# Fluid dynamics simulations

## Introduction

Simulation

In the previous workshops we made a design based on program, functionality, concept, location and architectural formal and spatial qualities. These criteria and constraints are only a few of the factors which are integrated in the design process. Because the design is made in a digital environment it can use a wide range of different software programs which can add additional support to the design process to further enhance the quality of the design. Due to the exponential development of computer power and fast development of more efficient software the range of capabilities which are available for the designer are increasing rapidly. Interesting is that simulation and analysis software is entering the realm of the concept design phase. This enables the designer to simulate and analyze in various ranges of accuracy physical properties of the design. Like loads within the structure, lighting conditions in spaces, energy analysis and wind simulations. Because this capability is available from the early stages of the design their data can be used to enhance the quality of the design decisions made in these early stages of the design process, thereby having a fundamental impact on the design. This is a part of performative design.

### Fluid Dynamics

voxels
wind simulation

Fluid Dynamic simulations are the latest member in a wide range of simulation and analysis tools which came available for the designer in the concept architectural design phase. The reason for this late entry is the complexity of the simulations. New development in the mathematics behind the calculations and increased computing power makes it now possible to simulate wind flow around object like buildings. Although the accuracy is still limited in the tools used in the concept design phase, the data generated from the simulations can give useful insight of positive or negative effects of air moving around or in the designed building. For more accurate analysis, calculations are made with advanced wind simulation software often in combination with a wind tunnel to verify the data generated by the software.

The Fluid Dynamics simulation uses a special type of geometry to make the fluid calculations possible, called voxels (volume pixels). These are 3 dimensional pixels, basically a set of cubes nicely stacked into a container. Every cube will contain the mathematically defined properties of a fluid and therefore behave like a particle of fluid. A cube (voxel) will behave like one mathematically defined object, calculating the forces, pressure and temperature placed on the cube and then emitting the result to its neighboring cubes. Transmitting effects of heat, pressure and movement throughout the container holding all the cubes. The container is used because it limits the volume which is calculated. This speeds the calculations up and it will define at its boundary the speed, pressure and heat of the fluid set by the user. The result from these settings will then cascade from the boundary layer through the cubes which will recognize if there are any objects within the container which will influence the calculations.

The size of the container and the size of the cubes(voxels) will determine the effort it will take to calculate the result. If the container remains the same and the voxel size decreases by half its length,width and height then the calculation time will increase 8 fold. 2 times 2 times 2. An option could be to use the same amount of voxels but decrease the size of the container. This has however its limits because fluids are effected by its surrounding and by their neighbor voxels, the less information is integrated from its surrounding and neighboring voxel the less accurate the result will be. This limitation at this moment is a result of the computing power it takes to calculate the simulation.

The Fluid Dynamic simulation can, for example, give information about:

• Potential for natural ventilation through various wind speed and pressures in and outside the building.
• Energy generation by optimizing the form of the building to focus the wind in an area where energy is extracted from it.
• Hindrance due to wind, where wind speeds are to high at certain areas around the building.

## Flow Design

For this course we use Flow Design. It is relatively simple to use Fluid Dynamic simulator which can help to determine how the wind moves around the designed building.

### Importing a file into Flow Design

Export geometry from Rhino to Flow design
Import Flow design

Flow Design has no model engine, so any geometry has to be imported from another software. In this case Rhino. Select the parts of the geometry you want to import into Flow Design and use:

Files » Export selected

One of the most robust methods, that is both available for Windows and Mac OS X versions of Flow Design, is the use of STL (polygon based) files. Rhino will convert the (poly)surfaces to polygons, which is the type of geometry STL uses. This conversion is called meshing.

• Select the .stl file type. Set the Tolerance for the STL meshing to 1 mm if you've modeled in millimeters in Rhino. Export the selected objects to the STL file.
If you've installed Moldflow Design Link (part of the Flow Design installation), you can directly import Rhino 3DM files. So alternatively you could also select the .3DM file type. In this case Design Flow will do the conversion from surfaces to polygons. When you import a 3DM (Rhino) file in Flow Design you might find your model brightly colored by Flow Design. If you don't want that, save your file as an STL or AutoCad drawing file.

This file can be opened in Flow Design. When Flow Design is opened it will ask for the user to import a file, to use an Demonstration model or a recently opened file. If a new file is made use the Import option and select the previously saved .STL or .3DM file.

