Ambiens is a 2D computational fluid dynamics (CFD) program, specifically designed to model air flow and temperatures in buildings. The software is valid for a 1 m thick cross section. Ambiens is produced by EDSL – now owned by Bentley – and is part of the TAS suite.
Though Ambiens is technically a piece of CFD software, it allows for rapid modeling (when you get used to its quirks) and very quick simulations (often less than 2 minutes). After playing with Ambiens for about an hour, you should be able to run relatively complex studies – which is a-typical of most airflow modeling software. So as quirky as the software is, the “bang for the buck” is quite good.
Here’s the typical workflow for thinking about approaching a natural ventilation study with Ambiens…
- Ask meaningful questions that can be answered in the smallest number of conditions possible.
- Create 2D sections for each condition in bitmap (.bmp) format.
- Import your bitmap. Set the scale and grid size.
- Trace the zone boundaries.
- Create inputs and outputs (inlets and outlets).
- Determine temperatures (surface and ambient).
- Add Gains.
- Run the simulation.
- Repeat the above steps for each condition.
- Synthesize the results.
1. Ask meaningful questions…
- Ask a meaningful question. By meaningful, your question should be something that is easily testable. For example: “What is the difference in air velocity in two naturally ventilated buildings, one being 60 ft wide and one being 90 ft wide?” A difficult question to answer would be something like: “What is the optimal natural ventilation strategy for residential building?” You can spend 30 minutes answering the first question, and days answering the second. Try to be realistic and methodical in your questioning.
- Conceive of your scenarios carefully. Do not attempt to test too many variables at one time. Try to divide up your conditions/scenarios so that you can draw logical “apples-to-apples” comparisons between each.
2. Create 2D sections…
- Create sectional conditions for each scenario you with to test. These 2D sections can either be hand drawn or extractions from your BIM model…for example:
- Draw in your assumptions about the probably airflow as vector arrows (shown above). These will help you remember your predictions and illustrate how the simulated performance deviates (or adheres) to your predictions.
- Optional: It is very helpful to draw dark lines around the zone you wish to test. Thicken the lines showing exactly which walls or partitions are critical to your study…for example:
- Scan your drawings (if necessary). If your sections are already raster images, you can skip this step.
- Convert the images to bitmap (.bmp) format. Ambiens only accepts bmp files. Scale each of your conditions so that they match each other. You may want to line up each of the backgrounds in Photoshop so that they all match and have the same resolution. This will help with making small changes to one model when swapping in and out different backgrounds.
- Once you have your conditions and background images, you are ready to launch Ambiens.
3. Launch Ambiens.
- Import your bitmap background. Navigate to File – Bitmap…
- Set the scale and grid size… Right-click anywhere in the empty grid background. This will bring up the Cell Details dialog box:
- The cell dimension is the size of each cell. In this example, we have set the cell dimension to 0.5 ft (or 6 inches).
- Set the number of X and Y to something that matches a given dimension. You are trying to line up the grid with a known dimension. For example, if you know that the floor-to-floor height is 15 ft, you will want to set the Y value to a number that matches 30 cells (if your cells are set to 6 inches) with the floor-to-floor in the background image.
4. Trace the zone boundaries …
- WARNING: Always draw CLOCKWISE!
- Pretend you are stretching a rubber-band around the edges of the zone you are modeling. Each “vertex” is like a tack you are stretching the rubber band around. In general, keep the model as simple as possible with as few features as possible; you can always add detail later. Start with the simple outline:
- To complete the zone, click the “Add Vertex” button when you are done, otherwise the tool assumes you are still adding vertices.
- Your zone does not have to be complete. If you would like to move around a vertex, click the “Move” button, then drag the vertex…
- Now that you have traced the outline, you will need to add detail. To add vertices, click the “Insert Vertex” button…
- And then click on the existing “walls” to add a vertex.
- Delete vertices using the “Delete” button…
- Keep adding detail…remember the rubber band metaphor. You will often need to stretch the walls around two vertices to create a floor…
- Remember, you only have one band, so you will have to be creative in creating your interior partitions and floors.
- Check to make sure you don’t have any duplicate vertices in the same cell location. It is difficult to tell, but a way to manually check is to use the Move tool to stretch the vertex away, and then replace it. If you pull it away and still see a vertex, you will have to delete the duplicate before you proceed.
- Once you are done, save your file. Save regularly. Now that you have your basic walls, it’s time to add inlets and outlets…
5. Create inlets and outlets…
- Inlets and outlets must be horizontal or vertical (not diagonal).
- Click on a small section of the wall to highlight it, then right-click and choose either inlet or outlet. You will probably need to add a few vertices to be able to get the size you want. When adding vertices, remember that the preceding few cells and the next few cells to your intended inlet or outlet must be parallel to the inlet or outlet. Use the “Move” tool to get the sizes correct.
