ANSYS FLUENT 12.0 User's Guide - 13.3.9 Solar Load Model

13.3.9 Solar Load Model

ANSYS 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:

automotive climate control (ACC) applications
human comfort modeling applications in buildings

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. ANSYS 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, ANSYS 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.

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 ANSYS 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 13.3.9 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 ANSYS FLUENT in the Radiation Model dialog box.

The solar ray tracing option allows you to include the effects of direct solar illumination as well as diffuse solar radiation in your ANSYS 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. ANSYS 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 . 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 ANSYS 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 faces and greater. The quad-tree refinement factor can be modified in the text interface. The default value of this parameter is 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 dialog box. 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 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 [ 23].

Transmissivity is given by

$T(\theta, \lambda) = T(0,\lambda) Tref(\theta)$

(13.3-4)

where

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

(13.3-5)

Reflectivity is given by

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

(13.3-6)

where

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

(13.3-7)

The constants used in Equations 13.3-4 and 13.3-6 are for coated glazings and are taken from Finlayson and Arasteh. [ 23]. The normal transmissivity and reflectivity, $T(0,\lambda)$ and $R(0,\lambda)$ are specified in the Wall boundary conditions dialog box.

Inputs

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

sun direction vector
direct solar irradiation
diffuse solar irradiation
spectral fraction
direct and IR absorptivity (opaque wall)
direct and IR absorptivity and transmissivity (semi-transparent wall)
diffuse hemispherical absorptivity and transmissivity (semi-transparent wall)
solar transmissivity factor
quad tree refinement factor
scattering fraction
ground reflectivity

The sun direction vector is the direction vector looking to the sun, from which the direct irradiation will be incident. You can enter the vector components ( X, Y, Z) and the direct and diffuse solar irradiation fluxes in the Radiation Model dialog box, or you can have these parameters derived from the solar calculator. These irradiation fluxes can also be specified using a user-defined function (Section 13.3.9). The spectral fraction is the final input in the Radiation Model dialog box. This defines the split of visible and infra-red (shortwave and longwave respectively) radiation, specifically the fraction of the direct irradiation flux that is in the visible band. These quantities can also be defined through the text interface.

The scattering fraction defines the amount of non-absorbed radiation that will be distributed (uniformly) across all participating surfaces. This is required as the solar load model does not track the rays beyond the first opaque surface. Therefore, a highly glazed space where incident radiation is likely to be reflected back out will have a low value. Conversely, a predominantly opaque (wall-bounded) space where reflected radiation is likely to be incident upon (and ultimately absorbed by) other opaque surfaces will have a high value. This parameter is defined through the text interface only, taking a default value of 1.0:

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

The ground reflectivity is used by the solar calculator to compute the background diffuse radiation intensity component contributed to by radiation reflected off the ground. This should be based on typical figures for the surface reflectivity of the outside ground surfaces. By default this is set to 0.2, but can be adjusted through the text-interface:

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

The quad-tree-refinement parameter determines the level of detail used by the shading algorithm. By default this is set to 7 which will generally work well, but can lie between 0 and 10. This is defined only through the text interface:

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

Further details on the text interface-only entries is provided later in this section (see Text Interface-Only Commands).

The wall related absorptivity and transmissivity parameters are entered in the Wall boundary conditions dialog box (under the Radiation tab) for the particular wall zones you wish to participate in solar ray tracing. On flow boundaries you have a solar transmissivity factor to allow you to attenuate the incoming solar flux, e.g. set to 1 for a fully open inlet or set to 0 for a light obscuring louvered inlet.

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 (which 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:

direct irradiation
diffuse irradiation
beam direction
beam width
diffuse fraction

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

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 the sun angle always have an opposite sign since they are quantities that are defined from opposite perspectives.

Solar Calculator

ANSYS 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 (latitude, longitude, time zone)
starting date and time
mesh orientation
solar irradiation method
sunshine factor

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 mesh orientation, you will need to specify the North and East direction vector in the CFD mesh. 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 dialog box that is accessible from the Radiation Model dialog box (Figure 13.3.22). Alternatively, you can enter the parameters using text interface commands (Section 13.3.9).

The following values are computed by the solar calculator and are displayed in the console whenever the solar calculator is used:

sun direction vector
sunshine fraction
direct normal solar irradiation at earth's surface
diffuse solar irradiation - vertical and horizontal surface
ground reflected (diffuse) solar irradiation - vertical surface

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.

