This study investigates the flow behaviors of the microflow in a sudden expansion microfluidic channel with a rectangular block structure. 2D and 3D numerical simulations are used to predict the vortex formation behavior and experimental approaches are adopted to confirm the simulated results. A novel microfluidic rectifier is proposed by operating the designed microfluidic device under opposite flow conditions. The performance of the flow rectifier is also evaluated under difference flow velocities. There are three parts finished in this thesis. Firstly, the vortex formation behavior is investigated for the microchannel with the block at different distances downstream the sudden expansion channel. The size of the fully developed vortices is measured and analyzed. Results show that the size of the vortex reaches stable while the distance between the block and sudden expansion channel is longer than 1000 £gm. Secondly, this study also investigates the sequence of the vortex formation under different flow velocity (Reynolds number). Results indicate that there are four stages for the vortex formation in the microfluidic channel. Vortices are formed firstly at the sudden expansion channel and then behind the block. Two small vortices are then formed once beside the block and then merge with the two big vortices behind the block under increasing velocity conditions. The flow becomes instable once the Reynolds number higher than 555, two symmetrical shedding flows are observed behind the block structure. This flow behavior is rarely observed in a microfluidic channel due to the big viscous force of the flow in the microchannel. Thirdly, this study measures the pressure drops for the forward and backward flows under different flow speeds. Results show that the vortex formation behavior in backward flow is different from it is in forward conditions. Two symmetric vortexes are formed beside the channel while the Reynolds number higher than 416. The squeezed vortices form a virtual valve structure and increase the flow resistance of the microflow, resulting in a high performance valve structure. The calculated results indicate that the diodicity (Di) of the designed microchannel is as high as 1.76 and 1.5 for the numerical result and experimental result, respectively. The rectifying performance of the developed microchip device is higher than the reported devices fabricated using delicate processes and designed. The results of this research will give valuable knowledge for the flow behavior in a microchannel and the design of microfluidic chips.

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It is well known that most fluid flows observed in nature or encountered in engineering applications are turbulent and involve separation. Fluid flows in turbines, diffusers and channels with sudden expansions are among the widely observed areas where separation substantially alters the flow field and gives rise to complex flow dynamics. Such types of flows are referred to as internal flows since they are confined within solid surfaces and predominantly involve the generation or utilization of mechanical power. However, there is also a vast variety of engineering applications where the fluid flows past solid structures, such as the flow of air around an airplane or that of water around a submarine. These are called external flows and as in the former case the downstream evolution of the flow field is crucially influenced by separation. The present doctoral thesis addresses both internal and external separated flows by means of direct numerical simulations of the incompressible Navier-Stokes equations. For internal flows, the wall-driven flow in a onesided expansion channel and the pressure-driven flow in a plane channel with a single thin-plate obstruction have been studied in the fully developed turbulent state. Since such geometrical configurations involve spatially developing turbulent flows, proper inflow conditions are to be employed in order to provide a realistic fully turbulent flow at the input. For this purpose, a newly developed technique has been used in order to mimic an infinitely long channel section upstream of the expansion and the obstruction, respectively. With this approach, we are able to gather accurate mean flow and turbulence statistics throughout each flow domain and to explore in detail the instantaneous flow topology in the separated shear layers, recirculation regions as well as the recovery zones. For external flows, on the other hand, the flow past a prolate spheroid has been studied. Here, a wide range of Reynolds numbers is taken into consideration. Based on the characteristics of the vortical structures in the wake, the flow past a prolate spheroid is classified as laminar (steady or unsteady), transitional or turbulent. In each flow regime, the characteristic features of the flow are investigated by means of detailed frequency analysis, instantaneous vortex topology and three-dimensional flow visualizations.

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