Flow structures around a butterfly-shaped low-aspect-ratio wing

In the present study, we numerically investigate three-dimensional flow structures around a butterfly-shaped low-aspect-ratio wing and their effect on the aerodynamic force at the Reynolds number of 1000 based on the wing chord length and free-stream velocity. When the angle of attack is less than 10°, the flow is steady and fully attached to the upper-wing surface, by which the lift force increases almost linearly with the angle of attack. As the angle of attack further increases, the flow around the wing becomes unsteady and contains the leading-edge, trailing-edge, wing-tip, and hairpin vortices. At these angles of attack, the drag force increases rapidly with increasing angle of attack due to massive separation at the leading edge, but the lift force increases gradually without abrupt fall-off. This is because the wing-tip vortices induce a strong downward flow interacting with the flow separated from the leading edge and delay subsequent vortex roll-up in the downstream. The wing-tip vortices themselves also produce low-pressure regions on the upper-wing surface and thus provide an additional lift force. The flows separated from the leading and trailing edges are eventually combined into pairs of hairpin vortices which travel downstream in the wake. The formation of the hairpin vortices above the upper-wing surface also generates lowpressure regions, and they are another source of the lift force.     

A study on the spray structure and evaporation characteristic of common rail type high pressure injector in homogeneous charge compression ignition engine

The purpose of this study was to investigate the spray structure and evaporation characteristics of common rail high pressure injector for use in a direct injection type HCCI (Homogeneous Charge Compression Ignition) engine. In this study, we measured the injection rate and visualized the spray structure of a HCCI injector according to injection conditions. The CFD simulation of the spray and the air fuel mixture formation in real engine conditions was also conducted using the VECTIS commercial code. In addition, we compared simulation results to experimental results.

From the spray experiment and simulation results, we found that the spray penetration was proportional to the back pressure by an exponent of 1/4. This is similar to Hiroyasu's experimental result. The fuel evaporation and air fuel mixture result indicate that the influence of the spray impingement with the ambient density was bigger than that of the intake pressure and temperature conditions in evaporation rate when the fuel was injected at the early stage of the compression stroke. The results also reveal that the fuel was uniformly distributed in the combustion chamber at this early injection time and the air fuel mixture was enhanced in this relatively rich region. However, when ambient density was kept constant, the fuel evaporation was sensitive to the influence of the intake temperature and pressure. As the fuel was injected at the later stage of the compression stroke, the fuel tended to concentrate in the bowl zone and to generate the lean air fuel mixture. From these results, it was confirmed that the air fuel mixture characteristics are sensitive to the impingement position of the injected fuel.     

Experimental investigation of nanoparticle formation characteristics from advanced gasoline and diesel fueled light duty vehicles under different certification driving modes

This paper mainly focused on the comparison of nanoparticle size distribution and number concentration level characteristics with gasoline and diesel fueled light duty vehicles. In the engine research, particle size distribution and number concentrations were analyzed by a DMS500 with engine parameters. Time-resolved particle number concentration levels from test vehicles were measured by a golden particle measurement system (GPMS) as recommended by a Particle Measurement Programme (PMP) on certification modes such as New European Driving Cycle (NEDC), Federal Test Procedure (FTP)-75, and Highway Fuel Economy Test (HWFET). In addition, particle emission characteristics from vehicles were analyzed by DMS500 during transient and high-speed driving conditions. From the results, we found that the formation of particles was highly dependent on vehicle speed and load conditions for each mode. The diesel vehicle equipped with a particulate filter showed substantial reduction of the total particle number whose number concentration was equivalent to that of the gasoline vehicle. The nucleation mode particles from gasoline fuel were mainly emitted; however, the accumulation mode particles from the diesel fuel were generally analyzed. This paper was recommended for publication in revised form by Associate Editor Kyoung Doug Min Simsoo Park received his B.S. and M.S. degrees from Seoul National University in 1977 and 1979, respectively, and a Ph.D. from the State University of New York at Stony Brook. He served as a Chief Research Engineer at Hyundai Motor Company, a Director for Publication of the KSME, a Technical Advisor of Hyundai-Kia Motor Company, and an Editing Director, Project Director, International Director, Accounting Director, and General Affair Director of KSAE. He is currently Vice President and Editor-in-Chief of IJAT at KSAE and a professor in school of mechanical engineering at Korea University. Hyungmin Lee received his B.S. degrees from Republic of Korea Naval Academy in 1997 and his M.S. degrees from Korea University in 2005, respectively. He served as an Operation Officer, Command Engineer Officer at various naval vessels. He is currently Ph.D. course in school of mechanical engineering at Korea University and his rank is a Lieutenant Commander of Korea Navy.     

