
Summary
You will enter the following information for a wall boundary:
Defining Thermal Boundary Conditions at Walls
When you are solving the energy equation, you need to define thermal boundary conditions at wall boundaries. Five types of thermal conditions are available:
If the wall zone is a ``twosided wall'' (a wall that forms the interface between two regions, such as the fluid/solid interface for a conjugate heat transfer problem) a subset of these thermal conditions will be available, but you will also be able to choose whether or not the two sides of the wall are ``coupled''. See below for details.
The inputs for each type of thermal condition are described below. If the wall has a nonzero thickness, you should also set parameters for calculating thinwall thermal resistance and heat generation in the wall, as described below.
You can model conduction within boundary walls and internal (i.e., twosided) walls of your model. This type of conduction, called shell conduction, allows you to more conveniently model heat conduction on walls where the wall thickness is small with respect to the overall geometry (e.g., finned heat exchangers or sheet metal in automobile underhoods). Meshing these walls with solid cells would lead to highaspectratio meshes and a significant increase in the total number of cells. See below for details about shell conduction.
Thermal conditions are entered in the Thermal section of the Wall panel (Figure 6.13.1), which is opened from the Boundary Conditions panel (as described in Section 6.1.4). To view the thermal conditions, click the Thermal tab.
Heat Flux Boundary Conditions
For a fixed heat flux condition, choose the Heat Flux option under Thermal Conditions. You will then need to set the appropriate value for the heat flux at the wall surface in the Heat Flux field. You can define an adiabatic wall by setting a zero heat flux condition. This is the default condition for all walls.
Temperature Boundary Conditions
To select the fixed temperature condition, choose the Temperature option under Thermal Conditions in the Wall panel. You will need to specify the temperature at the wall surface ( Temperature). The heat transfer to the wall is computed using Equation 6.138 or Equation 6.139.
Convective Heat Transfer Boundary Conditions
For a convective heat transfer wall boundary, select Convection under Thermal Conditions. Your inputs of Heat Transfer Coefficient and Free Stream Temperature will allow FLUENT to compute the heat transfer to the wall using Equation 6.1312.
External Radiation Boundary Conditions
If radiation heat transfer from the exterior of your model is of interest, you can enable the Radiation option in the Wall panel and set the External Emissivity and External Radiation Temperature.
Combined Convection and External Radiation Boundary Conditions
You can choose a thermal condition that combines the convection and radiation boundary conditions by selecting the Mixed option. With this thermal condition, you will need to set the Heat Transfer Coefficient, Free Stream Temperature, External Emissivity, and External Radiation Temperature.
ThinWall Thermal Resistance Parameters
By default, a wall will have a thickness of zero. You can, however, in conjunction with any of the thermal conditions, model a thin layer of material between two zones. For example, you can model the effect of a piece of sheet metal between two fluid zones, a coating on a solid zone, or contact resistance between two solid regions. FLUENT will solve a 1D conduction equation to compute the thermal resistance offered by the wall and the heat generation in the wall.
To include these effects in the heat transfer calculation you will need to specify the type of material, the thickness of the wall, and the heat generation rate in the wall. Select the material type in the Material Name dropdown list, and specify the thickness in the Wall Thickness field. If you want to check or modify the properties of the selected material, you can click Edit... to open the Material panel; this panel contains just the properties of the selected material, not the full contents of the standard Materials panel.
The thermal resistance of the wall is , where is the conductivity of the wall material and is the wall thickness. The thermal boundary condition you set will be specified on the outside of the thin wall, as shown in Figure 6.13.2, where is the fixed Temperature specified at the wall.
!! Note that for thin walls, you can specify only a constant thermal conductivity. If you want to use a nonconstant thermal conductivity for a wall with nonzero thickness, you should use the shell conduction model (see below for details).
Specify the heat generation rate inside the wall in the Heat Generation Rate field. This option is useful if, for example, you are modeling printed circuit boards where you know the electrical power dissipated in the circuits.
