#吱吱声#
[SQUEAKING]
#啵#
[POP]
来看看这个
Check this out.
我要在水池里拖拽盘子
I’m just gonna drag this plate through the water in the pool.
就像这样
That’s it.
看看泳池底部出现了什么
But look what forms– perfect black circles
完整的黑色圆圈
on the bottom of the pool.
很奇怪吧
And it’s so weird.
圈圈没有立即消失
They don’t die.
他们只是不断靠近
They just keep going right next to each other.
我把这个镜头调快了 因为这些圆圈
I sped this footage up because the circles
穿过泳池到达对面需要三分钟
took 3 minutes to cross the pool and didn’t
看上去并不会就这样消失
seem to be dying out.
这些圈随着运动变得更加清晰 一直穿越泳池
These got sharper as they went and made it all the way across.
太酷炫了
So cool.
凑近看
And look closer.
你会发现它们是怎么引起的
You can see what’s causing them.
有两个几乎不太明显的小漩涡
There are two almost unnoticeable dips
在水的表面上
in the surface of the water.
为什么他们没有消失
Why don’t they die out?
这些黑色的点是怎么形成的
And how do these dark spots form?
为什么两个小漩涡一起运动
And why do the dips move together?
好多问题啊
So many questions.
我们先来看看他们是什么
Let’s first look at what they are.
当我拽着盘子在水中移动的时候
When I dragged the plate through the water,
你会发现有小漩涡形成
you could see it forming little vortices,
就像你在水中划桨产生的漩涡一样
like you get when you move a paddle through the water.
涡流是由一种气体或液体做旋转运动
A vortex is a gas or a liquid moving
并随着我们称之为的涡流线一直向上
in circles with what we call a vortex line going up
穿过圆轨
through the circles.
涡流现象无处不在 龙卷风 流向下水道的水
They are everywhere– tornadoes, water flowing down
飓风 或是
the drain, hurricanes, or the movement
飞机两翼上方空气的运动
of air over an airplane wing.
涡流通常出现在流体速度有差异的位置
Vortices often form when there’s a difference in fluid speed,
比如速度快的风和速度慢的风之间
like fast wind moving over slow wind.
所以当你将盘子向前移动时 右面
As you move the plate forward, the water
靠近盘边缘的水受到摩擦力和盘子一起
right next to the boundary moves with the plate
运动
because of friction.
距离盘子较远的水则没有动
The water further out is not moving.
速度上的不同产生了剪力
It’s that difference in velocity that causes a shear force
力使水流变成涡流旋转
and starts the water spinning.
盘子前面的水受到两边的力也产生了速度差 加速了水流旋转
This is accentuated by the water in front of the plate being
而盘子后面的水
forced around the sides and the water
在盘子向前移动的过程中
behind the plate being sucked forward
被吸到前面
as the plate moves along.
合成的漩涡变得更强
In this coherent vortex, our dips
所以它能够穿过泳池 比水波走的更远
can travel much further across the pool than a ripple
而不消失
without dissipating.
为什么
Why?
因为 涡流的角动量不变
The vortex wants to keep its angular momentum.
他一边穿过水池 一边旋转
So it continues spinning as it moves through the water
只在有摩擦的时候损失部分能量
and only loses some energy through friction
而摩擦来自周围不同的水
to the surrounding stationary water.
左右两边的原理是相同的 因此有两个漩涡
So what we appear to have here is two vortices
在穿过泳池
moving across the pool.
但是他们同步运动 这样有些奇怪
But it seems weird that they stay right together.
当我们制造这个涡流时
Well, when we create this vortex,
我们给整个盘子周围的水一个速度
we’re forcing water to move around the whole plate,
并不仅仅是水的表面
not just on the surface of the water.
涡流线是弯曲的
Our vortex line is curved.
我们向前推盘子 制造了半个涡环
We’re creating a half vortex ring.
就和吐出的烟圈 泡沫环 甚至是爆炸中上升的烟雾一样
You know, like a smoke ring, bubble ring, or even the plume
只不过用盘子推出的涡流只有半个圈
rising up from an explosion, but half of that.
在水表面的这两个涡流
These two dips on the water’s surface
是由一个半圈涡流演变而成的
are just the ends of the half ring.
所以他们互相联系
They’re connected.
