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模拟潮湿的沙滩

Simulating Wet Sand | Two Minute Papers #160

亲爱的学霸们 这里是两分钟论文 我是Károly Zsolnai-Fehér
Dear Fellow Scholars, this is Two Minute Papers with Károly Zsolnai-Fehér.
做了近160期节目后 任何人都看得出来
After around 160 episodes into Two Minute Papers, I think it is no secret to anyone
我对流体模拟无可救药的热爱
that I am helplessly addicted to fluid simulations,
那么你们也许已经猜出这一期的内容了
so you can already guess what this episode will be about.
我打赌你们一定会和我初见它时一样
I bet you will be as spellbound by this beautiful footage of wet sand
被这一组漂亮的潮湿沙滩模拟镜头迷住的
simulations as I was when I’ve first seen it.
是的 在你们发问前 我也预先准备好了一些慢动作场景
Before you ask, yes, I have attempted to prepare some slow-motion action too!
正如你们想到的 模拟液体流动与求解
As you remember, simulating the motion of fluids involves solving equations that tell
速度和压力随着时间变化的方程相关
us how the velocity and the pressure evolves in time.
现在 我们所在的三维世界是一个连续体 但我们并不能求解各处所有的量
Now, the 3D world we live in is a continuum, and we cannot solve these quantities everywhere
因为那将耗费无穷的计算时间
because that would take an infinite amount of time.
为了减轻计算量 我们在虚拟世界中放入一个网格
To alleviate this, we can put a grid in our virtual world and obtain these quantities
并且只包含这些网格中的量
only in these gridpoints.
方案中网格的精度越高 动画就越真实
The higher the resolution the grid is, the more realistic the animations are, but the
不过计算时间上也就越显得不足
computation time also scales quite poorly.
我们在视觉效果行业几乎看不到这样漂亮的
It is really not a surprise that we have barely seen any wet sand
潮湿沙滩模拟 这也就不足为奇了
simulations in the visual effects industry so far.
这儿我们有高效的算法来处理这些场景 如你所见
Here, we have an efficient algorithm to handle these cases, and as you will see, this is
这不仅极度耗费计算量 同时稳定性方面也很棘手
not only extremely expensive to compute, but nasty stability issues also arise.
看看这里的一个例子
Have a look at this example here.
这些是关于不同内聚力值的沙地模拟
These are sand simulations with different cohesion values.
内聚力指把物质结合在一起的分子间作用力强度
Cohesion means the strength of intermolecular forces that hold the material together.
内聚力越强 沙子也就越难被打散 沙子的块头就越大
The higher cohesion is, the harder it is to break the sand up, the bigger the clumps are.
这是模拟的一个重要特性 因为沙块的
This is an important quantity for our simulation because the higher the water saturation of
水分饱和度越高 它就结合的越紧密
this block of sand, the more cohesive it is.
现在 如果我们尝试用传统方法在粗糙的网格上模拟这个效果
Now, if we try to simulate this effect with traditional techniques on a coarse grid, we’ll
我们将会遇到一个奇怪现象 就是模拟的时间越长
encounter a weird phenomenon: namely, the longer our simulation runs, the larger the
沙子的体积就会变得越大
volume of the sand becomes.
展示这个现象的一个好办法就是使用沙漏
An excellent way to demonstrate this phenomenon is using these hourglasses,
你可以在几个来回后就看清楚
where you can clearly see that after only a good couple turns,
里面沙子的数量在显著增长
the amount of sand within is significantly increased.
这特别有趣 因为一般在经典流体模拟中
This is particularly interesting, because normally, in classical fluid simulations,
如果网格方案有所不足 我们将会遇到的典型问题是水的体积损耗
if our grid resolution is insufficient, we typically encounter water volume dissipation,
它意味着模拟中的物质总量随着时间减少
which means that the total amount of mass in the simulation decreases over time.
这里却恰好相反 就像变戏法一样
Here, we have the exact opposite, like in a magic trick,
每一回合过后 体积变得越大
after every turn, the volume gets inflated.
这真是一个怪异和充满挑战性的难题
That’s a really peculiar and no less challenging problem.
这方面可以用一个更加精细的网格来改善
This issue can be alleviated by using a finer grid,
就如我们所知的 它是极度耗费计算量的
which is, as we know, extremely costly to compute, or,
或者作者提出一个办法 能解决体积问题
the authors proposed a volume fixing method to take care of this without
同时又不显著增加算法的运行时间
significantly increasing the execution time of the algorithm.
一定要看一看这篇论文 它深得我心
Make sure to have a look at the paper, which is certainly my kind of paper:
大量漂亮的物理学和关于这些方程解法的研究
lots of beautiful physics and a study on how to solve these equations
因此我们能够得到一个高效的潮湿沙地模拟器
so that we can obtain an efficient wet sand simulator.
同样别忘了 一天一篇流体论文 着迷躲一边
And also, don’t forget, a fluid paper a day keeps the obsessions away.
同时说一点关于两分钟论文衬衫的话
In the meantime, a word about the Two Minute Papers shirts.
我总是很高兴看到 你们这些同学发来的
I am always delighted to see you Fellow Scholars sending over photos of
自豪地穿着新获得的系列衬衫摆姿势拍的照片
yourselves proudly posing with your newly obtained shirts for the series.
非常感谢 请继续保持!
Thanks so much and please, keep them coming!
在美国 它们可以通过 twominutepapers.com 获得
They are available through twominutepapers.com for the US,
而对于欧洲以及世界其他地方 链接在视频描述中可见
and the EU and worldwide link is also available in the video description.
感谢您的收看和慷慨支持 我们下次再见!
Thanks for watching and for your generous support, and I’ll see you next time!

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视频概述

一个漂亮的潮湿沙滩模拟,以及其中遇到的物理学和算法问题。

听录译者

收集自网络

翻译译者

B11101001

审核员

审核团1024

视频来源

https://www.youtube.com/watch?v=4Df_BluxwkU

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