Geometries and Virtualwind Entities
Before you start to build a model, it is important to be familiar with Virtualwind's "Entities", and how they relate to various types of geometry. Here, only a brief overview of the three that relate to the SketchUp Plugin will be addressed, and the concept of "watertight geometry" will be introduced. This concept will be expanded in the next section.
Watertight Geometry: An Introduction
As far as Virtualwind is concerned, including the Virtualwind SketchUp Plugin, "watertight geometry" is geometry that obeys the rule "every edge must have exactly two faces". For now, it is sufficient to understand "watertight geometry" as referring to three-dimensional geometry, as opposed to two-dimensional (or one-dimensional) geometry.
An archetypical example of watertight geometry is a cluster of houses, or modern skyscraper. In contrast, a fence is a good example of the opposite, "non-watertight geometry". As you will see in the next section, the real definition is more subtle – for example, you can model a house as a "non-watertight object", and you can model a fence as a "watertight object" – but for now, think of watertight geometry as "something closed, like a building" and non-watertight geometry as "something open, like a fence or an awning."
Buildings and Canopies
"Buildings" and "Canopies" are the two types of "structural entities" in Virtualwind. If you model the Petronas Towers in Kuala Lumpur, it will become a series of Buildings and/or Canopies, depending on how it is modelled and how it is grouped.
The difference between Buildings and Canopies is whether the geometry passes or fails the golden rule of watertightness: "every edge must have exactly two faces". Watertight geometries generally become Buildings; non-watertight geometries always becomes Canopies. (You can make watertight geometry into a Canopy, but you generally would not want to; you can never make non-watertight geometry into a Building though – it has to be a Canopy.) Of the two, the Building Entity is the preferred Entity. Ideally, you'd want all structures to be Buildings: when calculating wind flows around structures, Virtualwind handles Building geometry the most accurately.
However, it is not always easy to make watertight geometry. In some cases, it is very difficult to model something as a watertight object. For instance, if you wanted to model the Eiffel Tower or the Golden Gate Bridge, you would have to model every beam and cable as a three-dimensional object, and you would have to be careful to have no interior faces. Doing that accurately might requires days of modelling time, whereas you might be able to sketch together a Canopy version of the same model in a matters of hours. In practice, however, cases like the Eiffel Tower are the exception rather than the rule: if you know from the outset that you will be running a model in Virtualwind, it is typically straightforward to make sure the model conforms to watertightness rules.
Terrain
As the name suggests, the "Terrain" Entity is intended to represent the surrounding terrain: the nearby bumps and dips in the landscape. You do not have to have a Terrain Entity in a model. In fact, if the land around a geometry of interest is mostly featureless, it is acceptable to not model the terrain. If you do want to include a terrain, however, it can be either watertight or non-watertight, but it has to meet certain requirements when it is not watertight. Technically speaking, its boundary cannot "self-intersect" when projected to the domain's bottom plane (Roughly speaking, the terrain surface, at least at its boundary, has to be a single-valued height function).