ANSYS 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

ANSYS 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:

$Edn = \frac{A}{e^\frac{B}{\sin(\beta)}}$

(13.3-8)

where and are apparent solar irradiation at air mass 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):

$Edn = S_{etrn} S_{unprime}$

(13.3-9)

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:

(13.3-10)

where is a constant whose values are given in Table 7 from Chapter 30 of the 2001 ASHRAE Handbook of Fundamentals, is the ratio of sky diffuse radiation on a vertical surface to that on a horizontal surface (calculated as a function of incident angle), and 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:

$Ed = C Edn \frac{(1 + \cos\epsilon)}{2}$

(13.3-11)

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:

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

(13.3-12)

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

Computation of Load Distribution

In calculating the solar load that will be incident on each surface, it is necessary to distinguish between the calculation of diffuse and direct solar loads. A direct load will be tracked from participating transmissive boundary surfaces and non-participating boundary surfaces, the former provides some opportunity to attenuate the incoming flux by absorption and reflection, while the non-participating surfaces allow the flux to enter without any drop in intensity. The direct load is then tracked through the model space until it is incident on an opaque surface, or it exists through a transmissive or non-participating boundary zone. During its passage, its intensity will be attenuated as it passes through participating semi-transparent internal walls, where some radiation may be absorbed and some may be reflected. The total amount of direct radiation which is reflected at internally facing surfaces will be added to the scattered radiation budget for further use later.

The diffuse load originates at participating transmissive boundary surfaces. It is these surfaces that permit diffuse radiation to enter, irrespective of their orientation relative to the direction vector. For each transmissive surface, some of the incoming diffuse load may be immediately absorbed and/or reflected to the outside. The rest is assumed to be transmitted inside and summed from all of these surfaces to give an initial diffuse budget. Onto this budget is added a fraction of the previously computed scattered radiation from the direct load, the fraction used is defined as an input to the model. This provides the total diffuse load. This is then uniformly distributed across all surfaces which are participating in the solar calculation, irrespective of whether they are opaque or semi-transparent. There is no scope to define local absorptivity for this distribution and no biasing with regards proximity to transmissive surfaces. Note that a non-participating boundary zone will allow direct load to enter the model space but will not provide an incoming quantity of diffuse load.

Note that the solar flux which is externally incident on an opaque surface will be completely disregarded, e.g. solar load on an opaque roof of a model whose internals only are modeled will not be included as a heat gain. Instead, this heat gain should be manually calculated and applied as a thermal condition, typically using a fixed heat flux or a radiation/mixed condition.

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 set up the solar load model (Section 13.3.9) and boundary conditions (Section 13.3.9) 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).

Note that you can compute the solar load at any time once you have set up the model by using the sol-on-demand text interface command (see Section 13.3.9 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 dialog box. ANSYS 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 ANSYS 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 the serial solver in ANSYS FLUENT and read (or set up) your case file.
2. Set up the solar load model (Section 13.3.9).
3. Set up the boundary conditions (Section 13.3.9)
4. Initialize the solution in the Solution Initialization task page.
Solar load data is computed at solution initialization for steady-state cases. The data will be written to the console, 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. Set up 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 ANSYS FLUENT and read (or set up) your case file.
2. Set up the solar load model which includes specifying the Time Steps per Solar Load Update in the Radiation Model dialog box (Section 13.3.9).
3. Set up the boundary conditions (Section 13.3.9).
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 13.3.9). 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 Equations dialog box, which is accessed from the Solution Controls task page.
6. Save the case file.
7. Initialize the solution. Solar load data is computed at solution initialization for steady-state cases and written to the console. See previous steady-state procedure.
8. Set the Max Iterations/Time Step to in the Run Calculation task page.
9. Run the simulation. As the solver iterates, ANSYS 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. 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.

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 13.3.9). 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 task page.
14. Run the transient simulation. 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. See this section in the separate UDF Manual for more information. After it is 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 dialog box. See Step 2 in Section 13.3.9 for details.

Setting Up the Solar Load Model

The solar load model is enabled in the Radiation Model dialog box (Figure 13.3.19).

Models Radiation Edit...

**Figure 13.3.19:** The **Radiation Model** Dialog Box

Solar load is available in the 3D solver only, and can be used to model steady and unsteady flows.

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 ANSYS 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.

To set up the solar load model, perform the following steps:

1.   Enable the solar load model in the Radiation Model Dialog Box.

(a)   To enable the solar ray tracing algorithm, select Solar Ray Tracing under Solar Load (Figure 13.3.20).

Figure 13.3.20: The Radiation Model Dialog Box (With Solar Load Model Solar Ray Tracing Option)

(b)   To enable the DO irradiation option, first select Discrete Ordinates under Model, and then select DO Irradiation under Solar Load (Figure 13.3.21).

Figure 13.3.21: The Radiation Model Dialog Box (With Solar Load Model DO Irradiation Option)
2.   Define the solar parameters.

(a)   Enter 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 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. (For more information on writing solar intensity UDFs, see this section in the separate UDF Manual.) 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 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. (For more information on writing solar intensity UDFs, see this section in the separate UDF Manual.) 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 13.3.20), 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 =

$\frac{V}{V + IR}$ (13.3-13)

where is the visible incident solar radiation, and 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 dialog box to open the Solar Calculator dialog box (Figure 13.3.22).

Figure 13.3.22: The Solar Calculator Dialog Box

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

i.   Enter a real number in degrees for Longitude. Values may range from to 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 to .



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 to under Time of Day. Enter an integer or floating point number for Minute.
The time of day is based on a 24-hour clock: hours and minutes corresponds to 12:00 a.m. and hours 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 Mesh Orientation as the vectors for North and East in the CFD mesh 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 = ).

