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13.3.18 Solar Load Model

FLUENT provides a solar load model that can be used to calculate radiation effects from the sun's rays that enter a computational domain. Two options are available for the model: solar ray tracing and DO irradiation. The ray tracing approach is a highly efficient and practical means of applying solar loads as heat sources in the energy equations. In cases where you want to use the discrete ordinates (DO) model to calculate radiation effects within the domain, an option is available to supply outside beam direction and intensity parameters directly to the DO model. The solar load model includes a solar calculator utility that can be used to construct the sun's location in the sky for a given time-of-day, date, and position. Solar load is available in the 3d solver only, and can be used to model steady and unsteady flows.



Introduction


Typical applications that are well-suited for solar load simulations include the following:

The effects of solar loading are needed in many ACC applications, where the temperature, humidity, and velocity fields around passengers (and drivers) are desired. ACC systems are tested for their capacity to cool down passenger compartments after they have been "soaked'' in intense solar radiation. FLUENT's solar load model will enable you to simulate solar loading effects and predict the time it will take to reasonably cool down the cabin of a car that has been exposed to solar radiation, as well as predict the time interval needed to lower the temperature in specified points and areas within the domain.

In the analysis of buildings, solar loading provides a significant burden on the cooling requirement in warm climates, particularly where architects want to use the aesthetics of glazed facades. Even in cooler climates, solar loading can provide a burden during warmer seasons where modern buildings are well insulated against thermal loss during winter months. As well as providing an engineer with a practical tool for determining the solar heating effect inside a building, FLUENT's solar load model will allow the solar transmission through all glazed surfaces to be determined over the course of a day, allowing important decisions to be made before undertaking any flow studies.



Solar Ray Tracing


The solar load model's ray tracing algorithm can be used to predict the direct illumination energy source that results from incident solar radiation. It takes a beam that is modeled using the sun position vector and illumination parameters, applies it to any or all wall or inlet/outlet boundary zones that you specify, performs a face-by-face shading analysis to determine well-defined shadows on all boundary faces and interior walls, and computes the heat flux on the boundary faces that results from the incident radiation.

figure   

The solar ray tracing model includes only boundary zones that are adjacent to fluid zones in the ray tracing calculation. In other words, boundary zones that are attached to solid zones are ignored.

The resulting heat flux that is computed by the solar ray tracing algorithm is coupled to the FLUENT calculation via a source term in the energy equation. The heat sources are added directly to computational cells bordering each face and are assigned to adjacent cells in the following order: shell conduction cells, solid cells, and fluid cells. Heat sources are assigned to one of these types of adjacent cells, only. You can choose to override this order and include adjacent fluid cells in the solar load calculation by issuing a command in the text user interface (see Section  13.3.18 for details). Note that the sun position vector and solar intensity can be entered either directly by you or computed from the solar calculator. Direct and diffuse irradiation parameters can also be specified using a user-defined function (UDF) and hooked to FLUENT in the Radiation Model panel.

The solar ray tracing option allows you to include the effects of direct solar illumination as well as diffuse solar radiation in your FLUENT model. A two-band spectral model is used for direct solar illumination and accounts for separate material properties in the visible and infrared bands. A single-band hemispherical-averaged spectral model is used for diffuse radiation. Opaque materials are characterized in terms of two-band absorptivities. A semi-transparent material requires specification of absorptivity and transmissivity. Values that you specify for transmissivity and absorptivity are defined for normal incident rays. FLUENT recomputes/interpolates these values for the given angle of incidence.

The solar ray tracing algorithm also accounts for internal scattered and diffusive loading. The reflected component of direct solar irradiation is tracked. A fraction of this radiative heat flux, called internally scattered energy is applied to all the surfaces participating in the solar load calculation, weighted by area. The internally scattered energy depends on the scattering fraction which is specified in the TUI, and whose default value is $1$. Depending on the reflectivity of the primary surface, the scattering fraction can be responsible for the inclusion (or exclusion) of a large amount of radiation within the rest of the domain.

Also included as internally scattered energy is the contribution of the transmitted component of diffuse solar irradiation (which enters a domain through semi-transparent walls depending upon the hemispherical transmissivity). The total value of internally scattered energy is reported to the FLUENT console. The ambient flux is obtained by dividing the internally scattered energy by the total surface area of the faces participating in the solar load calculation.

