Advances in Mechanical Engineering

In this work, we deal with high-order solver for incompressible flow based on velocity correction scheme with discontinuous Galerkin discretized velocity and standard continuous approximated pressure. Recently, small time step instabilities have been reported for pure discontinuous Galerkin method, in which both velocity and pressure are discretized by discontinuous Galerkin. It is interesting to examine these instabilities in the context of mixed discontinuous Galerkin–continuous Galerkin method. By means of numerical investigation, we find that the discontinuous Galerkin–continuous Galerkin method shows great stability at the same configuration. The consistent velocity divergence discretization scheme helps to achieve more accurate results at small time step size. Since the equal order discontinuous Galerkin–continuous Galerkin method does not satisfy inf-sup stability requirement, the instability for high Reynolds number flow is investigated. We numerically demonstrate that fine mesh resolution and high polynomial order are required to obtain a robust system. With these conclusions, discontinuous Galerkin–continuous Galerkin method is able to achieve high-order spatial convergence rate and accurately simulate high Reynolds flow. The solver is tested through a series of classical benchmark problems, and efficiency improvement is proved against pure discontinuous Galerkin scheme.

Gasoline compression ignition (GCI) is an effective way to achieve both high thermal efficiency and low emission. The combustion and emission performances of GCI and DCI (diesel compression ignition) were compared on a 2.0 L diesel engine equipped with Three-way catalyst-Lean NOx trap/Passive selective catalytic reduction (TWC-LNT/PSCR) aftertreatment system. In order to further clarify the advantages and disadvantages of GCI, this paper first studies the combustion and emission at 1500 rpm and braking average effective pressure (BMEP) of 4–9 bar. Secondly, six small map points of worldwide harmonized light vehicles test cycle (WLTC) are studied. The results show that the braking thermal efficiency (BTE) of GCI is lower than that of DCI at low load. When BMEP is greater than 5 bar, the BTE of GCI is significantly improved. GCI achieves a maximum BTE of 43%, which is 3% higher than DCI. Compared with DCI, the NOx emission of GCI is slightly lower, the smoke emission of filter smoke number (FSN) is significantly improved, and the CO and HC emissions are significantly increased. GCI engine equipped with TWC-LNT/PSCR system with high aftertreatment efficiency has the potential to meet China’s VI B emission regulations.

Bài tổng quan này phân tích thiết kế, hành vi cơ học, khả năng sản xuất, và ứng dụng của các cấu trúc lưới gradient được sản xuất bằng công nghệ đắp dần kim loại. Bằng cách thay đổi các tham số thiết kế như kích thước ô, chiều dài thanh, và đường kính thanh của các ô đơn vị trong cấu trúc lưới, một thuộc tính gradient được hình thành để đạt được các mức độ chức năng khác nhau và tối ưu hóa tỷ lệ giữa độ bền và trọng lượng. Cấu trúc lưới gradient cung cấp khả năng nén và độ rỗng có thể thay đổi; và có thể kết hợp hơn một loại ô đơn vị với các hình học khác nhau, dẫn đến các hành vi cơ học khác nhau qua từng lớp khi so với các cấu trúc lưới không gradient. Các kỹ thuật đắp dần có khả năng sản xuất các phần nhẹ phức tạp như cấu trúc lưới đồng nhất và gradient do đó, tạo ra sự tự do thiết kế cho các kỹ sư. Mặc dù có những ưu điểm này, việc sản xuất cấu trúc lưới theo kỹ thuật đắp dần cũng có những nhược điểm riêng. Các quy tắc và chiến lược để vượt qua các giới hạn này đã được thảo luận và các đề xuất cho nghiên cứu tương lai đã được đưa ra.

Hemiplegia, apoplexia, or traffic accidents often lead to unilateral lower limb movement disorders. Traditional lower limb rehabilitation equipments usually execute walk training based on fixed gait trajectory; however, this type is unsuitable for unilateral lower limb disorders because they still have athletic ability and initiative walking intention on the healthy side. This article describes a wearable lower limb rehabilitation exoskeleton with a walk-assisting platform for safety and anti-gravity support. The exoskeleton detects and tracks the motion of the healthy leg, which is then used as the control input of the dyskinetic leg with half a gate-cycle delay. The patient can undergo walk training on his own intention, including individual walking habit, stride length, and stride frequency, which likely contribute to the training initiative. The series elastic actuator is chosen for the exoskeleton because the torque output can be accurately detected and used to calculate the assisted torque on the dyskinetic leg. This parameter corresponds to the recovery level of a patient’s muscle force. Finally, the walk-assisting experiments reveal that the rehabilitation exoskeleton in this article can provide the necessary assisting torques on the dyskinetic leg, which can be accurately monitored in real time to evaluate a patient’s rehabilitation status.

A piezo-driven linear actuator based on the improved stick–slip principle is developed in this article. With the help of two piezo-stacks and flexure hinges, the preload force can be changed, so the designed actuator can realize relatively large linear ranges and large output force. The designed actuator mainly consists of the mover, the stator, two piezo-stacks, an adjusting stage and the base. The working principle and theoretical analysis are described. A prototype actuator was fabricated and a series of experiments were carried out to investigate the work characteristics of it. Experimental results indicate that the maximum speed is about 3.086 mm/s and the maximum output force is 0.98 N. They are both improved compared with the traditional stick–slip motion. Experimental results confirm that the proposed actuator can realize large output force relatively and different motion speeds with high accuracy under different driving voltages and frequencies.

An experimental study is conducted to cool the ambient air using a new humidification technique. A wind tunnel is built with a test section formed by four modified MiniModule membrane contactors. An ambient air passes over the membrane contactors (cross flow) while water pumps through the contactors. Air temperature and relative humidity are measured upstream and downstream of the membrane contactors array which was used to humidify and cool the outdoor air. Five average air velocities (3.03, 3.33, 3.95, 4.52, and 5.04 m/s) and four water flow rates (0.0, 0.013, 0.019, and 0.025 kg/s) are used. Air velocity is measured at different locations along the centerline of the cross section. Using the modified MiniModule membrane contactors array dropped the air temperature by a maximum and minimum of 10.77°C and 3.44°C, respectively, depending on the outdoor air. The corresponding maximum increase of the relative humidity is 4.65% which depends on the ambient condition. It is noticed that the evaporation process does not follow the isenthalpic lines therefore; heat transfers from the air as latent and sensible heats.

In this study, a mixed meta-modeling-based optimization method has been proposed and applied to a commercial vehicle for crashworthiness design subjected to the frontal crash. A full-scale finite element model of the commercial vehicle has been built and validated by a crash test. The front frame parts have been separated to build a sub-model for crashworthiness optimization. Sensitivity analysis has been performed to find the design factors contributing most to crash performance by using design of experiments. With the reduced dimensions of design space, meta-models of crashworthiness criteria (i.e. specific energy absorption, peak crush force, and peak crush acceleration) have been built by using polynomial response surface and radial basis function networks, respectively. The meta-models with higher global fidelity in design space have been adopted to formulate the multi-objective optimization problem of crashworthiness design, which has then been solved by using Non-dominated Sorting Genetic Algorithm-II. The obtained Pareto front has been discussed and validated with that achieved by Strength Pareto Evolutionary Algorithm 2. The normalized optimal solution from the Pareto front has resulted in 11.15% increase in specific energy absorption and 13.2% decrease in peak crush force for the frame and has led to an obvious improvement in occupant protection and energy absorption for the whole vehicle, verifying that the proposed method is effective for vehicle crashworthiness optimization.