In basic terms, the Transition Ratio control determines the rate at which adjacent elements grow. It is the volume-based size change between the last layer of elements in the inflation layer and the first elements in the tetrahedron region. The value of Transition Ratio is an ideal value and should produce accurate size change for inflation from a planar boundary. However, be aware that areas of strong curvature will introduce an inaccuracy into the size change. If proximity detection is activated, elements in proximity and elements with prism aspect ratios meeting the value defined by the Maximum Height over Base control will ignore this transition ratio.

The Transition Ratio control is applicable only when Inflation Option is Smooth Transition. Valid values for Transition Ratio are from 0 to 1. When Physics Preference is set to CFD and Solver Preference is set to CFX, the default for Transition Ratio is 0.77. For all other physics preferences, including CFD when Solver Preference is set to either Fluent or Polyflow, the default is 0.272.

The reason for this difference is because the Fluent and Polyflow solvers use a cell-centered scheme for transition ratio. In a cell-centered scheme, the fluid flow variables are allocated at the center of the computational cell, and the mesh-element is the same as the solver-element. In contrast, the CFX solver uses a vertex-centered scheme. In a vertex-centered scheme, the fluid flow variables are stored at the cell vertex, and the solver-element is a "dual" of the mesh-element. This means that the vertex of the mesh-element is the center of the solver-element. The difference between the two schemes is illustrated in the figure below.