modelNaturalConvection

Information

This tutorial gives step by step instructions for building and simulating a natural convection model. The model tests the coupled simulation of Buildings.ThermalZones.Detailed.CFD with the FFD program by simulating the natural convection in an empty room with only surface boundary conditions.

Case Description

The Rayleigh number is a dimensionless number associated with natural convection, defined as

R_a = g β (T_w-T_e)L³ ⁄ (ν α)

To get a Rayleigh number of 1E5, the flow properties are manually set as acceleration due to gravity g_z=-0.01 m/s², thermal expansion coefficient β=3e-3 K^-1, kinematic viscosity ν=1.5e-5 m²/s, thermal diffusivity α=2e-5 m²/s, and characteristic length L=1 m.

Figure (a) shows the schematic of the FFD simulation. The following conditions are applied in Modelica.:

• East wall: Fixed temperature at T_e=0°C,
• West wall: Fixed temperature at T_w=1°C,
• North & South wall, Ceiling, Floor: Fixed heat flux at 0 W/m².

Figure (a)

Figure (b) shows the velocity vectors and temperature contour in degree Celsius on the X-Z plane at Y = 0.5 m as simulated by the FFD.

Figure (b)

More details of the case description can be found in Zuo et al. (2012).

Step by Step Guide

This section describes step by step how to build and simulate the model.

1. Add the following component models to the NaturalConvection model:

   • Buildings.ThermalZones.Detailed.CFD. This model is used to implement the data exchange between Modelica and FFD. Name it as roo.
   • Buildings.BoundaryConditions.WeatherData.ReaderTMY3. Use weather data from OHare Intl. Airport, Chicago, Illinoi, U.S.A. Name it as weaDat.
   • Modelica.Blocks.Sources.Constant. Three models are needed to specify that internal radiation, internal convective heat gain and internal latent heat gain zero. Name these models as qRadGai_flow, qConGai_flow and qLatGai_flow, respectively.
   • Modelica.Blocks.Routing.Multiplex3. This block is used to combine three scalar signals to a vector. Name it as multiple_x3.
   • Buildings.HeatTransfer.Sources.FixedTemperature. Two models are needed to specify the temperatures on the east and west walls. Name them as TeasWal and TwesWal, respectively.
   Note that for the other four walls with adiabatic boundary conditions, we do not need to specify a zero heat flow boundary condition because the heat flow rate transferred through a heat port from the outside is zero if the heat port is not connected from the outside.

2. In the textual editor mode, add the medium and the number of surfaces as shown below:

   Buildings.ThermalZones.Detailed.CFD roo(
     package MediumA = Buildings.Media.GasesConstantDensity.MoistAirUnsaturated(
       T_default=283.15);
     parameter Integer nConExtWin=0;
     parameter Integer nConBou=0;
     parameter Integer nSurBou=6;
     parameter Integer nConExt=0;
     parameter Integer nConPar=0;
   
   

3. Edit roo as below:

    edeclare package Medium = MediumA,
   surBou(
     name={"East Wall","West Wall","North Wall","South Wall","Ceiling","Floor"},
     each A=1*1,
     til={Buildings.Types.Tilt.Wall,
       Buildings.Types.Tilt.Wall,
       Buildings.Types.Tilt.Wall,
       Buildings.Types.Tilt.Wall,
       Buildings.Types.Tilt.Ceiling,
       Buildings.Types.Tilt.Floor},
     each absIR=1e-5,
     each absSol=1e-5,
     boundaryCondition={
       Buildings.ThermalZones.Detailed.Types.CFDBoundaryConditions.Temperature,
       Buildings.ThermalZones.Detailed.Types.CFDBoundaryConditions.Temperature,
       Buildings.ThermalZones.Detailed.Types.CFDBoundaryConditions.HeatFlowRate,
       Buildings.ThermalZones.Detailed.Types.CFDBoundaryConditions.HeatFlowRate,
       Buildings.ThermalZones.Detailed.Types.CFDBoundaryConditions.HeatFlowRate,
       Buildings.ThermalZones.Detailed.Types.CFDBoundaryConditions.HeatFlowRate}),
     AFlo = 1*1,
     hRoo = 1,
     linearizeRadiation = false,
     useCFD = true,
     sensorName = {"Occupied zone air temperature", "Velocity"},
     cfdFilNam = "resources/Buildings/Resources/Data/ThermalZones/Detailed/Examples/FFD/Tutorial/NaturalConvection.ffd",
     nConExt = nConExt,
     nConExtWin = nConExtWin,
     nConPar = nConPar,
     nConBou = nConBou,
     nSurBou = nSurBou,
     T_start=273.15,
     samplePeriod = 60);
   