- Double-click on each inlet and outlet and enter an air velocity of each inlet and outlet. Each cell will have this velocity. For example, if an inlet has 2 cells, each with a velocity of 4.0 ft/s, then the total velocity of the inlet is 8.0 ft/s (2 * 4.0)…
- The model MUST conserve mass, which means that if you have the same number of cells for inlets as for outlets, the inlet and outlet air flow velocity will be equal. If you don’t have the same number of cells, then you will have to make the velocity balance. For example: 2 cells of inlet with 5.0 ft/s air flow is equivalent to 5 cells of outlet with 2.0 ft/s air flow.
- You must balance this velocity by hand…if we take the simple example (given in step c above) of 2 inlet cells each with 4.0 ft/s, and one outlet cell, then the outlet must be set to a total 8.0 ft/s…
- To test to see if you have conserved mass, click the “Initialize” button in the “Calculate” menu. If the mass balances, you will see the simulation initialize; if not, you will get a “Mass imbalance” error.
- Keep working until you can initialize the simulation.
6. Determine temperatures…
- Once the simulation initializes, you are not yet ready to run. Click the “Stop” button to return to the editing mode…
- Now you need to set the ambient domain temperature as well as the surface temperatures for walls, windows and radiators.
- Set the ambient domain temperature by double-clicking inside your zone (domain). This brings up the “Domain” dialog box…
- Set the Start Temperature to something reflecting your design condition…if you are testing on a summer day, this is the ambient temperature the room will have reached at a stable, unconditioned state.
- Now set the surface temperatures for each surface. Walls, windows, and radiators are essentially the same object type, but you can distinguish the two be adding vertices, then changing the color (right-click the segment to change its color). For example, you might color windows yellow. Once you have determined which surfaces are walls, windows and radiators, double-click each surface and set the temperatures, one-by-one.
7. Add Gains
- You will probably need to insert regions of heat gains to represent people, appliances, lighting, etc. This is done by clicking and dragging to make a gain rectangle. The size of the rectangle should represent a person or group of people for example.
- Confer with an engineer as to what the sensible (and latent) heat gains should be in your space depending on your building use and design condition.
8. Run the simulation…
- Once you have all the inlets and outlets balanced and the temperatures set, you are ready to run a simulation.
- Initialize the simulation (Calculate – Initialize).
- Click the “Play” button to start the simulation…
- Once the simulation begins, you will need to switch the display mode to one of the different value settings. For example, to display air velocity, switch the display to Speed.
- Ambiens can also display dry and wet bulb temperatures, relative humidity, as well as percentage of people dissatisfied (PPD). To change the display setting (which does not require recalculation), just use the drop-down menu in the main window…
- To display the values on the proper scale, you may need to alter the upper and lower values displayed in the Settings dialog box, found under Calculate – Settings)…
- To draw vector lines: during the simulation, in the Calculation Settings dialog, simply enter a non-zero value in the “Velocity Scale Factor” field. This is a nice way of quickly viewing speed and temperature at the same time…
9. Repeat the above steps for each condition…
- Break out each condition into a separate file folder, containing the background bitmap, as well as the Ambiens (.tai) file.
- You may be able to create different conditions (if they are not significantly different) by just using the “Save As…” feature and making small edits to the previous condition. For example, you may just be testing two different inlet sizes…so you can just drag around vertices and save into different .tai files.
- Be as organized as possible with the different conditions, giving them file names that reflect the conditions.
10. Synthesize the results…
- Though Ambiens can output film clips (in .AVI format), you will probably want to screen capture the results and composite on top of the backgrounds. Use Photoshop or a similar application.Photoshop works well for overlaying the resulting Speeds (or temperatures, etc). Use 60% opacity on your layers and line up the results on top of the backgrounds you used in Ambiens. This may seem like a redundant step, but it is a necessary one…this step also gives you an opportunity to label each condition well, giving it a unique name that you can compare to other conditions.
- Draw Clockwise: drawing the outlines in a clockwise fashion ensures that Ambiens knows which surfaces are inside and outside (not “reversed.”)
- Balancing inlets and outlets. Again, the model MUST conserve mass, which means that if you have the same number of cells for inlets as for outlets, the inlet and outlet air flow velocity will be equal. If you don’t have the same number of cells, then you will have to make the velocity balance. For example: 2 cells of inlet with 5.0 ft/s air flow is equivalent to 5 cells of outlet with 2.0 ft/s air flow.
- Another way of stating this is that the overall pressure must be zero; air going in equals the air going out. Inlets and outlets do not need to have the same number of cells, but the air flow multiplied by the number of cells on the inlet MUST equal the air flow multiplied by the number of cells on the outlets.
- Extraneous vertices. These are often hidden. This is frequently caused by accidentally clicking twice on the same spot when inserting a vertex to a wall. Clicking once places a vertex in a wall; clicking more than once places more than one vertex (though it will still look like a single vertex).
Notes and Resources
- You can change the parameters you are looking at during the calculations, but eventually the conditions stop and this is the state that you can use in your report as the stable conditions. Ambiens has been designed so that you can stop the calculations and change some of the input and output parameters and continue run the simulation.
- There are tutorial videos and help files that can be accessed through the help menu or through the TAS manager. The tutorial videos haven’t been updated since 2005. There is an issue with their playing uninterrupted in Internet Explorer. EDSL/Bentley promises to fix this in the next release.