Keep in mind that depending on the computing power of your laptop the capability to handle complex models can vary. If the model is to complex and it takes to much time to generate a useful simulation, you probably have to simplify the model in to simple surfaces with low detail.

Interface flow design

The program has very simple interface. It consists op a limited set of commands and is therefore also limited in the options of fine tuning the simulation. The interface consists of:

• A set of pulldown menu's with general commands like importing and exporting data.
• A toolbar with a set of icons related to the settings of the simulation
• A color scale related to the wind speed
• A Head up Display of the current settings of the simulation

navigating in flow design

There are several ways of navigating through the scene. One could use the mouse buttons and keyboard or the navigation icons visible in the scene.

Using the mouse and keyboard.

• Pan : Moving in a plane. Right mouse button
• Zoom: Scroll weel
• Rotate around the object: Left mouse button
• Pinching; For simulating a multitouch screen zoom in and out option the left mouse button and ALT can be used.
• Rolling For simulating a multitouch screen roll option the left mouse button and CRTL can be used.

#### Settings of the simulation

Setting

The settings of the simulation are limited. They focus around:

• The Resolution of the simulation. This option defines the amount of the voxels in the container and therefore also their size.If the resolution increases and the container remains the same size than the voxels size becomes smaller. The result is a more accurate wind simulation, but also more calculation time. When the file is imported the scale of the file is taken into account (for example the location file is made in MM) and the resolution is set to 100.

• The wind Tunnel Settings The wind speed can be set in this menu.This is also the only variable which can be set regarding the wind input values. Options for making the Tunnel (container) manipulators makes it possible to change the size of the Tunnel. Decreased size will also mean that the size of the voxels will decrease. In some cases the simulation can have problems when the size of the Tunnel is changed. The simulation can for example only be calculated in the top part of the Tunnel. Changing the size of the Tunnel again can solve this problem.

• Model Orientation When the model is imported it is placed according to the x-y-z values of the original file. The program itself has no actual North setting. That means that the user has to determine which is the likely direction of the wind and what wind direction might cause problems. By using the sliders the object will be rotated, not the Wind Tunnel.

• 2D or 3D The calculation can be done on a 2D plane or a 3D volume. In the first option the calculation only takes data into account which is within the 2D plane. Any wind effects next to the plane are not integrated into the calculation. This makes it faster to calculate. If the 3D option is selected, effects from objects outside the plane will also be taken into account. This makes it much more accurate but also much slower.

• Velocity or Pressure Flow Design can visualize the wind speed but also the wind pressure. The pressure can be interesting for example to see if the wind patterns around the building can be used for natural ventilation.

• Drag plot This option is not relevant for the assignment.

• Flow Lines The visualization of the flow lines. This can be made visible by a set of lines drawn on the simulation plane or a solid color image projected on the simulation plane. The movement of the flow lines might make the simulation easier to interpret.

• Lines The visualization of the wind speed can be further refined by selecting one of the options of representation of the lines.

• Settings Or the settings of the individual lines them selves can be changed in appearance.

• Side or Top The side or top option defines if the simulation plane is placed horizontally or vertically

The location of the plane can be changed by sliding the sphere at the edge of the Tunnel.

#### Getting started

Step by step

The use of Flow design is relatively simple. The software provides a virtual wind tunnel where models made in other 3D programs can be imported. There are no modelling tools available within the software so any alterations have to be made in the case of this course in Rhino. There are several steps to be made before useful data can be extracted.

• Import a 3D model : Export the 3D data from Rhino as a .DWG file (option : 2004 solid). Import this file into Flow Design.

• Rotate the location into a useful orientation The wind tunnel is fixed so the object has to be rotated change the wind direction. Use the Orientation option for that.

• If necessary change the size of the wind tunnel This can be done by using the Wind Tunnel Settings icon which gives you access to the menu. In the lower part of the menu the option is available to Show Wind Tunnel Manipulators By dragging the arrows the tunnel will be changed in size.

This can sometimes lead to an only partial display of the flow lines. This can often be remedied by changing the wind tunnel again.

• Place the simulation plane at the right level This can be done by dragging the sphere at the corner of the wind tunnel. It can take a while before the plane is updated, depending on the settings.

• Change the settings of the wind speed in necessary' The wind speed can be changed in the Wind Speed Settings

• If possible change the accuracy of the simulation If this is possible will depend on the computing power of your laptop. In the Simulation Settings the values can be changed so the voxel size decreases and therefore its accuracy increases.

• Save an image of the simulation In the File menu there is an option of saving an image of the simulation.

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