Effect of body angle on the aerodynamics of a rhinoceros beetle: Smoke-wire visualization in a wind tunnel

In the present study, flow structure around a live rhinoceros beetle in a tethered flight is investigated experimentally using a smoke-wire visualization technique in a wind tunnel with a free-stream velocity of 1.2 m/s, which is close to that of a typical flight speed. While varying the body angle (from 5 to 85°), the flow structures generated by the hindwings, elytra, and body are visualized along the spanwise direction. During the flapping period, the complex flow structures comprised leading-edge, trailing-edge, and tip vortices generated on the hindwing, but the flow structure is quite simple on the elytra (attached flow) and body (separated flow). As the body angle increases, these vortices convect in the downward direction, which matches the observation that the body angle of a hovering flight is larger than that of a forward flight. When the body angle matches the condition of forward flight, it is also found that the Strouhal number of a flapping hindwing is tuned to 0.4, which is known as an optimal value for thrust efficiency. Further, the effect of free-stream velocity (i.e., advance ratio) on the formation and evolution of these coherent vortical structures are investigated.

     

The thermo-mechanical behavior of brake discs for high-speed railway vehicles

Journal of Mechanical Science and Technology - The structure of a brake disc is coupled to the axle and rotates together with the wheel. A brake disc is a friction-type device that presses the pad...     

Optical and electronic properties of highly stable and textured hydrogenated ZnO:Al thin films

We have experimentally investigated the effects of hydrogen-annealing on the structural, electrical, and optical properties of Al-doped ZnO (ZnO:Al) thin films prepared by RF magnetron sputtering at room temperature. From the X-ray diffraction observations, the orientation of ZnO:Al films was found to be a c-axis in the hexagonal structure. We found that intentionally incorporated hydrogen plays an important role in n-type conduction as a donor, improving free carrier concentration and electrical stability. We simultaneously obtained improved optical transmission and enhanced absorption edge of the ZnO:Al film due to hydrogen-annealing. Our experimental data suggest the hydrogen-annealing process as an important role in the enhancement of electrical and optical properties, which is promising as a back reflector material for thin-film solar cells.     

Geometric direct search algorithms for image registration.

A widely used approach to image registration involves finding the general linear transformation that maximizes the mutual information between two images, with the transformation being rigid-body [i.e., belonging to SE(3)] or volume-preserving [i.e., belonging to SL(3)]. In this paper, we present coordinate-invariant, geometric versions of the Nelder-Mead optimization algorithm on the groups SL(3), SE(3), and their various subgroups, that are applicable to a wide class of image registration problems. Because the algorithms respect the geometric structure of the underlying groups, they are numerically more stable, and exhibit better convergence properties than existing local coordinate-based algorithms. Experimental results demonstrate the improved convergence properties of our geometric algorithms.     

Convergence acceleration of Navier-Stokes equation using adaptive wavelet method

An efficient adaptive wavelet method is proposed for the enhancement of computational efficiency of the Navier-Stokes equations. The method is based on sparse point representation (SPR), which uses the wavelet decomposition and thresholding to obtain a sparsely distributed dataset. The threshold mechanism is modified in order to maintain the spatial accuracy of a conventional Navier-Stokes solver by adapting the threshold value to the order of spatial truncation error. The computational grid can be dynamically adapted to a transient solution to reflect local changes in the solution. The flux evaluation is then carried out only at the points of the adapted dataset, which reduces the computational effort and memory requirements. A stabilization technique is also implemented to avoid the additional numerical errors introduced by the threshold procedure. The numerical results of the adaptive wavelet method are compared with a conventional solver to validate the enhancement in computational efficiency of Navier-Stokes equations without the degeneration of the numerical accuracy of a conventional solver.     

An investigation on the fuel behavior for a PFI type motorcycle engine

Recently, the electrically controlled fuel injection type motorcycle has been emphasized in order to meet regulations for exhaust emissions. However, there are many difficulties in selecting the control parameters because the pulsation phenomenon occurs in the intake port due to the higher speed operating range and the smaller layout than for a passenger car. Therefore, we investigated the injector spray characteristics which are applied to a 4-valve motorcycle gasoline engine. The spray characteristics were visualized by using a CCD camera synchronized with the stroboscope at 6000 rpm. Furthermore, we compared the simulation results using the VECTIS code with experimental results. The results showed that the trajectory of the spray was directed towards the lower wall of the intake port when the fuel was injected at closed valve timing. On the other hand, when the fuel was injected at open valve timing, a large portion of the fuel was lifted towards the upper half of the port. In addition, open valve injection makes fuel evaporation time short; this resulted in better mixture formation than a closed valve injection. From this result, we found that injection timing has a great effect on the mixture formation within a motorcycle cylinder.