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24/*! @brief class grad_Champ_Face_PolyMAC_MPFA for the calculation of the gradient This field is a Champ_Fonc_Face_PolyMAC_MPFA that calculates the gradient of a velocity field
25 *
26 * grad_u(f, n*D + dU) returns the gradient of the dU velocity component in the phase n at face f
27 * grad_u(nf_tot + e*D + dX, n*D + dU) returns the gradient of the dU velocity component in the phase n in element e along the dX component
28 * (i.e. = dU/dX)
29 * In 2D, the gradient at the element e in phase n is equal to :