A laminar flow of a viscous incompressible fluid with a constant pressure gradient (Poiseuille flow) is considered in a rectangular duct for different values of cross-sectional aspect ratio. A new method, significantly more efficient than the known ones, is proposed and justified for computing the critical Reynolds number of such a flow. The dependence of the critical Reynolds number on the cross-sectional aspect ratio is numerically studied. A theoretical justification of the obtained dependence is proposed
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