Note that Solar Ray Tracing is not a participating radiation model. It does not deal with emission from surfaces, and the reflecting component of the primary incident load is distributed uniformly across all surfaces rather than being local to the surfaces reflected to. If surface emission is an important factor in your case then you can consider implementing a radiation model (e.g., P1) in conjunction with Solar Ray Tracing.

Shading Algorithm

The shading calculation that is used for solar ray tracing is a straightforward application of vector geometry. A ray is traced from the centroid of a test face in the direction of the sun. Every other face is checked to determine if the ray intersects the candidate face and if the candidate face is in front of the test face. If both conditions are met, then an opaque face completely shades the test face. A semi-transparent face attenuates the incident energy.

A Barycentric coordinate formulation is used to construct triangle-ray intersections. A quadrilateral ray intersection method is used to handle the case when model surfaces contain quadrilaterals. A quad-tree preprocessing step is applied to reduce the ray tracing algorithm complexity that can lead to long runtime for $10^4$ faces and greater. The quad-tree refinement factor can be modified in the text interface. The default value of this parameter is $7$ which is sufficient to cover the entire spectrum of mesh sizes between one cell and five million cells. If the mesh is greater than five million cells, an increase in this parameter would reduce the CPU time needed to compute the solar loads.

Glazing Materials

Incident solar radiation can be applied to glass and plastic glazing materials of various types at wall boundaries, and the effects of coated glazings modeled using the solar ray tracing algorithm. To model solar optical properties, you will need to specify the transmissivity and reflectivity of the material in the Wall boundary conditions panel. You can obtain these values from the glass (or plastic) manufacturer or use data from another source (e.g., ASHRAE Handbook).

Glazing optical properties are dependent on incident angle, and the variation is significant for an incident angle greater than $40$ degrees. As the incident angle increases from zero, transmissivity decreases, reflectivity increases, and absorptivity increases initially due to lengthened optical path, and then decreases as more incident radiation is reflected. The shape of the property curve varies with glass type and thickness. This difference is more pronounced for coated glass or for a multiple-pane glazing system. It cannot be assumed that all glazing systems have a universal angular dependence.

For coated glazings, the spectral transmissivity and reflectivity at any incident angle are approximated in the solar load model from the normal angle of incidence (Finlayson and Arasteh, 1993).

Transmissivity is given by


 T(\theta, \lambda) = T(0,\lambda) Tref(\theta) (13.3-93)

where


 Tref(\theta) = a0 + a1 cos(\theta) + a^2 cos(\theta^2) + a3 cos(\theta^{3}) + a4 cos({\theta}^{4}) (13.3-94)

Reflectivity is given by


 R(\theta, \lambda) = R(0, \lambda)[1 - Rref(\theta)] + Rref(\theta) (13.3-95)

where


 Rref(\theta) = b0 + b1 cos(\theta) + b^2 cos(\theta^2) + b3 cos(\theta^{3}) + b4 cos({\theta}^{4}) - Tref(\theta) (13.3-96)

The constants used in Equations  13.3-93 and  13.3-95 are for coated glazings and are taken from Finlayson, E.U., Arasteh, D.K., Huizenga, C., Rubin, M.D., Reilly, M.S. 1993. WINDOW 4.0: Documentation of Calculation Procedures. Publication LBL-33493/TA-309. Lawrence Berkeley Laboratory, Energy and Environmental Division, Berkeley, CA. URL: http://btech.lpl.gov/papers/33493.pdf. $T(0,\lambda)$ and $R(0,\lambda)$ are specified in the Wall boundary conditions panel.

Inputs

The following inputs are required for the solar ray tracing algorithm:

You can enter the sun direction vector components ( X, Y, Z) and the direct and diffuse solar irradiation values as constants in the Radiation Model panel, or you can have these parameters derived from the solar calculator. Irradiation can also be specified using a user-defined function (Section  13.3.18.) The scattering fraction and quad tree refinement factor defaults are used by the solar ray tracing algorithm. Ground reflectivity is the overall reflectivity of the ground and is used to calculate the component of diffuse radiation that will result from radiation being reflected off the ground. You can modify the default values using text commands issued at the console window (see Section  13.3.18).

The absorptivity and transmissivity parameters are entered in the Wall boundary condition panel for the particular wall zones you wish to participate in solar ray tracing.