Thermal Conditions for TwoSided Walls
If the wall zone has a fluid or solid region on each side, it is called a ``twosided wall''. When you read a grid with this type of wall zone into FLUENT, a ``shadow'' zone will automatically be created so that each side of the wall is a distinct wall zone. In the Wall panel, the shadow zone's name will be shown in the Shadow Face Zone field. You can choose to specify different thermal conditions on each zone, or to couple the two zones:
Shell Conduction in Walls
To enable shell conduction for a wall, turn on the Shell Conduction option in the Wall boundary condition panel. When this option is enabled, FLUENT will compute heat conduction within the wall, in addition to conduction across the wall (which is always computed when the energy equation is solved). The Shell Conduction option will appear in the Wall panel for all walls when solution of the energy equation is active. For a boundary wall, the thermal conditions are applied as described above for thin walls.
!! You must specify a nonzero Wall Thickness in the Wall panel, because the shell conduction model is relevant only for walls with nonzero thickness.
!! Note that the shell conduction model has several limitations:
Defining Wall Motion
Wall boundaries can be either stationary or moving. The stationary boundary condition specifies a fixed wall, whereas the moving boundary condition can be used to specify the translational or rotational velocity of the wall, or the velocity components.
Wall motion conditions are entered in the Momentum section of the Wall panel (Figure 6.13.4), which is opened from the Boundary Conditions panel (as described in Section 6.1.4). To view the wall motion conditions, click the Momentum tab.
Defining a Stationary Wall
For a stationary wall, choose the Stationary Wall option under Wall Motion.
Defining Velocity Conditions for Moving Walls
If you wish to include tangential motion of the wall in your calculation, you need to define the translational or rotational velocity, or the velocity components. Select the Moving Wall option under Wall Motion. The Wall panel will expand, as shown in Figure 6.13.4, to show the wall velocity conditions.
Note that you cannot use the moving wall condition to model problems where the wall has a motion normal to itself. FLUENT will neglect any normal component of wall motion that you specify using the methods below.
Specifying Relative or Absolute Velocity
If the cell zone adjacent to the wall is moving (e.g., if you are using a moving reference frame or a sliding mesh), you can choose to specify velocities relative to the zone motion by enabling the Relative to Adjacent Cell Zone option. If you choose to specify relative velocities, a velocity of zero means that the wall is stationary in the relative frame, and therefore moving at the speed of the adjacent cell zone in the absolute frame. If you choose to specify absolute velocities (by enabling the Absolute option), a velocity of zero means that the wall is stationary in the absolute frame, and therefore moving at the speed of the adjacent cell zonebut in the opposite directionin the relative reference frame.
!! If you are using one or more moving reference frames, sliding meshes, or mixing planes, and you want the wall to be fixed in the moving frame, it is recommended that you specify relative velocities (the default) rather than absolute velocities. Then, if you modify the speed of the adjacent cell zone, you will not need to make any changes to the wall velocities, as you would if you specified absolute velocities.
Note that if the adjacent cell zone is not moving, the absolute and relative options are equivalent.
Translational Wall Motion
For problems that include linear translational motion of the wall boundary (e.g., a rectangular duct with a moving belt as one wall) you can enable the Translational option and specify the wall's Speed and Direction ( X,Y,Z vector). By default, wall motion is ``disabled'' by the specification of Translational velocity with a Speed of zero.
If you need to define nonlinear translational motion, you will need to use the Components option, described below.
Rotational Wall Motion
For problems that include rotational wall motion you can enable the Rotational option and define the rotational Speed about a specified axis. To define the axis, set the RotationAxis Direction and RotationAxis Origin. This axis is independent of the axis of rotation used by the adjacent cell zone, and independent of any other wall rotation axis. For 3D problems, the axis of rotation is the vector passing through the specified RotationAxis Origin and parallel to the vector from (0,0,0) to the ( X,Y,Z) point specified under RotationAxis Direction. For 2D problems, you will specify only the RotationAxis Origin; the axis of rotation is the direction vector passing through the specified point. For 2D axisymmetric problems, you will not define the axis: the rotation will always be about the axis, with the origin at (0,0).
Note that the modeling of tangential rotational motion will be correct only if the wall bounds a surface of revolution about the prescribed axis of rotation (e.g., a circle or cylinder). Note also that rotational motion can be specified for a wall in a stationary reference frame.
Wall Motion Based on Velocity Components
For problems that include linear or nonlinear translational motion of the wall boundary you can enable the Components option and specify the XVelocity, YVelocity, and ZVelocity of the wall. You can define nonlinear translational motion using a boundary profile or a userdefined function for the XVelocity, YVelocity, and/or ZVelocity of the wall.
