Most industrial fluids such as polymers, liquid crystals, and colloids contain suspensions of rigid particles that undergo rotation. However, the classical Navier-Stokes theory normally associated with Newtonian fluids is inadequate to describe such fluids as it does not take into account the effects of these microstructures. In this paper, the unsteady mixed convection boundary layer flow of a micropolar fluid past an isothermal horizontal circular cylinder is numerically studied, where the unsteadiness is due to an impulsive motion of the free stream. Both the assisting (heated cylinder) and opposing cases (cooled cylinder) are considered. Thus, both small and large time solutions as well as the occurrence of flow separation, followed by the flow reversal are studied. The flow along the entire surface of a cylinder is solved numerically using the Keller-box scheme. The obtained results are compared with the ones from the open literature, and it is shown that the agreement is very good. 1. Introduction The unsteady nature of a wide range of fluid flows of practical importance has received considerable attention in recent years. In many applications, the ideal flow environment around a device is nominally steady, but undesirable unsteady effects arise either due to self-induced motion of the body, or due to the fluctuations or nonuniformities in the surrounding fluid. On the other hand, some devices are required to execute time-dependent motion in order to perform their basis functions (McCroskey [1]). The fluid dynamic aspects of some of these problems can normally be approximated by small departures from steady behavior, and some cannot. In general, unsteady viscous phenomena play an important role in the reentry of space vehicles. Such phenomena, as for example, the growth of separated bubble or the displacement of the point of separation also appear in the study of flow around a helicopter blade or stalling of airfoil. The rotor blades of helicopters in forward flight translate more nearly in the plane of rotation than axially. This introduces still another type of unsteadiness, because relative to the individual blade elements, the local approaching air stream varies periodically with large-amplitude fluctuations in its magnitude, yaw angle, and chordwise incidence (McCroskey [1]). Fluid motion in the human blood vessels is also unsteady and appears to involve regions of reversed flow. Unsteady viscous flows have been studied quite extensively and all the characteristic features of unsteady effects are now more or less familiar to fluid
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