The CELL_AVERAGE method of calculating gradients is a carry-over from the initial unstructured solver in Wind-US 1.0. Average face values are first computed using an inverse distance weighted average among the cell-centered values from cells sharing the face. The Green-Gauss theorem is then used to compute the average gradient in the cell from the average face values. This has been found to be stable, but can be inaccurate, especially for high-aspect-ratio cells, and is not guaranteed to preserve an initially-linear arbitrary function in the computational mesh.

The LEAST_SQUARES method calculates gradients using a least squares fit to the flow field values in the region of the cell. Inverse distance weighting is used, with values obtained from neighboring faces or nodes. [Mavriplis, D. J. (2003) "Revisiting the Least-Squares Procedure for Gradient Reconstruction on Unstructured Meshes," AIAA Paper 2003-3986.] This procedure should be used whenever second-order accuracy is important, especially with highly-stretched meshes typically used for viscous problems. For example, for computing accurate drag values on a body at cruise conditions, using LEAST_SQUARES is essential.

The default setting is LEAST_SQUARES.