Laminar fuel-air counterflow diffusion flames (CFDFs) were studied using axisymmetric convergent-nozzle and straight-tube opposed jet burners (OJBs). The subject diagnostics were used to probe a systematic set of H m a -a i r CFDFs over wide ranges of fuel input (22 to 100% Ha), and input axial strain rate (130 to 1700 Us) just upstream of the airside edge, for both plug-flow and parabolic input velocity profiles. Laser Doppler Velocimetry (LDV) was applied along the centerline of seeded air flows from a convergent nozzle OJB (7.2 mm i.d.), and Particle Imaging Velocimetry (PIV) was applied on the entire airside of both nozzle and tube OJBs (7 and 5 mm i.d.1 to characterize global velocity structure. Data are compared to numerical results from a one-dimensional (1-D) CFDF code based on a stream function solution for a potential flow input boundary condition. Axial strain rate inputs at the airside edge of nozzle-OJB flows, using LDV and PIV, were consistent with 1-D impingement theory, and supported earlier diagnostic studies. The LDV results also characterized a heat-release hump. Radial strain rates in the flame substantially exceeded 1-D numerical predictions. Whereas the 1-D model closely predicted the max I min axial velocity ratio in the hot layer, it overpredicted its thickness. The results also support previously measured effects of plug-flow and parabolic input strain rates on CFDF extinction limits. Finally, the submillimeter-scale LDV and PIV diagnostics were tested under severe conditions, which reinforced their use with subcentimeter OJB tools to assess effects of aerodynamic strain, and fueVair composition, on laminar CFDF properties, including extinction. x = axial coordinate, cm. X(i) = mole fraction of species i, input jet.
Force measurements from experiments conducted in water on a flapping-and-pitching thin flat plate wing of semi-elliptic planform at low Reynolds numbers are reported. Time varying force data, measured using a force transducer, provide a means to understand the mechanisms that lead to enhanced performance observed in insect flight compared to fixed wing aerodynamics. Experimental uncertainties associated with low level (~1N) fluid dynamic force measurements on flapping-and-pitching wings are addressed. A previously proposed pitching mode in which the leading edge and trailing edge switch roles to allow using cambered airfoils has been shown to be viable, and may have advantages over the non-switching mode. The present data are part of a larger database planned to experimentally investigate various aspects of insect flight including another previously proposed idea that performance may be improved by flying at optimum reduced flapping frequency. The study has applications in micro air vehicle development.
Building on our previous work on transient, two dimensional simulations of the Navier-Stokes equations to investigate mixing enhancement by introducing the Lorentz force in MHD as a control parameter to create turbulentlike chaotic advection, this paper describes transient, three-dimensional simulations. Our approach differs from many previous analytical investigations by other workers based on potential flow and linearized stokes flow. A shallow disk-ring cylindrical microfluidic cell with gold electrodes deposited on the floor serves as a representative lab-on-a-chip. By applying a voltage across specific disk electrodes and a ring counter electrode, a current is established in the weak conductive solution. The current interacts with an externally applied magnetic field generating a Lorentz force that causes fluid motion. Velocity vectors, electric potential distributions and ionic current lines are presented. By switching on and off a pair of disk electrodes with a certain period T, a "blinking vortex" that induces chaotic advection is produced. Various particle trajectory-based analyses using extensive postprocessing of the simulation results show that the period T plays an important role in generating chaotic advection. Large periods provide efficient stirring which improves mixing performance. Taking a step further, we show that by having two pairs of disk electrodes that were subjected to a different on/off switching scheme, more complex chaotic motion can be generated, and the mixing region can be extended to almost the entire fluid domain. This study establishes CFD simulation of MHD at the microscale as a robust tool to develop efficient strategies for mixing by chaotic advection. The techniques developed in the present work are also applicable in MHD-based flow control in microfluidics for other applications such as pumping and steering fluid to target locations.
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