4. Connect the component as shown in the figure below.

   

5. Set the values for the following components:
   • Set qRadGai_flow, qConGai_flow and qLatGai_flow to 0.
   • Set TEasWal to 273.15 Kelvin.
   • Set TWesWal to 274.15 Kelvin.
6. Use the Simplified CFD Interface (SCI) to generate the input file for FFD.
   • Use a 20 x 20 x 20 uniform grid.
   • Set the time step size of the FFD to 10 seconds.
   • Generate the input files which have the default names input.cfd (mesh file) and zeroone.dat (obstacles file).
   • Rename the files as NaturalConvection.cfd and NaturalConvection.dat, respectively.
7. Revise the FFD parameter input file NaturalConvection.ffd (an example file is provided in Buildings/Resources/Data/ThermalZones/Detailed/Examples/FFD/Tutorial/):

     inpu.parameter_file_format SCI
     inpu.parameter_file_name NaturalConvection.cfd
     inpu.block_file_name NaturalConvection.dat
     prob.nu 1.5e-5 // Kinematic viscosity
     prob.rho 1 // Density
     prob.gravx 0 // Gravity in x direction
     prob.gravy 0 // Gravity in y direction
     prob.gravz -0.01 // Gravity in z direction
     prob.cond 0.02 // Conductivity
     prob.Cp 1000.0 // Specific heat capacity
     prob.beta 3e-3 // Thermal expansion coefficient
     prob.diff 0.00001 // Diffusivity for contaminants
     prob.alpha 2e-5 // Thermal diffusivity
     prob.coeff_h 0.0004 // Convective heat transfer coefficient near the wall
     prob.Temp_Buoyancy 0.0 // Reference temperature for calculating buoyance force
     init.T 0.0 // Initial condition for Temperature
     init.u 0.0 // Initial condition for velocity u
     init.v 0.0 // Initial condition for velocity v
     init.w 0.0 // Initial condition for velocity w
   

   Please note that some of the physical properties were manipulated to obtain the desired Rayleigh Number of 10⁵.

8. Store NaturalConvection.ffd, NaturalConvection.dat, and NaturalConvection.cfd at Buildings/Resources/Data/ThermalZones/Detailed/Examples/FFD/Tutorial.
9. Set simulation the stop time of the Modelica model 3600 seconds and choose for example the CVode solver.
10. Translate the model and start the simulation.
11. Post-process: click the Tecplot macro script Buildings/Resources/Image/Rooms/Examples/FFD/Tutorial/NaturalConvection.mcr that will generate the temperature contour and velocity vectors shown in the Figure (b). Note: Tecplot is needed for this.

Parameters

Components

Contents

Name	Description
MediumA	Medium model

Revisions

• September 16, 2021, by Michael Wetter:
  Removed assignment of parameter lat as this is now obtained from the weather data reader.
  This is for IBPSA, #1477.
• September 07, 2017, by Thierry Nouidui:
  Refactored the FFD C-code and revised the documentation. This is for issue 612.
• July 7, 2015 by Michael Wetter:
  Removed model for prescribed heat flow boundary condition as the value was zero and hence the model is not needed. This is for issue 439.
• July 25, 2014, by Michael Wetter:
  Revised documentation.
• June 27, 2014, by Wei Tian, Thomas Sevilla, Wangda Zuo:
  First implementation.

References

Wangda Zuo, Mingang Jin, Qingyan Chen, 2012.
Reduction of numerical viscosity in FFD model.
Journal of Engineering Applications of Computational Fluid Mechanics, 6(2), p. 234-247.