Laminar flow of micropolar fluid in rectangular microchannels

Compared with the classic flow on macroscale, flows in microchannels have some new phenomena such as the friction increase and the flow rate reduction. Papautsky and co-workers explained these phenomena by using a micropolar fluid model where the effects of micro-rotation of fluid molecules were taken into account. But both the curl of velocity vector and the curl of micro-rotation gyration vector were given incorrectly in the Cartesian coordinates and then the micro-rotation gyration vector had only one component in the z-direction. Besides, the gradient term of the divergence of micro-rotation gyration vector was missed improperly in the angular moment equation. In this paper, the governing equations for laminar flows of micropolar fluid in rectangular microchannels are reconstructed. The numerical results of velocity profiles and micro-rotation gyrations are obtained by a procedure based on the Chebyshev collocation method. The micropolar effects on velocity and micro-rotation gyration are discussed in detail.

Numerical and experimental research of flow control on an NACA 0012 airfoil by local vibration

A flow control technique by local vibration is proposed to improve the aerodynamic performance of a typical airfoil NACA 0012. Both wind-tunnel experiments and a large eddy simulation(LES) are carried out to study the effects of local vibration on drag reduction over a wide range of angles of attack. The application parameters of local vibration on the upper surface of the airfoil are first evaluated by numerical simulations.The mounted position is chosen at 0.065–0.09 of chord length from the leading edge.The influence of oscillation frequency is investigated both by numerical simulations and experiments. The optimal frequencies are near the dominant frequencies of shear layer vortices and wake vortices. The patterns of shear vortices caused by local vibration are also studied to determine the drag reduction mechanism of this flow control method. The results indicate that local vibration can improve the aerodynamic performance of the airfoil. In particular, it can reduce the drag by changing the vortex generation patterns.

Hydrodynamics characterization of a choanoid fluidized bed bioreactor used in the bioartificial liver system： Fully resolved simulation with a fctitious domain method

Choanoid fluidized bed bioreactors （CFBBs） are newly developed core devices used in bioartificial liver- support systems to detoxify blood plasma of patients with microencapsulated liver cells. Direct numerical simulations （DNS） with a direct-forcing/fictitious domain （DF/FD） method were conducted to study the hydrodynamic performance of a CFBB. The effects of particle-fluid density ratio, particle number, and fil- ter screens preventing particles flowing out of the reactor were investigated. Depending on density ratio, two flow patterns are evident： the circulation mode in which the suspension rises along one sidewall and descends along the other sidewall, and the non-circulation mode in which the whole suspension roughly flows upward. The circulation mode takes place under non-neutral-buoyancy where the particle sedimentation dominates, whereas the non-circulation mode occurs under pure or near-neutral buoy- ancy with particle-fluid density ratios of unity or near unity. With particle-fluid density ratio of 1.01, the bioartificial liver reactor performs optimally as the significant particle accumulation existing in the non-circulation mode and the large shear forces on particles in the circulation mode are avoided. At higher particle volume fractions, more particles accumulate at the filter screens and a secondary counter circulation to the primary flow is observed at the top of the bed. Modelled as porous media, the filter screens play a negative role on particle fluidization velocities; without screens, particles are fluidized faster because of the higher fluid velocities in the jet center region. This work extends the DF/FD-based DNS to a fluidized bed and accounts for effects from inclined side walls and porous media, providing some hydrodynamics insight that is important for CFBB design and operation optimization.