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Automatic near-wall treatment automatically switches from wall-functions to a low-Re near wall formulation as the mesh is refined.
One of the well known deficiencies of the model is its inability to handle low turbulent Reynolds number computations. Complex damping functions can be added to the model, as well as the requirement of highly refined near-wall grid resolution (y+ < 0.2) in an attempt to model low turbulent Reynolds number flows. This approach often leads to numerical instability. Some of these difficulties may be avoided by using the model, making it more appropriate than the model for flows requiring high near-wall resolution (e.g., high wall heat transfer, transition). However, a strict low-Reynolds number implementation of the model would also require a near wall grid resolution of at least y+ < 2. This condition cannot be guaranteed in most applications at all walls. For this reason, a new near wall treatment was developed by ANSYS CFX for the based models that allows for a smooth shift from a low-Reynolds number form to a wall function formulation. This near wall boundary condition, named automatic near wall treatment in ANSYS CFX, is used as the default in all models based on the -equation (standard , Baseline , SST, -Reynolds Stress).
To take advantage of the reduction in errors offered by the automatic switch to a low-Re near wall formulation, you should attempt to resolve the boundary layer using at least 10 nodes when using these models.