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07 November 2018 09:00
"I'm a heart / dandelion in the past / I climbed into a breeze / I'm going to your side."
In 1985, a song by singer Park Mi-kyung entitled "Dandelion Hall" is part of the lyrics. The taste for food and singing seems to be somewhat regressive, but that seems to be the case. Even though there have been a lot of hits in the last generation, my favorite song is the 8090, which was young. After using YouTube, I often watch videos at that time and listen more often.
Among the songs of the previous generation, the works ranked among today's famous songs seem to be as good as the songs. For example, if you listen to a song and listen to the lyrics, it will be a "poem".
Until this song was released, the dandelion would have been a "flower not knowing the name of a single pin" on the road to many people. Through the words of this song, Dandelion Hall has become a symbol of the desire to go to the elderly while being carried by the wind.
However, there is another serious problem to say "jade tea" in the lyrics of this song. It's called 'dandelion hall'. because Hall is a Korean word that means spore. Of course, seeds such as dandelion seeds should be called "dandelion seeds". Specifically, it is called 'dandelion fruit', but that's called 'seed'. because it has no pulp and the dry skin is wrapped around the seeds.
A dandelion flower (left: actually a flowering of 100 flowers) and a seed (actually a fruit) attached to the end of a peduncle after the formation of the flower. This dandelion was a seed used in the experiment. – Provided by Wikipedia
Perhaps the lyricist at that time, after the dandelion bloomed, about 100 seeds (fruits) were formed and the ball was formed. Unfortunately, however, in the Korean dictionary, Hall points only to spores, and since there is no similar meaning, I will write "Dandelion seed".
It has the same function as a parachute
A generation ago, when Korean musicians gave artificial immortality to dandelion seeds, British scientists recently discovered dandelion seeds and found a place in the history of fluid mechanics, a field of physics. Scientists from the University of Edinburgh in the UK have discovered the hydrodynamic mechanism by which dandelion seeds can fly off the flowerbed and "fly over".
Plants have developed various strategies for spreading seeds. In the case of dandelion, this has created a dirt-like structure that maximizes resistance to gravity. A bundle of hair (called a lid) on the other side of the seed causes resistance to the air. If the seed is a person, the head is a parachute.
The majority of dandelion seeds fly off the peduncle and fall to the ground without exceeding 2 m. But the lucky ones (when it's hot, dry or windy) travel more than 100 km.
But the parachute is a garment, while the hair is a structure with about 100 fine filaments extending from one side to the other of the seed. The length of the filaments is 7.4 mm on average, but the average diameter is 16.3 μm (micrometer, 1 μm = 1/1000 mm), which is only one-fifth of the hair diameter. If you look at the bristles from above, you will not be able to get the airborne effect of the parachute just after the filament lock.
For reference, the value obtained by dividing the area of a section of a section by the total area is called "porosity" and the porosity of the bristles on the top is 0.916 on average. It is said that the proportion of the surface occupied by 100 filaments represents only 8.4% of the disk.
The researchers conducted a wind tunnel experiment to see how these mediocre dandelion seeds could withstand the air and fly for a long time. The dandelion seeds were placed in a cylinder of wind that was blowing up and down, and the airflow around the seeds was observed with the speed of the air adjusted to levitate.
As a result, we found that a small swirling ring had been created and held at the top of the lid. The vortex had the effect of reducing the wind speed on the yaw rod by 10%, which allowed the pressure to be lowered and the lift increased due to lift.
The average termination speed obtained from the wind tunnel test (the final velocity at which objects falling into the fluid that can generate a resistive force can reach) is only 39.1 cm per second. For reference, the termination speed of raindrops with a diameter of 2 mm reaches 7 m per second.
The researchers used these data to calculate the resistivity of dandelion seeds. The resistivity factor is proportional to the resistance force (identical to the gravity and the opposite direction) and inversely proportional to the square of the end velocity of the object and the blocked section. In the case of dandelion seeds, the diameter of the disc is 13.8 mm, but the cross section is only 12.6 mm 2, which is plugged by a porosity of 0.916. That's the number of resistors.
