Numerical modeling of heterogeneous flow fields around rigid objects with special reference to particle paths, strain shadows and foliation drag

With the help of two-dimensional numerical models this paper investigates t hree aspects of heterogeneous deformation around rigid objects: (1) the nat ure of particle paths: (2) the development of strain shadow zones; and (3) the drag patterns of passive markers. In simple shear, spherical objects de velop typically a concentric vortex motion, showing particle paths with an eye (double-bulge)-shaped separatrix. The separatrix has no finite dimensio n along the central line, parallel to the shear direction. Under a combinat ion of pure shear and simple shear, the particle paths assume a pattern wit h a bow-tie shaped separatrix. With increase in the ratio of pure shear to simple shear (S-r), the separatrix around the object shrinks in size. The a xial ratio of the object (R) is another important factor that controls the geometry of particle paths. When R < 1.5, the loci of a particle close to t he object form an elliptical shell with the lent: axis lying along the cent ral line. With increase in axial ratio R, the loci form a doublet elliptica l shell structure. Objects with R > 3 do not show closed particle paths, bu t give rise to elliptical or circular spiral particle paths. The development of strain shadow zones against equant rigid bodies depends strongly on the strain ratio S-r. When S-r = 0 (simple shear), they develop opposite to the extensional faces of the object, forming a typical sigma - type tail. The structure has a tendency to die out with an increase in the pure shear component of the bulk deformation (S-r). The initial angle of th e long axis of the object with the shear direction (phi) and the axial rati o of the object (R) determine the development of strain shadow zones near i nequant rigid objects. Objects with large R and phi between 60 and 120 degr ees form pronounced zones of low finite strain, giving rise to strain shado w structures. A geometrical classification of diverse drag patterns of pass ive markers around rigid objects is presented along with their conditions o f formation. (C) 2001 Elsevier Science B.V, All rights reserved.