DO Irradiation


The solar load model's discrete ordinates (DO) irradiation option provides you with an easy means of applying a solar load directly to the DO model. Unlike the ray tracing solar load option, the DO irradiation method does not compute heat fluxes and apply them as heat sources to the energy equation. Instead, the irradiation flux is applied directly to semi-transparent walls (that you specify) as a boundary condition, and the radiative heat transfer is derived from the solution of the DO radiative transfer equation.

The following inputs are required for DO irradiation at semi-transparent walls:

In the Wall boundary condition panel for each semi-transparent wall you want to participate in DO irradiation, you can specify that the beam direction and total irradiation be derived from the solar parameters (e.g., solar calculator) that you set (or compute) in the Radiation Model panel. This is done by checking the Use Beam Direction from Solar Parameters and Use Total Irradiation from Solar Parameters boxes. When selected, FLUENT sets the beam width (the angle subtended by the sun) to the default value of $0.53$ degrees for DO irradiation.

figure   

Note that the sign of the beam direction that is needed for the DO model is opposite the sun direction vector that is entered or derived from the solar parameters. The beam direction in the DO model is the direction of external radiation (e.g., radiation coming from the sun), while the sun direction vector in the solar load model points to the sun. Incident radiation and sun angle always have an opposite sign since they are quantities that are defined from opposite perspectives.



Solar Calculator


FLUENT provides a solar calculator that can be used to compute solar beam direction and irradiation for a given time, date, and position. These values can be used as inputs to the solar ray tracing algorithm or as semi-transparent wall boundary conditions for discrete ordinates (DO) irradiation.

Inputs/Outputs

Inputs needed for the solar calculator are:

Global position consists of latitude, longitude, and time zone (relative to GMT). The time of day for a transient simulation is the starting time plus the flow-time. For grid orientation, you will need to specify the North and East direction vector in the CFD grid. The default solar irradiation method is Fair Weather Conditions. Alternatively, you can choose the Theoretical Maximum method. The sunshine factor is simply a linear reduction factor for the computed incident load that allows for cloud cover to be accounted for, if appropriate.

You can specify these inputs in the Solar Calculator panel that is accessible from the Radiation Model panel (Figure  13.3.34). Alternatively, you can enter the parameters using text interface commands (Section  13.3.18).

The following values are computed by the solar calculator and are displayed on the console window whenever the solar calculator is invoked:

Direct normal solar irradiation is computed using the ASHRAE Fair Weather Conditions method, when this option is selected in the solar calculator. (Note: Equation 20 and Table 7 from Chapter 30 of the 2001 ASHRAE Handbook of Fundamentals.) The theoretical maximum values for direct normal solar irradiation and diffuse solar irradiation are computed using NREL's Theoretical Maximum method, when this option is selected. In practice, these values are unlikely to be experienced due to atmospheric conditions.

FLUENT computes the diffuse solar irradiation components (vertical and horizontal) internally for each face in the domain. When the Theoretical Maximum method is chosen, these diffuse irradiation values provide estimates for the maximum vertical and horizontal surface effects.

Theory

FLUENT provides two options for computing the solar load: Fair Weather Conditions method and Theoretical Maximum method. Although these methods are similar, there is a key difference. The Fair Weather Conditions method imposes greater attenuation on the solar load which is representative of atmospheric conditions that are fair -but not completely clear.

The equation for normal direct irradiation applying the Fair Weather Conditions Method is taken from the ASHRAE Handbook:


 Edn = \frac{A}{e^\frac{B}{\sin(\beta)}} (13.3-97)

where $A$ and $B$ are apparent solar irradiation at air mass $m=0$ and atmospheric extinction coefficient, respectively. These values are based on the earth's surface on a clear day. $\beta$ is the solar altitude (in degrees) above the horizontal.

The equation for direct normal irradiation that is used for the Theoretical Maximum Method is taken from NREL's Solar Position and Intensity Code (Solpos):


 Edn = S_{etrn} S_{unprime} (13.3-98)

where $S_{etrn}$ is the top of the atmosphere direct normal solar irradiance and $S_{unprime}$ is the correction factor used to account for reduction in solar load through the atmosphere.

The calculation for the diffuse load in the solar model is based on the approach suggested in the 2001 ASHRAE Fundamental Handbook (Chapter 20, Fenestration). The equation for diffuse solar irradiation on a vertical surface is given by:


 Ed = C Y Edn (13.3-99)

where $C$ is a constant whose values are given in Table 7 from Chapter 30 of the 2001 ASHRAE Handbook of Fundamentals, $Y$ is the ratio of sky diffuse radiation on a vertical surface to that on a horizontal surface, and $Edn$ is the direct normal irradiation at the earth's surface on a clear day.

