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Boundary condition input

• Run a IDA ICE simulation. Explore and inspect the result and choose an instant (time and date) for the CFD case.
  • The output in an IDA ICE simulation is the input for boundary conditions in the CFD calculations.
  • Note all default settings in the different IDA ICE simulations. E.g. internal gains are default 0 % in Heating load calculation.
• Please note that CO₂ emissions are not affected by the percentage setting for internal gains in the Simulation settings. 

CFD tab

• Select type of "Zone simulation" under "Input selection".
• Enter time and date under "Zone simulation instant", this instant (time and date) will be calculated in the CFD.
  • CFD tab doesn't automatically choose the instant that is presented under IDA ICE tab Summary (for e.g. the Heating simulation).
• Add the zones of interest, use button "+". 

Table "Boundary input and sources"

• Visualize "Temperature, °C" by clicking the symbol of the white box next to the text. In the visualization, white surfaces are adiabatic.
• Checkboxes "Ignore automatic leaks" and "Create missing":
  • Generally keep default setting for "Ignore automatic leaks", i.e. keep the "Ignore automatic leaks" enabled. Automatic leaks is a feature specific to IDA ICE, whereas balance in the CFD simulation is achieved through other methods.
  • "Create missing terminals" creates terminals that needs to be deleted if not wanted in later stages. Primarily check this one for test cases only. Inspect model in the 3D tab if missing air terminals are created, and evaluate their position.
• Column "Heat flux, W" is convective heat. Therefor it might differ from total power input of an object in IDA ICE simulation.
• The table needs to be refreshed (button "REFRESH") when changes are made in IDA ICE for the zone, for correct presentation of data. A refreshed table shows the summarized boundary condition input for the upcoming CFD case.
• * (asterisk) after an object means that the no 3D object is assigned to the thermal IDA ICE object.
• When objects are missing descriptions in the table for "Boundary input and sources", a View->Refresh might be needed in the 3D tab.

Smallest area outflow surface is defined as a pressure outlet

A CFD simulation needs a reference for pressure. When one or more outflows are available (exhausts, leaks, openings, ...), the smallest of these will be assigned as a pressure outlet, which sets the reference value and also acts to balance the mass flow in the CFD simulation.
The pressure outlet boundary may, especially if it is a large opening, have varying and sometimes uneven velocity distribution; However, the solver will make sure that mass balance is maintained. Earlier implementations selected the largest outflow for pressure reference, but since the variations in velocity over a large, pressure controlled boundary condition may become unrealistic this is changed.

An open IDA ICE window in IDA ICE CFD

An opened window in IDA ICE results in two flows, represented by two mass flows that may have the same direction (e.g. both flow in to the zone), or the opposite direction. To represent this in the CFD case, the window is divided in two equally sized parts, upper and lower, split by the horizontal center line of the window.

If there is an outflow vector in IDA ICE, and its meshed surface (boundary, half of the window opening) is the largest of all outflow boundaries, this boundary will become the pressure outlet boundary. If there exists a larger outflow surface in the CFD, both air flows through the window will be fixed, given by the zone simulation.

The Introduction Exercise - Boundary condition input

• Run a Heating design simulation under tab "Simulation" and request necessary output:
  • Click tab "Simulation".
  • Click the blue CFD text to request necessary output for the CFD simulation.
  • Click "Setup" next to "Heating design".
    • Enter "100" under "Internal gains".
    • Click "Run".

• Click tab "CFD".
  • Choose "Heating" under "Zone simulation".
  • Set instant time to 05:17.
  • Click button "+" and "Add all zones" (you only have one zone in this example).

• Table with heading "Boundary input and sources":
  • Inspect the input to the CFD. Is it as expected?
  • Sort the table by "Type" by clicking on the column header "Type".
  • The * after Zone.Light means that the heat is not connected to a geometry, it will be evenly distributed.
  • Please note: Column "Heat flux, W" is convective heat. Therefor it might differ from your input, which is all heat generated from an object.

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