(g)   Click Apply.
The solar calculator output parameters are computed and the results are reported in the console. 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 ANSYS 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 13.3.9), you will need to set up boundary conditions for boundary zones that will participate in solar loading.

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 dialog box (e.g., Velocity Inlet) and click the Radiation tab (Figure 13.3.23).

Figure 13.3.23: The Velocity Inlet Dialog Box

(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)   Enter a value between 0 and 1 for the Solar Transmissivity Factor. This will allow you to control the amount of solar irradiation entering the domain. By reducing the solar transmissivity factor from 1 to 0.5, you can effectively cut the total internal energy source entering the domain by half.



Note that the solar transmissivity factor is applied to both direct
and diffuse solar irradiation components.

(d)   Click OK.
2.   Set the boundary condition for each wall boundary zone that you want to include in solar loading.

(a)   Open a Wall boundary condition dialog box 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 13.3.24). Then enable the Participates in Solar Ray Tracing option and enter constant values for Direct Visible and Direct IR absorptivity.



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

Figure 13.3.24: The Wall Dialog Box

ii.   For a semi-transparent wall, select semi-transparent from the drop-down list for BC Type (Figure 13.3.25). 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 13.3.25: The Wall Dialog Box



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. These parameters are properties of the glazed unit and
should be provided by the glazing manufacturer. The direct
components are based on normal incident radiation ( ANSYS FLUENT
adjusts this for the actual angle of incidence). Most manufacturers
present this information in a slightly different way so it may be
necessary to seek guidance from the supplier. Another useful
source of data can be found in the ASHRAE Fundamentals
Handbook, chapter on Fenestration.

iii.   Click OK.

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

(13.3-14)

DO Irradiation

1.   For DO irradiation, all boundary conditions are set up 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 dialog box and click the Radiation tab (Figure 13.3.26).

Figure 13.3.26: The Wall Dialog Box

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

(c)   Enable the Use Beam Direction from Solar Load Model Settings option, under Solar BC Options, to have the values for beam direction applied from the Solar Load Model settings in the Radiation dialog box.



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 Direct and Diffuse Irradiation from Solar Load Model Settings option to have the solar calculator output be applied for direct and diffuse irradiation.
When Use Direct and Diffuse Irradiation from Solar Load Model Settings is enabled, the beam width will automatically be set to degrees - the angle subtended by the sun.

(e)   Click OK.

Text Interface-Only Commands

ANSYS 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 Ray Tracing Data

It is possible to direct ANSYS 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= ).
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 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 ANSYS 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 ANSYS 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= ).
2. Enter the filename, in quotations.

The text interface command for autosave-read-data for a file named solar and a frequency of 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 ANSYS 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 13.3.9 for details.

Specifying the Scattering Fraction

You can modify the default scattering fraction ( ) 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 and .

The text interface command for specifying a scattering-fraction of 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 ANSYS 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 ANSYS FLUENT which is shell, solid, fluid.

Specifying Quad Tree Refinement Factor

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

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

Ground reflectivity $\rho_g$ (Equation 13.3-12) 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 .

/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 ANSYS 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 (see Section 13.3.9 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.

solar heat flux (i.e., sum of visible and IR absorbed solar flux on opaque walls)
absorbed visible and IR solar flux (semi-transparent walls only)
reflected visible and IR solar flux (semi-transparent walls only)
transmitted visible and IR solar flux (semi-transparent walls only)

These quantities are available for postprocessing of solar loading at wall boundaries and can be displayed as contours of Wall Fluxes in the Contours dialog box. 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 ANSYS FLUENT session, after you have set up 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 13.3.9 for details).

Solar heat flux, for example, can be displayed for surfaces using the Contours dialog box. A sample dialog is shown below (Figure 13.3.27).

Graphics and Animations Contours Set Up...

**Figure 13.3.27:** The **Contours** Dialog Box

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 ANSYS FLUENT.
2. Set up the automatic execution of solution commands in the Execute Commands dialog box 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 picture image file (.tiff) during the solution process in the Execute Commands dialog box.
Calculation Activities (Execute Commands) Create/Edit...
3. Initialize and run the solution.
4. Animate the .tiff files using an external animation tool.

The following commands entered in the Execute Commands dialog box will direct ANSYS FLUENT to display contours of solar heat flux, align the camera with the current direction of the sun, and then generate a picture image file (.tiff) of the solar heat flux contour every 300 time steps during the unsteady simulation. See Figure 13.3.28.

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

**Figure 13.3.28:** The **Execute Commands** Dialog Box

Reporting and Displaying Solar Load Quantities

ANSYS 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:

Solar Heat Flux
Transmitted Visible Solar Flux (semi-transparent walls)
Transmitted IR Solar Flux (semi-transparent walls)
Reflected Visible Solar Flux (semi-transparent walls)
Reflected IR Solar Flux (semi-transparent walls)
Absorbed Visible Solar Flux (semi-transparent walls)
Absorbed IR Solar Flux (semi-transparent walls)

See Chapter 31 for their definitions.

Previous: 13.3.8 Postprocessing Radiation Quantities
Up: 13.3 Modeling Radiation
Next: 13.4 Modeling Periodic Heat