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A two-perfect-fluid model of an anisotropic fluid is presented. The energy-momentum tensor associated with the sum of two perfect fluids, one perfect and one null fluid, and two null fluids is examined. Special attention is devoted to the study of the stress tensor. The special case wherein the two perfect fluids are irrotational is studied. A relation between the Einstein equations for this particular case and the Einstein equations for a massless complex scalar field is found. The general solution of Einstein equations for an anisotropic fluid constructed with two-null-fluid components in the plane-symmetric case is discussed. The energy-momentum tensor of a cloud of strings and the energy-momentum tensor of an anisotropic fluid formed by two null fluids are compared.
Patricio S. Letelier (Fri,) studied this question.