19.5 Isovalue Adaption

Some flows may contain flow features that are easy to identify based on values of a certain quantity. For instance, wakes represent a total pressure deficit, and jets are identifiable by a region of relatively high-velocity fluid. Since it is known that these regions also contain large gradients of important flow quantities (such as $k$ and $\epsilon$ in turbulent flows), it is convenient to perform an isovalue adaption on the relevant flow quantity than to refine on gradients of the individual flow variables.

The isovalue adaption function allows you to mark or refine cells inside or outside a specified range of a selected field variable function. The mesh can be refined or marked for refinement based on geometric and/or solution vector data. Specifically, any quantity in the display list of field variables can be used for the isovalue adaption. Some examples of how you might use the isovalue marking/adaption feature include the following:

• Create masks using coordinate values or the quadric function.

• Refine cells that have a velocity magnitude within a specified range.

• Mark and display cells with a pressure or continuity residual outside of a desired range to determine where the numerical solution is changing rapidly.

The approach used in isovalue adaption function is to compute the specified value for each cell (velocity, quadric function, centroid $x$ coordinate, etc.), and then visit each cell, marking for refinement the cells that have values inside (or outside) the specified ranges.

An example of a problem in which isovalue adaption is useful is shown in Figure  19.5.1. The mesh for an impinging jet is displayed along with contours of $x$ velocity. An isovalue adaption based on $x$ velocity allows refinement of the mesh only in the jet (Figure  19.5.2).

Note:   When adapting to isovalues take care to prevent large gradients in cell volume. This can affect accuracy and impede convergence ( this section in the separate User's Guide). To rectify large gradients in cell volume, adapt to cell-volume change, as demonstrated in Section  19.7.2.

Figure 19.5.1: Impinging Jet Mesh Before Adaption

Figure 19.5.2: Impinging Jet Mesh after Isovalue Adaption