Flows past a circular cylinder have subjected to intensive studies over a century due to their simple geometry and complex flow features. However, most conventional investigations have been only focused on the flows around an infinite or very long cylinder with the end effects being neglected. In practice the cylinders are always finite and usually terminated by the endplates whose effects on the flows are not well understood until now. Meanwhile, there is an increased interest in investigating the flow behavior of ferrofluids in the presence of an external magnetic field due to the promising potential applications, widely in mirco-electro-mechanical system (MEMS), microfluidics and biomedical devices. Till now, there are only limited studies on three-dimensional laminar steady f...[ Read more ]

Flows past a circular cylinder have subjected to intensive studies over a century due to their simple geometry and complex flow features. However, most conventional investigations have been only focused on the flows around an infinite or very long cylinder with the end effects being neglected. In practice the cylinders are always finite and usually terminated by the endplates whose effects on the flows are not well understood until now. Meanwhile, there is an increased interest in investigating the flow behavior of ferrofluids in the presence of an external magnetic field due to the promising potential applications, widely in mirco-electro-mechanical system (MEMS), microfluidics and biomedical devices. Till now, there are only limited studies on three-dimensional laminar steady flows of electrically-nonconducting and incompressible magnetic fluids subjected to external magnetic field.

Hence, the motivations of this study are (1) to investigate the effects of endplates on the regular flow past a cylinder through the interaction of two boundary layers on the endplate and cylinder walls and (2) to further characterize the effects of magnetic body force on the interaction of the two boundary layers of ferrofluids flows.

Firstly, three-dimensional laminar flows of regular fluids past a circular cylinder bounded by two parallel endplates have been investigated via the direct numerical simulation method (DNS). Due to the presence of the endplates, these flows are characterized by two independent parameters: (1) the Reynolds number Re_D, based on the cylinder diameter, free-stream mean velocity and the kinematic viscosity of fluid, and (2) the aspect ratio Г, of the distance between two parallel endplates to the diameter of cylinder. The numerical results with Г ranging from 0.2 to 50 depicts the effects of endplates on three-dimensional flow topologies with two flow regimes at low and intermediate Re_D, corresponding respectively to steady separation with symmetrical vortex spirals and three-dimensional oblique laminar vortex shedding. Besides, the parameters characterizing the flow behavior, i.e., critical Re_D of the onset of separation, vortex spiral size, separation angle, Strouhal number, the force coefficients on the cylinder and endplates, etc. are also presented and discussed. Moreover, some results of experimental works have been provided to validate the numerical results.

Secondly, in presence of an external magnetic field, the three-dimensional laminar steady boundary layer flows of ferrofluids past a finite cylinder in a narrow channel are numerically studied. The interaction of ferrofluids and the imposed magnetic field leads to a significant change in flow patterns. These changes are shown graphically with different magnetic field strengths and cylinder aspect ratio. The variations in velocity distributions on the flow symmetries with the effects of the magnetic field are also discussed.