The equation for diffuse solar irradiation for surfaces other than vertical surfaces is given by:


 Ed = C Edn \frac{(1 + \cos\epsilon)}{2} (13.3-100)

where $\epsilon$ is the tilt angle of the surface (in degrees) from the horizontal plane.

The equation for ground reflected solar irradiation on a surface is given by:


 Er = Edn (C + \sin\beta) \rho_g \frac{(1 - \cos\epsilon)}{2} (13.3-101)

where $\rho_g$ is the ground reflectivity. The total diffuse irradiation on a given surface will be the sum of $Ed$ and $Er$ when the input for diffuse solar radiation is taken from the solar calculator. Otherwise, if the constant option is selected in the Radiation panel, then the total diffuse irradiation will be the same as specified in the panel.



Running Solar Load Using a Serial Solver


When you want to run a steady-state solution with solar load enabled on a serial solver, you simply setup the solar load model (Section  13.3.18) and boundary conditions (Section  13.3.18) for your case, and then run the simulation. The solution data file will contain the solar fluxes that you can use for postprocessing. For a steady-state solution, the solar loads are computed on initialization. If you want to initially solve a case without solar loading (say, for stability) and then add the effects of solar loading afterward, you will need to enable the solar load model through the text user interface (TUI).

figure   

Note that you can compute the solar load at any time once you have setup the model by using the sol-on-demand text interface command (see Section  13.3.18 for details).

When you want to run a transient solar load simulation on a serial solver, the process is the same as for the steady-state case but you will need to specify the additional Time Steps per Solar Load Update parameter in the Radiation Model panel. FLUENT will re-compute the sun position and irradiation and update solar loads with this specified frequency.



Using Solar Load in the Parallel Solver


The solar ray tracing algorithm is not parallelized in FLUENT. As a result, you will have to generate solar data for the case in serial mode, and then use that data in your parallel simulation. Follow the separate procedures below for steady-state and transient simulations, respectively.

Steady-State Simulation

The general process for a steady-state solar load simulation in parallel is outlined below:

1.   Start a serial solver in FLUENT and read (or setup) your case file.

2.   Setup the solar load model (Section  13.3.18).

3.   Setup the boundary conditions (Section  13.3.18)

4.   Initialize the solution in the Solution Initialization panel.

Solar load data is computed at solution initialization for steady-state cases. The data will be written to the console window, as shown in the example below:

Internally Scattered Energy [W]: 2.29688e-05, Ambient Flux [W/m^2]:
 0.000314448
Boundary ID: 11, Integral Energy Source [W]: 3.843255e-08
Boundary ID: 10, Integral Energy Source [W]: 1.922111e-08
Boundary ID: 8, Integral Energy Source [W]: 1.642018e-06
Boundary ID: 1, Integral Energy Source [W]: 1.126829e-04
Boundary ID: 3, Integral Energy Source [W]: 1.537705e-07
Boundary ID: 4, Integral Energy Source [W]: 3.074602e-07
Total Integral Energy Source [W]: 1.148438e-04
Compute Time:  1 sec

5.   Save the case and data files.

6.   Start the parallel solver and read the case and data files.

7.   Setup and run your parallel steady-state simulation.

Transient Simulation

The general process for a transient solar load simulation in parallel is outlined below.

1.   Start a serial solver in FLUENT and read (or setup) your case file.

2.   Setup the solar load model which includes specifying the Time Steps per Solar Load Update in the Radiation Model panel (Section  13.3.18).

3.   Setup the boundary conditions (Section  13.3.18).

4.   Enable the autosave file capability in the text interface that will write separate solar data file(s) at specified time intervals to be used by the parallel solver. (See autosave-solar-data in Section  13.3.18).

figure   

Make sure that the frequency you specify for autosaving solar load data is the same as for updating. If you choose to make these frequencies different, then the autosave time step should be a multiple of the solar load update time step.

5.   Disable all transport equations in the Solution Controls panel.

6.   Save the case file.

7.   Initialize the solution.

figure   

Solar load data is computed at solution initialization for steady-state cases and written to the console window. See previous steady-state procedure.

