## 14.8. Additional Variables, Wall Distance Variables, and Boundary Distance Variables

A single scalar equation is added for each Additional Variable. A single scalar equation is also added whenever the Wall Distance variables or Boundary Distance variables are referenced in a CFX Expression Language (CEL) expression, or when these quantities are required for a built-in model (for example, two equation-turbulence or boundary-distance-based mesh-stiffness models). Note that Wall Distance values and Boundary Distance values are derived from the solutions of the Wall Scale equations and Boundary Scale equations, respectively. Detailed information on the wall scale equation is available at Wall and Boundary Distance Formulation in the CFX-Solver Theory Guide.

Expect a CPU cost increase of approximately 20% for each Additional Variable over the solution of the U-Mom, V-Mom, W-Mom, and P-Mass equations for a single-phase single-component case. Increases in memory requirements are small.

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