Wall Motion for TwoSided Walls
As discussed earlier in this section, when you read a grid with a twosided wall zone (which forms the interface between fluid/solid regions) into FLUENT, a ``shadow'' zone will automatically be created so that each side of the wall is a distinct wall zone. For twosided walls, it is possible to specify different motions for the wall and shadow zones, whether or not they are coupled. Note, however, that you cannot specify motion for a wall (or shadow) that is adjacent to a solid zone.
Defining Shear Conditions at Walls
Three types of shear conditions are available:
Note that all moving walls are noslip; the other shear conditions are relevant only for stationary walls.
The noslip condition is the default, and it indicates that the fluid sticks to the wall and moves with the same velocity as the wall, if it is moving. The specified shear and Marangoni stress boundary conditions are useful in modeling situations in which the shear stress (rather than the motion of the fluid) is known. Examples of such situations are applied shear stress, slip wall (zero shear stress), and free surface conditions (zero shear stress or shear stress dependent on surface tension gradient). The specified shear boundary condition allows you to specify the , , and components of the shear stress as constant values or boundary profiles. The Marangoni stress boundary condition allows you to specify the gradient of the surface tension with respect to the temperature at this surface. The shear stress is calculated based on the surface gradient of the temperature and the specified surface tension gradient. The Marangoni stress option is available only for calculations in which the energy equation is being solved.
Shear conditions are entered in the Momentum section of the Wall panel, which is opened from the Boundary Conditions panel (as described in Section 6.1.4). To view the shear conditions, click the Momentum tab.
Modeling NoSlip Walls
You can model a noslip wall by selecting the No Slip option under Shear Condition. This is the default for all walls in viscous flows.
Specified Shear
In addition to the noslip wall that is the default for viscous flows, you can model a slip wall by specifying zero or nonzero shear. To specify the shear, select the Specified Shear option under Shear Condition (see Figure 6.13.5). You can then enter , , and components of shear under Shear Stress. Wall functions for turbulence are not used with the Specified Shear option.
Marangoni Stress
FLUENT can also model shear stresses caused by the variation of surface tension due to temperature. The shear stress applied at the wall is given by
where is the surface tension gradient with respect to temperature, and is the surface gradient. This shear stress is then applied to the momentum equation.
To model Marangoni stress for the wall, select the Marangoni Stress option under Shear Condition (see Figure 6.13.6). This option is available only for calculations in which the energy equation is being solved. You can then enter the surface tension gradient ( in Equation 6.131) in the Surface Tension Gradient field. Wall functions for turbulence are not used with the Marangoni Stress option.
Modeling Wall Roughness Effects in Turbulent WallBounded Flows
Fluid flows over rough surfaces are encountered in diverse situations. Examples are, among many others, flows over the surfaces of airplanes, ships, turbomachinery, heat exchangers, and piping systems, and atmospheric boundary layers over terrain of varying roughness. Wall roughness affects drag (resistance) and heat and mass transfer on the walls.
If you are modeling a turbulent wallbounded flow in which the wall roughness effects are considered to be significant, you can include the wall roughness effects through the lawofthewall modified for roughness.
LawoftheWall Modified for Roughness
Experiments in roughened pipes and channels indicate that the mean velocity distribution near rough walls, when plotted in the usual semilogarithmic scale, has the same slope ( ) but a different intercept (additive constant in the loglaw). Thus, the lawofthewall for mean velocity modified for roughness has the form
where and
where is a roughness function that quantifies the shift of the intercept due to roughness effects.
depends, in general, on the type (uniform sand, rivets, threads, ribs, meshwire, etc.) and size of the roughness. There is no universal roughness function valid for all types of roughness. For a sandgrain roughness and similar types of uniform roughness elements, however, has been found to be wellcorrelated with the nondimensional roughness height, , where is the physical roughness height and . Analyses of experimental data show that the roughness function is not a single function of , but takes different forms depending on the value. It has been observed that there are three distinct regimes:
According to the data, roughness effects are negligible in the hydrodynamically smooth regime, but become increasingly important in the transitional regime, and take full effect in the fully rough regime.
