Wang and Wen (2000) attempted to extend the model of Park et al. (1986) to include cases with varying 'knee' sharpness (i.e., n ≠ ≠ 2 ). However, in so doing, the proposed model was no longer rotationally invariant (isotropic). Harvey and Gavin (2014) proposed an alternative generalization of the Park-Wen model that retained the isotropy and still allowed for n ≠ ≠ 2 , viz.