a year, try to move as far as possible in solving them, in understanding the physical meaning of this or that phenomenon or in building its mathematical model, and then they meet in “physics battles”, this is something... something like a debate tournament, when one of the teams defends its model for solving a particular problem, and the other acts as an opponent. I was the captain of the USU Scientific Research Center team at the international TUFF in 1997 ( here is a detailed story about how it happened ), and the problem about the spreading stream was in my area of responsibility, I prepared it and would have to report if our team was called to it .
Many years have passed, but in general I seem to have been able to remember the model and, in general, the logic for solving the problem. I wanted to write yesterday, but there was a lot of running around Tallinn and a lot of meetings, and I just didn’t have time. But now there is no longer any need: the hysteria seems to have passed, and many balanced publications have appeared, the best of which is probably the post by physicist Kirill Pankratov . (And I highly recommend Ivan Kolpakov’s digest on Lente.Ru ). Pankratov’s post contains the same conclusion that I recalled: when water spreads over the surface, a hydraulic jump necessarily occurs in a certain place with the formation of a high wave; This is the place where the lower layer of water slows down, and subsequent flows begin to “roll” and layer on it. Turn on the water tap in the bathroom and make sure. As far as I remember the experimental data, the height of the “shaft” can be 50-100 times greater than the thickness of the water film of the diverging jet. Now, instead of a stream of water from the tap, turn on the shower and direct it not at a flat surface, but at a curved one, for example, at the place where the side surface of the bathtub meets the bottom - and you will see the same hydraulic jump, only not in a circle, and in the lower part, at the bottom of the bath.
Alas, instead of a shower there was a downpour, instead of a bath there was a mountain range, in a semicircle like a bucket, covering the valley. And the hydraulic jump occurred in the unfortunate city of Krymsk. (The distance at which it is formed depends on many parameters, first of all, on the volume of falling water. The city has been standing in this bucket for a long time, but unfortunately it was necessary for a strictly defined confluence of rainfall parameters to happen in order for the wave to form precisely in the city itself, not below it and not above it).
In general, what was happening in the information space left an extremely difficult impression. That very uncritical thinking to the fullest, the inability to question and check the simplest facts with your mind. The fact that the Neberdzhaevsky reservoir does not have a lower gate. The fact that it contains about 7 million cubic meters, and the area of Krymsk, easily measured using Google Maps, is more than 20 sq. km., That is, even if we assume that the entire reservoir moved at once to Krymsk - which would not have happened even in the case of instant its descent - this would give a thickness of the water layer of 35 cm. The fact that the catchment area of the Adagum River, coinciding with this very bucket, is hundreds of square kilometers, and with 300 mm of precipitation, this is tens of millions of cubic meters of water, many times more than in the reservoir, so the “overflow” that took place, could not make any contribution to the overall picture of the disaster... To understand all this, you don’t even need to have experience in solving school problems in physics, you only need arithmetic and common sense. But alas... And in the end, all this nonsense discredits many good and correct thoughts that are expressed mixed with it.
Local authorities have their own repertoire. Their precipitation rate is sometimes two months, sometimes five months, sometimes annual. They could not explain the situation with the reservoir; they denied overflows. Well, the peak of everything - “we couldn’t go around all the houses in three hours.” Notification - creeping line on TV. At night. Yeah, priceless. The habit of lying and the habit of hoping for chance... (It would be interesting to know how many such meteorological warnings about possible flooding as a result of heavy rains have been over the past few years. Surely, many, and every time they “got around.” But this time it didn’t nothing happened).
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All morning today we have been discussing with the Volunteer Movement volunteers how we can help Krymsk from Yekaterinburg. The problem is: transporting cargo from Yekaterinburg to Krymsk will take from 3-4 days (if we find a gazelle or a truck on our own and negotiate privately) to 1 week (if through a transport and logistics company), plus 3-4 days for collecting the cargo. We cannot predict what will actually be needed there in 1-2 weeks - it’s easy to assume, let’s say, that there will be a lot of clothing there, and something else that is not obvious will be sorely lacking. Therefore, after a heated discussion, we decided not to collect things, but to transfer money to the volunteer headquarters working at the site of the natural disaster, here are its coordinates . It’s “more impressive” to collect things here in Yekaterinburg, but managing money on the spot, taking into account operational expediency, is still much more effective.