求指教：Peric的书第八章Complex Geometries中讲BC的一段不明白如何实现

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Ferziger，Peric的Computational methods for fluid dynamics一书中第8章8.10节讲Impermeable Walls中一段（紧承(8.79)）：Alternatively we can use the velocity gradients at cell centers(Gauss theorem),extrapolate them to the center of wall cell face,calculate the shear stresses and calculate the shear force component from above.想了半天，不知道该怎么把速度梯度extrapolate to the center of wall cell face？还是因为太简单了，作者都不愿意详细说？总之，望高手赐教，不胜感激！