(D) View of dandelion (a) and top (pappus) seeds (b), enlarged image of the filament (c) Below, a wind tunnel test image showing a stable vortex at a wind speed of 60 % of termination rate (e) and termination rate. (G) If winds of the same velocity as e are sent to a large section disk, it becomes an unstable state in which eddies are released by turbulence. On the other hand, when a wind at the same speed as f is blown on a porcelain disk with a level of porosity, a stable vortex is maintained on the upper face. – Provide the "nature"
A 8.6 mm diameter disc should be made to form a disc with the same mbad (same strength) as the dandelion seed and with a completely clogged section (porosity 0) to reach the same final velocity. The diameter is 38% smaller than that of the brown blade, but it is capped at 74 square meters. Dandelion seeds have a much more economical structure due to the fact that the pod structure has a large padding structure and the resistance coefficient is four times or more. So how could dandelion seeds slow down the rhythm of the end with a bunch of filamentous filaments?
Do not swim but swim
The Reynolds number is another important parameter of fluid dynamics. The Reynolds number is the ratio between the inertia force and the viscosity force. When the Reynolds number of an object placed on a fluid is small, the flux becomes stable in a certain state and, if it is important, it is likely to become an unstable state such as turbulence.
The Reynolds number is proportional to the speed of movement of the object (or fluid) and to the characteristic length of the object (in the case of a dandelion seed, the diameter of the lid ) and is inversely proportional to the density of the fluid (in this case, air). The average dandelion seed Reynolds number at the terminating rate is 357, which is greater than the number of Reynolds 222 of fouled discs having the same resistances and termination rates (due to the longer characteristic length). According to this, the air around the dandelion seeds that falls at the final speed seems to be quite unstable, but that is not the case. Why
The researchers found the answer in the Reynolds number of the clavicle filament. Reynolds can also be seen when the hairs of a dandelion seed are visible on a disc, but Reynolds can be targeted for individual filaments. In this case, the characteristic length is the diameter of the filament, which is only 16.3 μm on average. The Reynolds number is therefore very small (less than 1) and the viscosity of the fluid (air in this case) therefore plays an important role.
As a result, the airflow around the filament is slowed down and the spacing between the filaments is only a few hundred micrometers, giving a semi-permeable membrane as a whole. As a result, a stable vortex ring is formed with a slight distance to the back (upper face of the head) and an uplift occurs. The researchers have succeeded in reproducing the same phenomenon in the wind tunnel experiment by treating the silicon disk to obtain the level of porosity of the dandelion.
Insects with a length of about 1 mm can also fly with wings arranged in filament. Because the Reynolds number is very small, the viscosity of the air plays an important role. It's an insect of the genus Arescon, a kind of fly fairy. – Provided by Wikipedia
Evenly spaced hair systems, such as dandelion hair filament arrangements, have a very small Reynolds number, so even viscous fluids, such as air, have a more viscous influence. Therefore, in this case, even if you do not make the film, you can see the effect of the film, you can save the material and you can lose a little weight.
It is the same principle that very small insects can fly with thin wings. The photo on the right shows an insect wing in Arescon, one of the fairy flies, a parasitic fly. This structure will have less lift than the membrane wings, but less material for the wings and less force when you wave the wings.
When the dandelion seeds are fixed and the wind of the terminal velocity is blown down (corresponding to a free fall), we see that the spiral loop is formed and is stably maintained on the seed fly. The average termination rate of dandelion seeds is only 39.1 cm per second, and long stay in the air has been found to cause lift with the swirling ring, which increases air resistance. .
As you can see on the scale at the bottom left, it is a very small and light insect whose body length is less than 1 mm. It is more appropriate for them to swim than to fly in the sky.
The document containing the results of the study was published in the journal Nature on October 18 and was also covered in the editorial of the same issue. This is a great example of the fact that we can sometimes find amazing discoveries in a familiar environment. The editorial says, "There are in the world many unknowns smaller than the atom, larger than the galaxy and billions of years old. But we still have secrets in objects that we think are obvious, "he concluded quoting a poem by the English poet William Blake.
Sky in a wild flower – "Auguries of Innocence"
※ About the author
Kang Suk-ki, scientific columnist ([email protected])) He worked as a researcher at the LG H & H Research Institute and worked as a journalist at Dong-A Science from 2000 to 2012. He has been an independent writer since September 2012. There are works such as "Café Scientifique de Gangseokgi "(1-7)," Finding the Origin of Life Sciences ", etc., and the translated works include" Antimatter "," Heart Story "," Proof:
07 November 2018 09:00
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