User's Guide

Virtualwind   →   Virtualwind Entities   →   Atmospheric Condition

Atmospheric Condition

Each Virtualwind study has one Atmospheric Condition entity and this entity has only one part: the Wind. As can be found in its model item, the Wind specifies the wind speed, wind direction, and wind profile. It also defines other parameters that are important for numerically solving the simulation, such as the wind resolution, maximum simulation time, and the time step used in the solving procedure.


A Domain must exist in the simulation before an Atmospheric Condition can be created. To create an Atmospheric Condition, access the Entities  →  Atmospheric Condition  →  Create Atmospheric Condition sub-menu item from the Virtualwind main menu. An Atmospheric Conditions item, as shown above, will then appear in the Entities & Monitors tree. Once created, it cannot be deleted, but you can modify its settings as explained below.

Wind Speed, Direction, and Profile

In general, wind speed varies at different heights above the ground. The wind speed specified in the wind model here needs to refer to the speed at a certain height. This height is determined by the wind profile. Virtualwind supports three types of wind profiles, namely Urban, Suburban, and Open, to represent the wind properties in urban, suburban, and rural areas, respectively. The table shown below illustrates the heights where the wind speed (in unit m/s) is measured.


Wind Profile Height Defining the Wind Speed
Urban 500m
Suburban 500m
Open 275m

The wind direction is provided in degrees whose value is relative to the X direction of the world coordinate system. The Actor item of the wind mainly controls the appearance of the inflow arrow in the viewport, such as the size, shading, color, wireframe, and transparency. You can also change the position of the inflow arrow by inputting different X, Y, and Z coordinates relative to the world coordinate system.

Wind Resolution

The wind resolution is used for Domain discretization purposes: the solver solves the air flow within a Domain that needs to be discretized to small rectangular cells (or cages). A fine resolution will generate larger number of cells for the Domain than a coarse resolution. Virtualwind provides five default wind resolutions: COARSE, MEDIUM, FINE, VERY_FINE, and SUPER_FINE for you to choose from and it automatically determines the numbers of cells (measured in the X, Y, and Z axes of the Domain, respectively) for the chosen resolution. You can also customize the wind resolution by choosing CUSTOM and setting the numbers of cells in the three axes of the domain as you wish. When you do this, however, you should make sure that the following constraints are met:

As a general guideline, you may use the coarse resolution (e.g., 32 x 32 x 32) for a quick-and-dirty solution or a pre-pass of your investigation, but you may want to use the fine resolution (e.g., 128 x 128 x 128) for a reasonably accurate simulation. You need to be careful, however, if you go beyond the fine resolution as the memory and time costs for executing the simulation may become prohibitive.

Maximum Aspect Ratio for Cages

The cages are rectangular cells, whose maximum aspect ratio is defined as the maximum ratio among the sizes measured in X, Y, and Z directions.

Estimated Memory

A rough estimation on how much RAM is required to run the simulation. The memory usage depends on the wind resolution and the number and the types of Monitors you set up in the simulation.

Maximum Time and the Time Step

The maximum time (in seconds) defines a time period [0, MAX_TIME] (starting at time 0 and ending at the maximum time) in which you want to simulate the air flow. The time step (also in seconds), again, is a parameter for discretization: it defines the interval of a series of time spot that the simulation is actually solved. For instance, if you specify a maximum time of 125 seconds and a time step of 0.1 second, the solver will then solve your simulation at times 0s, 0.1s, 0.2s, ..., 124.8s, 124.9s, and 125s.

Virtualwind automatically determines a proper time step for every simulation. It is recommended that you do not modify its value unless you are certain that it will not crash the solver.