| Abstrakt: |
The primary instability of the steady two-dimensional flow past rectangular cylinders moving parallel to a solid wall is studied, as a function of the cylinder length-to-thickness aspect ratio AR=L/D and the dimensionless distance from the wall g=G/D. For all AR, two kinds of primary instability are found: a Hopf bifurcation leading to an unsteady two-dimensional flow for g≥0.5, and a regular bifurcation leading to a steady three-dimensional flow for g<0.5. The critical Reynolds number Rec,2-D of the Hopf bifurcation (Re=U∞D/ν, where U∞ is the free stream velocity, D the cylinder thickness and ν the kinematic viscosity) changes with the gap height and the aspect ratio. For AR≤1, Rec,2-D increases monotonically when the gap height is reduced. For AR>1, Rec,2-D decreases when the gap is reduced until g≈1.5, and then it increases. The critical Reynolds number Rec,3-D of the three-dimensional regular bifurcation decreases monotonically for all AR, when the gap height is reduced below g<0.5. For small gaps, g<0.5, the hyperbolic/elliptic/centrifugal character of the regular instability is investigated by means of a short-wavelength approximation considering pressureless inviscid modes. For elongated cylinders, AR>3, the closed streamline related to the maximum growth rate is located within the top recirculating region of the wake, and includes the flow region with maximum structural sensitivity; the asymptotic analysis is in very good agreement with the global stability analysis, assessing the inviscid character of the instability. For cylinders with AR≤3, however, the local analysis fails to predict the three-dimensional regular bifurcation. [ABSTRACT FROM AUTHOR] |
