RealFlow在线教程翻译（11）——SPH - Particles (Liquid) （液体粒子）

（以下英文部分及图片皆来源于RealFlow官方文档：http://support.nextlimit.com/pages/viewpage.action?pageId=22217380）

This type represents the standard “watery” substances, but is also used for highly viscous fluids, like chocolate or honey. RealFlow's liquids simulate very fast and they are perfectly suited to all kinds of projects with a demand for high detail, because they show even the finest drops and splashes. The liquid particle-based approach can be used for any field of operation where traditional fluids are required. In combination with daemons they open the door to a pretty-much endless variety of effects and applications.

该类粒子（Particles (Liquid)）用来模拟基本的液态物质，有时也用来仿真类似巧克力和蜂蜜的高粘度流体。RF的液体仿真非常快并且对所有对细节要求较高的项目都非常适用，因为它会展现最佳的液滴与水花。基于液体粒子的方法可以用在需要传统液体的任何操作领域。通过与daemons结合，开启了一扇通往无限效果与应用的大门。

Type（类型）

This submenu allows you to define the fluid’s behaviour. By default, RealFlow generates fluid particles used for all kind of liquids. Fluid particles can interact with other fluid particles and support self-collision. They are affected by any of RealFlow’s internal or imported objects, including RealWave surfaces.

可通过此子菜单定义流体行为。默认情况下，RF生成用作所有类型液体的粒子。流体粒子之间可以相互作用也支持自碰撞。它们受包括RealWave surfaces在内的RF所有内部对象与导入对象的影响。

“Gas” is used to create substances like air. Gas particles tend to very strong expansion and high velocities. RealFlow gases are not grid-based and therefore behave completely different from other solvers.

“Gas”用于创建像空气一类的物质。Gas粒子易于扩散并且速度很快。RF气体不是基于网格的，所以其表现与其他求解器完全不同。

“Liquid” is RealFlow’s standard setting and provides parameters for all watery or high-viscous substances. It is surely the most often used type and is suitable for all kinds of particle-object interactions, but especially for small to mid-range simulations. For larger projects, you should consider using the new grid-based fluid solver "Hybrido".

“Liquid”是RF的标准设置，为所有类似水的或其他高粘度物质提供参数。所以它理所当然是最常使用的类型，并且适用于所有类型的粒子相互作用，特别是对小至中等尺度的仿真非常适用。对于更大的项目，可以考虑使用基于网格的新流体求解器“Hybrido”。

“Dumb” particles are perfectly suited for fast calculations of secondary effects, like spray or foam. Please keep in mind that these particles cannot react with each other and will not be affected by other emitters.

“Dumb”粒子特别适用于次生效果（如水花和泡沫）的快速计算。请记住，这些粒子不能相互作用，并且不会被其他emitter影响。

“Elastics” establishes a so-called spring-mass system between the particles, making them behave like a soft body. This option can be used for interesting effects like jelly-like fluids or expanding and contracting substances. Please note that elastic particles are not ruled by RealFlow’s soft body solver – that is a completely different system and can only be used with objects.

“Elastics”在粒子间建立了一个所谓的弹性质量系统，使粒子表现地类似于soft body（柔体）。该选项可以用来实现果冻一类的流体或扩散与收缩物质等有趣的效果。请注意，弹性粒子并不被RF的柔体求解器约束，它是一个完全不同的系统，并且只能用作objects。

“Script” gives you the opportunity to write your own fluid behaviour with Python scripting or C++. Choosing this type adds an “Edit” button to the menu. Clicking on “Edit” opens a new scripting window with a predefined function that is used to enter your source code. By default, this window does not contain any executable code.

“Script”可以让你通过Python脚本或C++来自定义流体行为。选择该类型会在菜单中新增一个“Edit”按钮。点击“Edit”将开启一个新的带有预定义的功能的脚本窗口，用于输入源代码。默认情况下，该窗口不含任何可执行代码。

Resolution（分辨率）

With this setting you can increase or decrease the amount of particles, and therefore it is one of the most important parameters. “Resolution” especially depends on scene scale and emitter scale, but it also affects the fluid’s mass and therefore depends on “Density”, too. By default, resolution is 1.0. A volume of 1 x 1 x 1 units filled with 1,000 particles has a mass of exactly 1,000 kg. In other words: Each particle has a mass of exactly 1.0 kg. By raising “Resolution”, an individual particle’s mass will be lowered and vice versa. You can monitor the relationship between mass and density under