In FLUENT, the whole roughness regime is subdivided into the three regimes, and the formulas proposed by Cebeci and Bradshaw based on Nikuradse's data [ 34] are adopted to compute for each regime.
For the hydrodynamically smooth regime (
):
For the transitional regime ( ):
where is a roughness constant, and depends on the type of the roughness.
In the fully rough regime (
):
In the solver, given the roughness parameters, is evaluated using the corresponding formula (Equation 6.134, 6.135, or 6.136). The modified lawofthewall in Equation 6.132 is then used to evaluate the shear stress at the wall and other wall functions for the mean temperature and turbulent quantities.
Setting the Roughness Parameters
The roughness parameters are in the Momentum section of the Wall panel (see Figure 6.13.6), which is opened from the Boundary Conditions panel (as described in Section 6.1.4). To view the wall roughness parameters, click the Momentum tab.
To model the wall roughness effects, you must specify two roughness parameters: the Roughness Height, , and the Roughness Constant, . The default roughness height ( ) is zero, which corresponds to smooth walls. For the roughness to take effect, you must specify a nonzero value for . For a uniform sandgrain roughness, the height of the sandgrain can simply be taken for . For a nonuniform sandgrain, however, the mean diameter ( ) would be a more meaningful roughness height. For other types of roughness, an ``equivalent'' sandgrain roughness height should be used for .
Choosing a proper roughness constant ( ) is dictated mainly by the type of the given roughness. The default roughness constant ( ) was determined so that, when used with  turbulence models, it reproduces Nikuradse's resistance data for pipes roughened with tightlypacked, uniform sandgrain roughness. You may need to adjust the roughness constant when the roughness you want to model departs much from uniform sandgrain. For instance, there is some experimental evidence that, for nonuniform sandgrains, ribs, and wiremesh roughness, a higher value ( ) is more appropriate. Unfortunately, a clear guideline for choosing for arbitrary types of roughness is not available.
Note that it is not physically meaningful to have a mesh size such that the walladjacent cell is smaller than the roughness height. For best results, make sure that the distance from the wall to the centroid of the walladjacent cell is greater than .
Defining Species Boundary Conditions for Walls
By default, a zerogradient condition for all species is assumed at walls (except for species that participate in surface reactions), but it is also possible to specify species mass fractions at walls. That is, Dirichlet boundary conditions such as those that are specified at inlets can be used at walls as well.
If you wish to retain the default zerogradient condition for a species, no inputs are required. If you want to specify the mass fraction for a species at the wall, the steps are as follows:
The boundary condition type for each species is specified separately, so you can choose to use different methods for different species.
If you are modeling species transport with reactions, you can, alternatively, enable a reaction mechanism at a wall by turning on the Reaction option and selecting an available mechanism from the Reaction Mechanisms dropdown list. See Section 13.1.4 more information about defining reaction mechanisms.
Defining Reaction Boundary Conditions for Walls
If you have enabled the modeling of wall surface reactions in the Species Model panel, you can indicate whether or not surface reactions should be activated for the wall. In the Species section of the Wall panel (Figure 6.13.7), turn the Surface Reactions option on or off.
Note that a zerogradient condition is assumed at the wall for species that do not participate in any surface reactions.
Defining Radiation Boundary Conditions for Walls
If you are using the P1 radiation model, the DTRM, the DO model, or the surfacetosurface model, you will need to set the emissivity of the wall ( Internal Emissivity) in the Radiation section of the Wall panel. If you are using the Rosseland model you do not need to set the emissivity, because FLUENT assumes the emissivity is 1. If you are using the DO model you will also need to define the wall as diffuse, specular, or semitransparent. See Section 11.3.16 for details.
Defining Discrete Phase Boundary Conditions for Walls
If you are modeling a discrete phase of particles, you can set the fate of particle trajectories at the wall in the DPM section of the Wall panel. See Section 21.10 for details.
Defining Multiphase Boundary Conditions for Walls
If you are using the VOF model and you are modeling wall adhesion, you can specify the contact angle for each pair of phases at the wall in the Momentum section of the Wall panel. See Section 22.6.16 for details.
Defining Boundary Conditions for UserDefined Scalars
If you are using any userdefined scalars in your simulation, you can specify boundary conditions for them in the UDS section of the Wall panel. See the separate UDF Manual for details about userdefined scalars.