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Stella Program Notes

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   1 STELLA program One document at a time Most applications can have several documents open at the same time and each document is usually represented by one window. STELLA, however, can only have one document open  but each document can have several windows. If one model is open, the item Open is therefore grayed out in the File menu, while Close is not, telling you that you can't open another model before you close the currently open model. In STELLA, each document can have several windows . The basic window where you enter your model is always open, but you can also have windows with output in table or graph format and these windows you may open and close as you wish. Remember to save your work now and then . Although computer crashes are rare, computer hang ups are not. As soon as you have done a few changes to your work, save it. The four views : The basic window can show you four different views of your model, 1.   Model 2.Equations 3. Map 4. High-level map  You choose between the views by clicking on the arrows to the left and on the globe/x2 sign (The Map/model toggle). When starting to work with STELLA, the model view is by far the most useful view. Model view : With the model view you can build your model, enter your equations, and enter comments:   The named tools are the most important tools. Equation view : The equation view shows your equations and with this view you can also change existing equations and comments: Map view : With this view you can build your model and enter comments  High level map view : With this view you can group your model into submodels (not demonstrated here)     2 The tools A STELLA model consists of four basic items 1.   Reservoirs 2.   Flows 3.   Converters 4.   and Connectors A reservoir   is a place where things are stored, e.g. a bathtub is a store of water. A flow defines the inflow/outflow from a reservoir. A converter can hold information like constants, unit conversions, and different kind of variables. A connector is an information flow. Beside these four basic items STELLA provides some other practical tools: 1.   Graph - To make graphs 2.   Table - To create tables 3.   Hand - To select, open and move things around  4.   Bomb - To delete (blow away) unwanted model items 5.   Ghost - To create aliases  Initial value  A reservoir needs an initial value. The initial value can either be a number or the initial value of any other item in the model In the student edition of STELLA, there are two kinds of reservoirs, Reservoirs and Conveyors. In the research model there is also Queue and Oven. Non-negative You may choose if the reservoir can hold negative values or not. The default in STELLA 5 is non-negative (se picture), i.e. that the reservoir only can hold positive values. This is very handy when simulating e.g. concentrations, temperature and similar state-variables who clearly can not be negative   3 Conveyor You may change the reservoir into a conveyor. A conveyor has a certain residence time. If there is only one outflow from the conveyor, the outflow is simply what came in one residence time  ago. If you however create a second outflow, the second outflow will be a tap flow which removes a certain fraction  of whats in the conveyor. The outflow (not the tap flow) will then be whatever is left  of what came in one residence time ago. Selecting more than one item 1) Press the mousebutton and drag until the items you want to select are inside the dotted box 2) Release the mouse button Moving things Everything in the model window can be moved around. We can thus change this picture to this by moving names and moving connectors. To move a reservoir, converter or flow, simply select it and drag it to wherever you want it. Moving names 1) Click and drag the name of a converter/reservoir/flow to the wished position   4 2) Release the mousebutton Converters Convertors are multipurpose objects. It easier to defining what they are not used for than defining what they are used for. Convertors are not  used for reservoirs and flows  Convertors are  used for 1.   Unit conversions 2.   Constants 3.    All kinds of different variables 4.   Input graphs  Moving connectors 1) Let the hand tool point at the little circle at the beginning of the connector arrow 2) Press the mouse button and drag the little circle to the wished position 3) Release the mousebutton Create an inflow to a reservoir 1) Choose the flow tool 2) Press the mousebutton down somewhere outside a reservoir 3) Continue to hold the mousebutton down and drag the cursor until the reservoir gets highlighted
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