8.   Set the Max Iterations per Time Step to $1$ in the Iterate panel.

9.   Run the simulation. As the solver iterates, FLUENT will write separate data files for the time step frequency that you specified in the autosave command, and will report it to the console window. The data files will be saved in your working directory and will be identified by the time step number that is appended to the file name. For example, solar_data002.dat will contain the solar data for the second time step.

figure   

The autosave solar data files cannot be used for postprocessing.

10.   Start the parallel solver.

11.   Read the case file.

12.   Enable the autoread file capability in the text interface that will direct the solver to automatically read the 'autosaved' solar data file(s) that were generated during the serial session. (See autoread-solar-data in Section  13.3.18).

figure   

Make sure that the frequency you specify for autoreading solar load data is the same that you specified for autosaving and updating solar data. If you choose to make these frequencies different, then the autosave time step should be a multiple of the update time step, and the autoread time step should be a multiple of autosave.

13.   Make sure that the equations you want to solve for in your parallel simulation are set in the Solution Controls panel.

14.   Run the transient simulation.

figure   

Note that the solar flux data that will be available at the end of the solution process is for the last time step that was read using the autoread frequency.



User-Defined Functions (UDFs) for Solar Load


You can write a user-defined function (UDF) to specify direct and diffuse solar intensity using the DEFINE_SOLAR_INTENSITY macro. Click here to go to the UDF Manual for details. Once interpreted or compiled, you can hook your intensity UDF for direct or diffuse solar irradiation by selecting user-defined in the drop-down lists for these parameters in the Radiation Model panel. See Step 2 in Section  13.3.18 for details.



Setting Up the Solar Load Model


Graphical User Interface

The solar load model is enabled in the Radiation Model panel (Figure  13.3.31).

Define $\rightarrow$ Models $\rightarrow$ Radiation...

Figure 13.3.31: The Radiation Model Panel (With Solar Load Model Solar Ray Tracing Option)
figure

The solar load model has two options: Solar Ray Tracing and DO Irradiation. Solar Ray Tracing can be applied as a standalone solar loading model, or it can be used in conjunction with one of the FLUENT radiation models (P1, Rosseland, Discrete Transfer, Surface-to-Surface, Discrete Ordinates). DO Irradiation is available only when the Discrete Ordinates (DO) radiation model is enabled.

1.   Enable the solar load model in the Radiation Model panel.

(a)   To enable the solar ray tracing algorithm, select Solar Ray Tracing under Solar Load (Figure  13.3.32).

Figure 13.3.32: The Radiation Model Panel
figure

(b)   To enable the DO irradiation option, first select Discrete Ordinates under Model, and then select DO Irradiation under Solar Load (Figure  13.3.33).

Figure 13.3.33: The Radiation Model Panel (With Solar Load Model DO Irradiation Option)
figure

2.   Define the solar parameters.

(a)   Enter read values for the X, Y, and Z components of the Sun Direction Vector. Alternatively, you can choose to have this vector computed from the solar calculator by enabling the Use Direction Computed from Solar Calculator option.

(b)   Specify the illumination parameters.

i.   Enter a value for Direct Solar Irradiation under Illumination Parameters. This parameter is the amount of energy per unit area in $W/m^2$ due to direct solar irradiation. This value may depend on the time of year and the clearness of the sky. Make your selection in the drop-down list next to Direct Solar Irradiation and either enter a constant value, have the value computed from the solar calculator, or specify it using a user-defined function. (See Section  here for details on writing solar intensity UDFs.) For transient simulations, you have the additional option of specifying a time-dependent piecewise-linear and polynomial profile for direct solar irradiation.

ii.   Enter a value for Diffuse Solar Irradiation, which is the amount of energy per unit area in $W/m^2$ due to diffuse solar irradiation. This value may depend on the time of year, the clearness of the sky, and also on ground reflectivity. Make your selection in the drop-down list next to Diffuse Solar Irradiation and either enter a constant value, have the value computed from the solar calculator, or specify it using a user-defined function. (See Section  here for details on writing solar intensity UDFs.) For transient simulations, you have the additional option of specifying a time-dependent piecewise-linear and polynomial profile for diffuse solar irradiation.

iii.   If you are using the Solar Ray Tracing solar load model (Figure  13.3.32), then you will need to enter a value for Spectral Fraction. The spectral fraction is the fraction of incident solar radiation in the visible part of the solar radiation spectrum. The spectral fraction is not used for DO irradiation since the DO implementation is intended only for a single band.

