彩虹有时会以两条同心圆弧形态出现。内弧称为主虹，外弧称为副虹或霓。

Rainbows sometimes appear as two concentric bows with the inner one called the primary bow and the outer one the secondary bow.

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该法适用于区域内点、围绕边界的尖角点、边界或内部同心圆弧及同心圆四种特殊区域的网格改进，为实际工程结构应力梯度突变或变化较大处网格细化提供了十分方便的手段。

The scheme adapts to refine the meshes of special regions, such as interior points, around corner points, arcs and circles, which are usually failure regions of parts because of great stress gradient.

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