通过本选项可以增加或减少粒子的数量，所以它是最重要的参数之一。“Resolution”取决于scene规模与emitter规模，但是它也影响流体的质量，所以也取决于“Density”（密度）。默认情况下，该参数为1.0。一个1 x 1 x 1单位的体积填充1000个粒子，其质量为1000kg。换句话说：每个粒子每个粒子重1.0kg。通过增大“Resolution”，每个单独粒子重量将减小，反之亦然。可以通过以下路径观察质量与密度的关系

Statistics > Particle mass

Dependent on your scene, “Resolution” can dramatically increase, and values of 1,000 or even much higher are sometimes necessary. Another interesting issue – especially for scripting and plugin development – is the relation between “Resolution” and a particle’s radius, ruled by this formula:

“Resolution”可以根据所需场景显著增加，1000或更大有时也是需要的。另一个有趣的问题——是特别针对脚本或插件开发的——“Resolution”和粒子半径之间的关系，由以下公式确定：

radius = 1.0 / (1000.0 · Resolution^1/3)

Density（密度）

“Density” is defined as mass per volume unit and is expressed in kilograms per cubic metre [ kg/m3 ]. Water, for example, has a density of 1000kg/m3. “Density” is different for each substance and for some fluids, like crude oil or honey, there are only average values available, because they consist of variable amounts of ingredients.

“Density”定义为单位体积的质量，单位是kg/m3。以水为例，密度为1000kg/m3。不同物质与流体的“Density”不同，例如原油或蜂蜜，只能得到密度的平均值，因为它们由不同组分组成。

Int Pressure（内部压力）

Each fluid shows a more or less strong tendency to expand. With gases this behaviour can be observed best, but even liquid substances have this tendency. “Int Pressure” simulates the forces between nearby particles, and very high values make the fluid fill a greater volume. If “Int Pressure” is set to 0.0 the particles lose their fluid behaviour.

不同流体或多或少会展现出扩散的趋势。气体的此种趋势最为明显，但是即使是液态物质也有此类趋势。“Int Pressure”模拟相邻粒子间的此种作用，该值越大流体所能填充的体积越大。如果将该值设为0，那么粒子将失去流体特征。

Ext Pressure（外部压力）

This is “Int Pressure’s” counterpart and tries to limit a fluid’s expansion tendency. With very high values it is possible to compress the fluid. Gases, for example, should have very low “Int Pressure” and rather high “Ext Pressure” values to prevent them from flying away. Both parameters are used to fine tune a fluid’s appearance and behaviour, therefore they are fundamentally important for a realistic simulation.

此值与“Int Pressure”相对应，其试图限制流体的扩散趋势。如果此值过高，可能会压缩流体。对于气体而言，其“Int Pressure”应该很小，而“Ext Pressure”应该很大，从而限制气体粒子飞散。以上两个参数都被用于调整流体的外观与行为，因此，若想仿真得到更加逼真的效果，它们很重要。

Viscosity（粘度）

Each fluid has a certain amount of viscosity, even water. It defines the tendency of particles to stick together. With very high values you can observe the typical strings in viscous fluids when they are torn apart. Substances with very high viscosity are honey, tar or syrup, for example. Fluids with low viscosity are alcohol, many solvents or liquid gases. Exaggerated settings can lead to misbehaving particles with high velocities.

每种流体都有一定的粘度，甚至是水。该参数定义了粒子附着在一起的趋势。若该值很大，当黏流被分开时可以看到典型的细流。高粘度的物质有蜂蜜、焦油和糖浆，低粘度的有酒精、溶剂和液态气体。若将该值设置的过大，会产生速度很大的异常粒子。

Surface tension（表面张力）

On a fluid’s surface we can observe forces that keep the outmost molecules together, creating a kind of skin. Some insects, such as water striders, can even walk on this tight skin. Surface tension can also prevent water from infiltrating cloth to dissolve the dirt. For that reason, detergents contain special substances to reduce the fluid’s surface tension”. These are called surfactants or tensides. A water drop’s shape is mostly a result of these forces.