Spectral Fraction =

\frac{V}{V + IR} (13.3-102)

where $V$ is the visible incident solar radiation, and $V + IR$ is the total incident solar radiation (visible plus infrared).

3.   Use the solar calculator to compute solar beam direction and irradiation.

(a)   Click Solar Calculator... in the Radiation Model panel to open the Solar Calculator panel (Figure  13.3.34).

Figure 13.3.34: The Solar Calculator Panel
figure

(b)   In the Solar Calculator panel, define the Global Position by the following parameters:

i.   Enter a real number in degrees for Longitude. Values may range from $-180$ to $180$ where negative values indicate the Western hemisphere and positive values indicate the Eastern hemisphere.

ii.   Enter a real number for Latitude in degrees. Values can range from $-90^{\circ}$ (the South Pole) to $90^{\circ}$ (the North Pole), with $0^{\circ}$ defined as the equator.

iii.   Enter an integer for Timezone that is the local time zone in hours relative to Greenwich Mean Time (+-GMT). This value can range from $+12$ to $-12$.

figure   

Note that you must specify all three Global Position parameters for
the solar calculator.

(c)   Define the local Date and Time by the following parameters:

i.   Enter an integer for Day and Month under Day of Year.

ii.   Enter an integer for Hour that ranges from $0$ to $24$ under Time of Day. Enter an integer or floating point number for Minute.

The time of day is based on a 24-hour clock: $0$ hours and $0$ minutes corresponds to 12:00 a.m. and $23$ hours $59.99$ min corresponds to 11:59.99 p.m. For example, if the local time was 12:01:30 a.m., you would enter 0 for Hour and 1.5 for Minute. If the local time was 4:17 p.m., you would enter 16 for Hour and 17 for Minute.

(d)   Define the Grid Orientation as the vectors for North and East in the CFD grid system of coordinates.

(e)   Select the appropriate Solar Irradiation Method. The Fair Weather Conditions is the default method.

(f)   Enter an integer for Sunshine Fraction (default = $1$).

(g)   Click Apply.

The solar calculator output parameters are computed and the results are reported in the console window. The default values are shown below:

Fair Weather Conditions:
Sun Direction Vector:  X: -0.0785396, Y: 0.170758, X: 0.982178
Sunshine Fraction: 1
Direct Normal Solar Irradiation (at Earth's surface) [W/m^2]:
881.635
Diffuse Solar Irradiation - vertical surface: [W/m^2]:
152.107
Diffuse Solar Irradiation - horizontal surface: [W/m^2]:
118.727
Ground Reflected Solar Irradiation - vertical surface: [W/m^2]:
96.4649

4.   For transient simulations, enter the Time Steps Per Solar Load Update under Update Parameters. The number of time steps that you specify will direct the FLUENT solver to update the solar load data for the specified flow-time intervals in the unsteady solution process.



Setting Boundary Conditions for Solar Loading


Once you have defined the solar parameters for the solar load model (Section  13.3.18), you will need to setup boundary conditions for boundary zones that will participate in solar loading.

Define $\rightarrow$ Boundary Conditions...



Solar Ray Tracing


1.   Set the boundary condition for each inlet and exit boundary zone that you want to include in solar loading.

  

(a)   Open the inlet or exit boundary condition panel (e.g., Velocity Inlet) and click the Radiation tab (Figure  13.3.35).

(b)   Enable the Participates in Solar Ray Tracing option. (The default is enabled for all boundary conditions.) If you deactivate solar ray tracing by disabling this option the surface will be ignored and the solar ray will pass through it with no interaction, regardless of the boundary condition type.

(c)   Click OK.

Figure 13.3.35: The Velocity Inlet Panel
figure

2.   Set the boundary condition for each wall boundary zone that you want to include in solar loading.

(a)   Open a Wall boundary condition panel and click the Radiation tab.

(b)   Define the wall as opaque or semi-transparent. An opaque wall will not allow any solar radiation to pass through it, while a semi-transparent surface will allow a portion of the solar radiation to pass through it.)

i.   For an opaque wall, select opaque from the drop-down list for BC Type (Figure  13.3.36). Then enable the Participates in Solar Ray Tracing option and enter constant values for Direct Visible and Direct IR absorptivity.

figure   

Absorption in the visible and infrared portions of the spectrum
define the surface material for the opaque wall.

