Applied Mechanics and Materials Vols. 284-287

Abstract: When a cannonball collides with sea water, the resulting impact force influences the cannonball trajectory, depending on launching angle, initial firing speed, incident angle, and cannonball nose-cone shape. In this study, the effect of nose-cone shape of a cannonball on impact and ricocheting behavior was investigated. During collision, the flow was assumed to be non viscous and incompressible, and a source panel method was used to determine velocity potential and pressure coefficient. The nose-cone shape was expressed by Haack-series. It was found that as the radius of curvature of a cannonball nose-cone decreases, a high impact force is resulted, and that depending on impact force and impulse, the ricocheting distance varies. The results obtained in this study can be used in researching and developing new cannonballs.

Abstract: The purpose of this study was to develop mathematical models for air-cooled chillers and their components using innovative varied row configurations as a parameter analysis model followed by a simulation of actual operational performance. In this manner, we were able to observe the increase in performance of air-cooled chillers and the energy transfer efficiency of individual components. This study found that the innovative varied row configuration (Type C) can increase the COP of air-cooled chiller by 6.7% over that of traditional condensing-coil configuration (Type A) with an increase in total irreversibility and the irreversibility of the condenser of 8.4% and 4.1%, respectively; Type C can increase the COP of air-cooled chiller by 3.3% over that of the best condensing-coil configuration (Type B) with an increase in total irreversibility and the irreversibility of the condenser of 1.6% and 4.6%, respectively. We believe that the results of this research can provide an important basis of reference for future design of air-cooled chiller units.

Abstract: This paper presents an investigation of spacing fluctuation characterization (SFC) of Hard Disk Drive (HDD) using thermal asperity (TA) sensor. Firstly, the spacing fluctuation monitored by TA sensor is correlated with that monitored by a Laser Doppler Vibrometer (LDV). Secondly, a series of experiments are conducted to study the factors affecting the spacing fluctuation, including disk distortion, head crown sensitivity, ZH sensitivity. Good disk assembly and small crown sensitivity and ZH sensitivity will be helpful to reduce spacing fluctuation.

Abstract: High-speed and high-temperature are the characteristics of the flow field in scramjet engine; the regular non-slip wall boundary condition requires zero speed at wall; in the same time, the material temperature limit does not allow high wall temperature; therefore the velocity gradient and temperature gradient in the engine boundary layer are huge. If these gradients are too large, the traditional assumption of the local thermal equilibrium in the fluid will fail, the Navier-Stokes equations are no longer valid in the boundary layer. For the first time, the non-equilibrium flow phenomena in Scramjet engine is studied here. Appropriate turbulence model and fine grid are used to analyze the turbulent boundary layer of the Hyshot scramjet engine with three different operating conditions. The result of the CFD simulation shows that the local Knudsen number in the engine boundary layer is greater than the critical value with the operating conditions 40Km/Ma8 and 30Km/Ma8; they are non-equilibrium flow and the Navier-Stokes equations fails. Special treatment of the boundary conditions are needed for these kinds of flow. With the operating condition of 20Km/Ma6, the local thermal equilibrium condition is observed and conventional CFD method is valid.

Abstract: Slimming of a smart phone is one of the key technologies in the current smart-phone industry. Because the vibration actuator is the thickest part of a smart phone, a slim linear vibration actuator has been developed. For the analysis of the vibration characteristic of a smart phone, experiment has been used one of the most popular techniques, but it consumes a lot of cost and time. Accordingly, as an alternative to experiments, mathematical modeling and analysis can be applied. In this paper, a mathematical modeling is created for analysis of the vibration characteristics of a smart-phone applied with a horizontally vibrating linear actuator. In addition, the result of the mathematical analysis is compared with that of the experiment for verification.

Abstract: The ball-end cutter plays an important role in machining free-form surfaces due to its adaptability to a surface. It is widely used in aerospace, automobile, and die/mode industries. Thus, the demand for higher cutting performance and lower cost of the ball-end cutter has been increasingly focused. In this paper, based on normal helix cutting edge geometric model, the cone wheel, which was used to grind the rake and flank face of ball-end cutter, was designed. Then, we use homogenous coordinate transformation to derive for the ball-end cutter and establish the mathematical model for grinding this type cutter in terms of grinding parameters. Finally, the cutting angles of ball-end cutter were discussed.

Abstract: This paper proposes an analytical method to evaluate the mechanical efficiency of the bicycle transmission hub. A 16-speed transmission hub, which consists of two transmission units and one differential unit, is presented first. By applying the concept of fundamental circuits, a step by step analysis process is then described to numerically calculate the angular speed, ideal torque and power flow, and actual torque and power flow by considering the gear-mesh loss of each link of the planetary gear train. Based on the power flow diagram, the mechanical efficiency at each speed of the transmission hub can be estimated.

Abstract: The unibody of LED lampshade is the expansion of a lampshade array from single-LED in order to decrease assembly time, the forming technique is complicated. This study employ solid model to build micro mold, and find out the optimal design parameters, such as gate position, running system, cooling system and holding pressure system. The improvement of gate system can reach flowing equilibrium when molten plastic flows into cavity, and no welding line is produced. In the cooling stage, the cooling speed at each cavity is also steady and balance in order to reduce residual stress and lens defects. This study utilizes multi-gate system and multi-stage holding pressure aided forming to decrease welding line and residual stress. This study improves traditional mold design to fit more multi-cavity LED mold design which includes cooling system redesigning, cavities layout, flowing balance in runner and reducing shear stress. The Taguchi method and orthogonal array are used in the study for each group of experiment; many factors are applied to obtain the minimum shear stress, minimum warp, reduction of flow mark and uniform stress distribution. The results can provide for industry reference.

Abstract: This study investigated the heat transfer characteristics of LED heat sink and the development process technology of graphite heat sink with micro-sized metal powders. Employing the reverse engineering technology, the three-dimension LED heat sink entity was rebuilt and the heat transfer characteristics of LED heat sink were analyzed by CFD numerical simulation and experimental measurement. The numerical results were validated with experimental results and it showed a good agreement. The experimental and simulation results showed that the heat dissipation of LED device could be removed by natural convection effectively. The difference between the maximum temperature and minimum temperature of cooling efficiency was 10°C. For the process technology development of LED graphite heat sink, the graphite powder, metal powder and resin were mixed in specific ratios. The vacuum casting, vacuum pressure casting and rapid die technology were used to manufacture LED graphite heat sink. The experimental results showed that the LED graphite heat sinks developed in this study have advantages of low cost, light weight and attractive appearance as compared with the heat sink of aluminum alloy, and the overall heat transfer capacity is still within acceptable range.

Abstract: A numerical simulation model to obtain the extra-low internal thermal resistance for submount of LED is presented. The 3-D numerical model for calculating thermal resistance is demonstrated on examining the aspect ratio of submount, the contact ratio between chip and submount, and the surrounding condition. According to the analysis by accurate numerical simulations of heat transfer; the appropriate dimensions for different conditions of ambient are determined. The thickness of submount should be over a specially designated value. Besides, a higher contact ratio between heat source and submount, a larger external convective effect, and the heat dissipated from symmetry axis of submount will decrease the spreading thermal resistance.