这也是他们同步运动的原因
And that’s why they stay together.
要证明两个漩涡之间是一个半圆的涡流 只需
We can see this if we put a little food coloring
在一个漩涡的末尾加入食用色素
into the ends.
色素从一个漩涡的末尾运动到另一个漩涡上
The food coloring travels from one end to the other.
你可以看到完整的半圆
And you can see the entire half ring.
太酷了
So cool.
我们刚刚不用气泡就制作了一个泡沫环
We just made a bubble ring without the bubble.
这也同样告诉你一些有趣的事
This also tells you something interesting.
半环并不是波
The half ring is not a wave.
波是通过媒介传播干扰运动
A wave is a disturbance moving through a medium that
它只传递能量不转移物质
transfers energy but doesn’t transfer matter.
当我们滴入食品着色剂时
When we added food coloring, we saw clearly
我们看得出涡流是传递物质的
that the vortex is transferring matter,
就像吐出的烟圈
just like a smoke ring does.
所以遗憾的是 这并不是波
So sadly, it’s not a wave.
它比波要更酷
It’s cooler than a wave.
现在我们回到这个半环的涡流
Now back to this half-ring vortex.
它还奇怪吗
Isn’t it weird?
你之前可能没有看到过一半的烟圈
You’ve probably never seen half of a smoke ring before.
为什么呢
Why?
因为根据赫姆霍兹定律
Well, according to Helmholtz’s law,
涡流线不可能在液体中产生和消失
a vortex line cannot start or end in a fluid.
所以涡流线要么近似为圆
So a vortex line either wants to close into a loop
或者要么在边缘衰减至零
or end at a boundary.
如果你想把一个烟圈分成两半
If you tried to cut a smoke ring in half,
烟圈要么消散没有了 或者更酷的
it would either break up or, even cooler,
分裂成两个较小的烟圈
it would break up into two smaller smoke rings.
我不知道会不会分成两个圈
I don’t know if that happens.
如果是 那一定棒极了
It would be awesome.
由于涡流线不能终止于液体 他就只能终止于表面
Since the line can’t end in the fluid, it ends at the surface.
你几乎从没见过半个涡流
You almost never see these half vortices
因为他们必须发生在
though because they have to occur
两种媒介的交界处
on the boundary of two media.
若你能想到其他方法 使整个半圆显现出来
If you can think of another way to make the entire half
像烟圈或者泡沫圈一样 请告知我
ring visible, like a smoke ring or a bubble ring, let me know.
那么现在 为什么两个小涡流导致了泳池底部酷炫的两个黑点
So now, why does our vortex create those cool dark spots?
重力 向心力和表面张力三力平衡
Well, the balance between gravity, centripetal forces,
制造了像透镜模样的
and surface tension creates a funnel shape
漏斗状凹陷
which acts like a lens.
透镜的工作原理是 把光线聚焦到一个点
Lenses often work by focusing light to a point,
像这个菲涅尔透镜 就能在太阳光下聚焦 融化硬币
like this Fresnel lens, which can melt pennies in the sun.
好像离题了
But I digress.
言归正传 我们的涡流棱镜透镜组把远离中心的太阳光
Our vortex lenses focus the sunlight away from the center,
像这样聚焦
like so.
所以你看到在暗圆的外围有一个明亮的圆
And you have a bright ring surrounding a dark circle.
这个亮圆使得暗圆更明显
The ring highlights the dark circle further.
并且你看到的这些黑点更清晰
And you get these incredibly clear black spots.
酷毙了
So cool.
现在我强烈推荐你们在家做这个试验
Now I highly recommend trying this at home,
特别是当你在游泳池游泳的时候
especially if it’s too cold to swim in the pool.
以下是一些建议
Here are some tips.
一 水面必须非常平静
The water has to be very still.
即便是微风也使得实验难度大大提高
Even a small breeze makes it harder to create these.
二 必须有太阳 否则观测不到什么
The sun has to be out, or it won’t work.
还有一些有趣的事 你也可以试试
And you can try some other really cool things.
比如让涡流沿着泳池壁运动
Send your vortices at a wall.
你将看到黑点散开
You’ll see the dark spots spread out.
或试着使它们碰撞
Or try colliding two of them.
实验愉快
Happy physics-ing.
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