在液体表面可以观察到使最外层分子连成一体并形成一层表面的作用力。一些如水黾一样的昆虫甚至可以在这层表面上行走。表面张力还可以阻止水渗入衣物溶解污渍。所以，会在洗涤剂中加入特殊物质以降低水的表面张力（使水能够渗入衣物，不至于聚集在一起，从而实现溶解污渍的作用），这些物质被称作表面活性剂。水滴的形状大部分也是表面张力作用的结果。

Interpolation（插值）

In some cases it is sufficient to raise the number of particles by selecting “Interpolation” instead of simulating the entire scene again. This feature gives you the possibility to generate more particles from an already cached BIN file sequence, but that is not the real idea behind this feature: “Interpolation” was originally introduced to simulate until a certain frame with low “Resolution”, stop, adjust to a higher value and resume. RealFlow analyses the fluid and places new particles where they will be safe. The new particles are not just filling the gaps, they completely satisfy the fluid engine’s equations.

在某些情况时，可以通过选择“Interpolation”选项来增加粒子的数量，而不必对整个scene重新仿真一遍。该特征使得通过现有缓存的BIN文件序列生成更多粒子成为可能，但是在以下情况时该方法不适用：最开始引入“Interpolation”仿真，直至某低“Resolution”帧停止，调整至高“Resolution”，继续。RF分析流体并将新粒子放在安全的地方。新生粒子不仅仅用于填充缝隙，它们完全满足流体引擎方程。

RealFlow offers three options: “None”, “Local” and “Global”. Of course, “None” does not create any new particles and disables this function. The second options creates the new particles only within the existing fluid cloud with rather high accuracy. Please keep in mind that there are always some particles that will not fit. This is a good choice when the fluid should keep its shape. “Global” can be used when you do not have to worry about the original shape. The new particles will be created with some tolerance leading to slightly fuzzy edges. This solution is perfect for larger amounts of fluids, for example inside water tanks.

RF提供三个选项：“None”，“Local”和“Global”。当然，“None”不创建任何新粒子并使方程失效。第二个选项仅在现存流体云中高精度地创建新粒子。请记住总有一些粒子不会适用。当流体应当维持其形状时是一个好选项。当不必担心初始形状时可以使用“Global”选项，此时，生成新粒子时会有一定容差，并且导致边缘略微有些模糊。该选项适用于大量流体，如水箱内部。

Another important thing about interpolation is fluid-object interaction. If a fluid is close to an object, it is very likely that new particles will suddenly appear inside the body. RealFlow does not check for collisions with objects and only examines the fluid.

关于插值另一件重要的事是fluid与object的相互作用。如果fluid接近object，新粒子很可能会突然出现在body里面。RF不检查与object的碰撞，只检查fluid。

Compute Vorticity（计算涡量）

"Vorticity" hardly contributes to a fluid simulation, but it is a very important and interesting feature when you want to create rendered images. When you read out the BIN files' "Vorticity" channel and add it to add a shader you can create stunning and naturally looking results. The RenderKit mesh engine also supports "Vorticity", as well as the RFRK particle shaders. By default, this property is not simulated and you have to change the value to "Yes" if you want to make use of it. Please note that a particle's vorticity cannot be used to create the typical vortices of gas, fire or smoke.

“Vorticity”对于流体仿真意义不大，但是当你想要创建渲染图时它将是一个重要且有趣的特征。当你读取BIN文件的“Vorticity”部分并着色时，你会创建一个令人吃惊又看起来很自然的结果。RenderKit（渲染工具）网格引擎也支持“Vorticity”和RFRK粒子着色器。默认情况下，仿真时不包含该属性，如果想使用它需要将该值变为“Yes”。请注意，粒子的vorticity不能被用于创建气体、火焰与烟雾的典型涡。

Max Particles（最大粒子）

The pre-adjusted default value is also the maximum value, but it can be changed any time, if you need less. Under “Statistics” you can see two entries counting particles: "Existing Particles" and "Emitted Particles". “Max Particles” is connected to the latter value, and if you want to stop the emitter to create more particles you can enter a smaller value. Please note that "Max particles" is based on the total amount of emitted particles. Let's say you have entered "500,000", so even if you currently have only 1,000 particles in your scene, but have already deleted 499,000 particles, RealFlow will stop creating new particles.

之前调整过的默认值也是最大值，但是如果想变小可以随时更改。在“Statistics”下有两处入口计数粒子："Existing Particles" （现存粒子）和 "Emitted Particles"（发射粒子）。“Max Particles”和后者相连，如果想停止emitter继续生成更多粒子，可以输入一个更小的值。应注意“Max particles”是基于已经发射的粒子的总数的。比如说，你输入“500000”，那么即使现在scene中仅有1000个粒子，并且已经删除了499000个粒子，RF也会停止创建新粒子。