
Turbulent flows are significantly affected by the presence of walls. Obviously, the mean velocity field is affected through the noslip condition that has to be satisfied at the wall. However, the turbulence is also changed by the presence of the wall in nontrivial ways. Very close to the wall, viscous damping reduces the tangential velocity fluctuations, while kinematic blocking reduces the normal fluctuations. Toward the outer part of the nearwall region, however, the turbulence is rapidly augmented by the production of turbulence kinetic energy due to the large gradients in mean velocity.
The nearwall modeling significantly impacts the fidelity of numerical solutions, inasmuch as walls are the main source of mean vorticity and turbulence. After all, it is in the nearwall region that the solution variables have large gradients, and the momentum and other scalar transports occur most vigorously. Therefore, accurate representation of the flow in the nearwall region determines successful predictions of wallbounded turbulent flows.
The

models, the RSM, and the LES model are primarily valid for turbulent core flows (i.e., the flow in the regions somewhat far from walls). Consideration therefore needs to be given as to how to make these models suitable for wallbounded flows. The SpalartAllmaras and

models were designed to be applied throughout the boundary layer, provided that the nearwall mesh resolution is sufficient.
Numerous experiments have shown that the nearwall region can be largely subdivided into three layers. In the innermost layer, called the "viscous sublayer'', the flow is almost laminar, and the (molecular) viscosity plays a dominant role in momentum and heat or mass transfer. In the outer layer, called the fullyturbulent layer, turbulence plays a major role. Finally, there is an interim region between the viscous sublayer and the fully turbulent layer where the effects of molecular viscosity and turbulence are equally important. Figure 4.12.1 illustrates these subdivisions of the nearwall region, plotted in semilog coordinates.
In Figure 4.12.1, , where is the friction velocity, defined as .
Wall Functions vs. NearWall Model
Traditionally, there are two approaches to modeling the nearwall region. In one approach, the viscosityaffected inner region (viscous sublayer and buffer layer) is not resolved. Instead, semiempirical formulas called "wall functions'' are used to bridge the viscosityaffected region between the wall and the fullyturbulent region. The use of wall functions obviates the need to modify the turbulence models to account for the presence of the wall.
In another approach, the turbulence models are modified to enable the viscosityaffected region to be resolved with a mesh all the way to the wall, including the viscous sublayer. For the purposes of discussion, this will be termed the "nearwall modeling'' approach. These two approaches are depicted schematically in Figure 4.12.2.
In most highReynoldsnumber flows, the wall function approach substantially saves computational resources, because the viscosityaffected nearwall region, in which the solution variables change most rapidly, does not need to be resolved. The wallfunction approach is popular because it is economical, robust, and can be reasonably accurate. It is a practical option for the nearwall treatments for industrial flow simulations.
The wallfunction approach, however, is inadequate in situations where the lowReynoldsnumber effects are pervasive and the assumptions underlying the wall functions cease to be valid. Such situations require nearwall models that are valid in the viscosityaffected region and accordingly integrable all the way to the wall.
ANSYS FLUENT provides both the wallfunction approach and the nearwall modeling approach.
Wall Functions
Wall functions are a set of semiempirical formulas and functions that in effect "bridge'' or "link'' the solution variables at the nearwall cells and the corresponding quantities on the wall. The wall functions comprise
Depending on the choice of turbulent model, ANSYS FLUENT offers three to four choices of wallfunction approaches: