Flow-roughness heterogeneity: critical obliquity and salient parameters*

Large-scale roughness heterogeneity aligned orthogonal to a prevailing flow forms an internal boundary layer (IBL), while roughness heterogeneity aligned parallel to a prevailing flow induces counter-rotating secondary cells, as has been well established in prior research; little research on flow response to oblique heterogeneity is available. We have used large-eddy simulation to perform comprehensive parametric assessment on the effect of obliquity, element height, and spacing between rows of elements. Results show persistent IBL-like flow patterns, even as the obliquity angle increases significantly from a canonical orthogonal arrangement. The flow patterns in this regime are monotonic, before abruptly collapsing at a critical obliquity. This critical obliquity is caused by successive element sheltering, whereby the frontal area of “leeward” obstacles is sheltered and roughness elements behave as a rough strip. Results of the parametric study are used to record effective roughness length a posteriori and develop of a prognostic roughness model generalized for aggregate roughness, frontal area index, and obliquity angle.