Fluent网格质量
 |
The minimum value that results from calculating Equation 5.1 (p. 131 ) and Equation 5.2 (p. 131 ) for all of the faces is then defined as the orthogonal quality for the cell. Therefore, the worst cells will have an orthogonal quality closer to 0 and the best cells will have an orthogonal quality closer to 1. Figure 5.22: The Vectors Used to Compute Orthogonal Quality (p. 132) illustrates the relevant vectors, and is an example where Equation 5.2 (p. 131 ) produces the minimum value and therefore determines the orthogonal quality.
网格的正交质量，从公式上看出，当其为0时，两个向量平行，为1时是正交，所以接近1时质量最好！
 |
Another important indicator of the mesh quality is the aspect ratio. The aspect ratio is a measure of the stretching of a cell. It is computed as the ratio of the maximum value to the minimum value of any of the following distances: the normal distances between the cell centroid and face centroids (computed as a dot product of the distance vector and the face normal), and the distances between the cell centroid and nodes. For a unit cube (see Figure 5.23: Calculating the Aspect Ratio for a Unit Cube (p. 133)), the maximum distance is 0.866, and the minimum distance is 0.5, so the aspect ratio is 1.732. This type of definition can be applied on any type of mesh, including polyhedral.
国内把aspect ratio翻译为长宽比似乎不是很确切，精确的定义如上，其比例越接近1越好，一般情况下不超过5
 |
Skewness is defined as the difference between the shape of the cell and the shape of an equilateral cell of equivalent volume. Highly skewed cells can decrease accuracy and destabilize the solution. For example, optimal quadrilateral meshes will have vertex angles close to 90 degrees, while triangular meshes should preferably have angles of close to 60 degrees and have all angles less than 90 degrees. A general rule is that the maximum skewness for a triangular/tetrahedral mesh in most flows should be kept below 0.95, with an average value that is significantly lower. A maximum value above 0.95 may lead to convergence difficulties and may require changing the solver controls, such as reducing under-relaxation factors and/or switching to the pressure-based coupled solver.
Skewness（偏斜度）应该低于0.95
高Skewness的单元对计算收敛影响很大，很多时候计算发散的原因就是网格中的仅仅几个高Skewness的单元。
举个例子：共有112,000个单元，仅有7个单元的Skewness超过了0.95，在进行到73步迭代时计算就发散了！
具体参见ANSYS Fluent Users Guide 5.2.2. Mesh Qulity