Figure 13.3.36: The Wall Panel
figure

ii.   For a semi-transparent wall, select semi-transparent from the drop-down list for BC Type (Figure  13.3.37). Then, enable the Participates in Solar Ray Tracing option and enter constant values for Direct Visible, Direct IR, and Diffuse Hemispherical absorptivity and transmissivity.

Figure 13.3.37: The Wall Panel
figure

figure   

Absorption and transmittance in the visible and infrared portions
of the spectrum, as well as the "shading'' formulation ( Diffuse
Hemispherical
), define the surface material for a semi-transparent
wall.

iii.   Click OK.

figure   

FLUENT will calculate the reflectivity as the difference between one and the sum of absorptivity and transmissivity:


 reflectivity = 1 - (absorptivity + transmissivity) (13.3-103)



DO irradiation


1.   For DO Irradiation, all boundary conditions are setup as normal for the DO model, except that now you can select semi-transparent boundary surfaces which will provide a source of solar irradiation.

(a)   Open a Wall boundary condition panel and click the Radiation tab (Figure  13.3.38).

Figure 13.3.38: The Wall Panel
figure

(b)   Select semi-transparent from the drop-down list for BC Type.

(c)   Enable the Use Beam Direction from Solar Parameters option Solar BC Options to have the values for beam direction applied from the Solar Parameters panel.

figure   

Note that the sign of the beam direction that is needed for the DO model is opposite the sun direction vector that is entered or derived from the solar parameters. The beam direction in the DO model is the direction of external radiation (e.g., radiation coming from the sun), while the sun direction vector in the solar load model points to the sun. Incident radiation and sun angle always have an opposite sign since they are quantities that are defined from opposite perspectives.

(d)   Enable the Use Total Irradiation from Solar Parameters option to have the solar calculator output be applied for total irradiation. Note that total irradiation is the sum of the direct and diffuse irradiation values. When Use Total Irradiation from Solar Parameters is enabled, the beam width will automatically be set to $0.53$ degrees - the angle subtended by the sun.

(e)   Click OK.



Text Interface-Only Commands


FLUENT has provided some additional commands for solar load setup that are only available in the text interface. These commands are present below.

Automatically Saving Solar Data

It is possible to direct FLUENT to automatically save solar load data to a generic file that you can examine or use in an external program. This is done by executing the text command autosave-solar-data from the text interface.

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ autosave-solar-data

1.   Enter the Solar Data File Frequency (default= $0$).

2.   Enter the filename, in quotations.

3.   Choose to write file in binary format.

The text interface command for autosave-solar-data for a file named solar and a frequency of $1$ is shown below:

/define/models/radiation/solar-parameters> autosave-solar-data
Autosave Solar Data File Frequency [0] 1
Enter Filename [""] "solar"

Automatically Reading Solar Data

When you are executing a transient simulation in parallel FLUENT and you want to take solar loading conditions into consideration, you can use autoread-solar-data text command to automatically read the solar load data file you generated during a serial run into parallel FLUENT. This is done by executing the text command autoread-solar-data from the text interface.

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ autoread-solar-data

1.   Enter the Solar Data File Frequency (default= $0$).

2.   Enter the filename, in quotations.

The text interface command for autosave-read-data for a file named solar and a frequency of $1$ is shown below:

/define/models/radiation/solar-parameters> autosave-solar-data
Autosave Solar Data File Frequency [0] 1
Enter Filename [""] "solar"
Use Binary Format for Reading Data Files [yes]

Aligning the Camera Direction With the Position of the Sun

When the solar load model is enabled, you can direct FLUENT to align the camera direction with the sun position using the text interface command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ sol-camera-pos

This command is useful when you are executing a transient simulation and you want to capture an image of your model with solar load parameters displayed (such as solar heat flux) as the sun position changes with time in order to create an animation. See Section  13.3.18 for details.

Specifying the Scattering Fraction

You can modify the default scattering fraction ( $1$) using the text interface command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ scattering-fraction

The scattering fraction is the amount of direct radiation that has been reflected from opaque surfaces (after entering through the transparent surfaces) that will be considered to remain within the space and be evenly distributed among all surfaces. The value is between $0$ and $1$.

The text interface command for specifying a scattering-fraction of $0.5$ is shown below:

/define/models/radiation/solar-parameters> scattering-fraction
Scattering Fraction [1] .5

Applying the Solar Load on Adjacent Fluid Cells

You can direct FLUENT to apply the solar load that is computed from the solar ray tracing algorithm to adjacent fluid cells by issuing the following command at the text interface:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ sol-adjacent-fluidcells

The text interface command is shown below:

/define/models/radiation/solar-parameters> sol-adjacent-fluidcells
Apply Solar Load on adjacent Fluid Cells? [no] y

This command allows you to apply solar loads to adjacent fluid cells only, even if solid or shell conduction zones are present. By applying the solar load on adjacent fluid cells, you are overruling the default order of the adjacent cell assignment in FLUENT which is shell, solid, fluid.

Specifying Quad Tree Refinement Factor

You can modify the default value ( $7$) for the maximum quad tree refinement factor in the solar ray tracing algorithm using the text command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ quad-tree-parameters

The text interface command is shown below, when a new maximum refinement value of $10$ is specified:

/define/models/radiation/solar-parameters> quad-tree-parameters
Maximum Quad-Tree Refinement [7] 10

Specifying Ground Reflectivity

You can modify the default value ( $0.2$) for the ground reflectivity using the text command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ ground-reflectivity

Ground reflectivity includes the contribution of reflected solar radiation from ground surfaces. It is treated as part of the total diffuse solar irradiation when the solar calculator is used in conjunction with the Diffuse Solar Irradiation illumination parameter. The default value is $0.2$.

/define/models/radiation/solar-parameters> ground-reflectivity
Ground Reflectivity [0.2] 0.5

Additional Text Interface Commands

Some solar load commands that are available in the graphical user interface are also made available in the text interface. For example, you can turn the solar load model on using the text command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar?

You can also enter the solar calculator parameters in the text interface by executing the command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-calculator

Once invoked, you will be prompted to enter the solar calculator input parameters.

To set the illumination parameters, select this option from the solar-parameters menu:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ illumination-parameters

And finally, you can direct FLUENT to compute the solar load on demand, by issuing the text command:

define $\rightarrow$ models $\rightarrow$ radiation $\rightarrow$ solar-parameters $\rightarrow$ sol-on-demand

When the command is initiated, the solar data are written to the console window (see Section  13.3.18 for a sample).



Postprocessing Solar Load Quantities


The following solar load quantities can be used to visualize the illuminated areas and shadows created by solar radiation.

These quantities are available for postprocessing of solar loading at wall boundaries and can be displayed as contours of Wall Fluxes in the Contours panel. For steady-state simulations, the solar flux data is computed at solution initialization and is available for postprocessing. You can also compute the solar load at any time during your FLUENT session, once you have setup the model and applied boundary conditions. To compute the solar load on demand, you can issue the sol-on-demand command in the text interface (see Section  13.3.18 for details).

Solar heat flux, for example, can be displayed for surfaces using the Contours panel. A sample panel is shown below (Figure  13.3.39).

Display $\rightarrow$ Contours...

Figure 13.3.39: The Contours Panel
figure

Solar Load Animation at Different Sun Positions

The solar camera alignment command is useful when you want to take timed pictures of solar loading effects of your model during transient simulations, and later create animations of the image files using an external program. Follow the procedure below.

1.   Read (or set-up) your transient case file in FLUENT.

2.   Set-up the automatic execution of solution commands in the Execute Commands panel that will: 1) display solar load parameter graphics, 2) re-position the solar camera such that the view is aligned with the instantaneous sun direction, and 3) generate a hardcopy image file (.tiff) during the solution process in the Execute Commands panel.

Solve $\rightarrow$ Execute Commands...

3.   Initialize and iterate the solution.

4.   Animate the .tiff files using an external animation tool.

The following commands entered in the Execute Commands panel will direct FLUENT to display contours of solar heat flux, align the camera with the current direction of the sun, and then generate a hardcopy image file (.tiff) of the solar heat flux contour every 300 time steps during the unsteady simulation. See Figure  13.3.40.

/di/cont solar-heat-flux ,,
/def/mod/rad/solar-para/sol-camera-pos
/di/hc "flux-%t.tiff"

Figure 13.3.40: The Execute Commands Panel
figure

Reporting and Displaying Solar Load Quantities

FLUENT provides some additional solar load variables that you can use for postprocessing when your model includes solar ray tracing. You can generate graphical plots or alphanumeric reports of the following variables:

In the Wall Fluxes... category:

See Chapter  30 for their definitions.


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