With growing avionics applications, the transmission of avionics data flows has been increasing in real-time avionics networks of aircraft. An Avionics Full DupleX switched Ethernet (AFDX), standardized as ARINC 664, is chosen as the backbone network for distributed real-time avionics systems as it offers high throughput and does not require global clock synchronization. Estimating the end-to-end transmission delay to validate the network performance is essential for both certification and industrial research. Because of the various waiting times caused by the backlog (i.e., the pending packets in the output port of the visited switch), it is necessary and reasonable to compute the worst-case end-to-end transmission delay to validate network performance. Several approaches have been designed to compute the upper boundaries of end-to-end transmission delays, such as the Network Calculus approach and the Trajectory approach. In this paper, we focus on a new approach, Forward end-to-end delay Analysis (FA). This approach iteratively estimates the maximum backlog (i.e., number of pending packets) in each switch visited along the transmission path, so that the worst-case end-to-end transmission delay can be computed and the network performance evaluated. We also present the termination condition for this iterative estimation. The experiments demonstrate that this approach achieves a more accurate evaluation of transmission performance than the Network Calculus approach. A comparison with the exact upper boundaries obtained using the Model Checking approach shows the pessimism (i.e., overestimation) in FA. This paper analyses the reasons for that pessimism, and proposes future research.
The mechanism causing high-frequency combustion instability was presumed to be the progression of combustion at an off-design location; that is, in the vicinity of the faceplate. Experiments were carried out to prove this. In the experiments, water was used to simulate the propellant and the increase in pressure caused by off-design combustion was simulated by injecting nitrogen gas. Pressure increase and decay were measured to within approximately 1 ms, and the maximum local pressure was approximately twice the pressure prior to increasing the pressure. To estimate the pressure change in actual engines, a simplified simulation calculation model was constructed. After calibration using the experimental results, the time and amplitude of pressure change were on the same order of magnitude of those of the engines. The present study results show that the off-design combustion model can be a cause of the high-frequency combustion instability experienced when using liquid rocket engines.
The aerodynamic characteristics of a wing in a propeller slipstream were investigated at a low Reynolds number. The effects of propeller position and rotation direction on the wing were clarified by aerodynamic measurements and particle image velocimetry. The propeller positions were the center and tip of the wing model, whereas the rotation directions were clockwise and counterclockwise. The center propeller configuration with a clockwise rotation showed a constant pitching moment and increased the lift-to-drag ratio. This was caused by the high-speed propeller slipstream (i.e. 12 and 10 m/s on the upwash and downwash sides, respectively) and the wingtip vortex effect on the slipstream separation. The separation point at an angle-of-attack of 18° was delayed from x/c ≈ 0.1 to 0.3 by the wingtip vortex. Hence, the following two factors must be considered to enhance the aerodynamic characteristics of a Mars airplane: (i) the ratio of the area of the upwash and downwash sides of a wing in a propeller slipstream, and (ii) the effect of the wingtip vortex on the propeller slipstream.
This study proposes an uncertainty estimation method for the nodal displacement of a truss structure. The truss structure has been used for space structures that require high accuracy, so it is necessary to consider the effects of minute physical uncertainties of the structures. This study considers two types of physical uncertainties: member length uncertainty and position uncertainty due to backlash at the connecting nodes. A structure model is created using a space deployable structure as an example of a truss structure that requires high shape accuracy. Shape accuracy is evaluated using the distortion angle, which is defined as the error with respect to the ideal truss deployment direction. In the first part of the study, the distortion angle due to member length uncertainty is estimated by analyzing the equilibrium of the truss structure model with an uncertain member length. The confidence interval of the distortion angle is then clarified to be linearly related to the magnitude of the uncertainty by applying Monte Carlo simulation, where the uncertainty follows an independent normal distribution. An efficient estimation method for the distortion angle is then established followed by the theoretical derivation of the probabilistic distribution of the distortion angle as a Rayleigh distribution. In the second part of the study, the effect of backlash uncertainty at the connecting nodes is investigated. The backlash is modeled using a virtual cable element having a natural length equal to the backlash size. Finally, the allowable uncertainty range to satisfy a required accuracy is estimated by analyzing the proposed distortion angle while considering both types of uncertainty.
The shaftless ducted rotor (SDR) is a new type of ducted rotor system built using a ducted-rotor-motor integration design. Based on unstructured sliding grid technology, the effects of four key parameters, such as rotor disk height, number of blades, the center hole radius and the blade root mounting angle, on the aerodynamic characteristics of the SDR were investigated. The calculation results show that the thrust performance gradually improves as the rotational speed increases. It is different from traditional ducted rotors where the rotor is installed near the lip of the duct to obtain the optimal aerodynamic performance, the rotor of the SDR is installed away from the duct entrance to obtain the best aerodynamic performance. Additionally, the number of blades is increased to improve peak thrust and ducted thrust performance; however, this results in decreasing the power load. The central hole radius of the SDR should not be too small, as this can result in an excessive blocking effect when the central airflow passes through the paddle. Finally, when the rotational speed is constant, increasing the blade root mounting angle results in a large increase in thrust, but this decreases the power load as well.
Efficient operation is important to make full use of the capabilities of China’s space station. Determining the stochastic impacts of emergencies on the operational scenario of the space station is critical for successful implementation. However, few studies have assessed the uncertainties in the operational processes of the space station. To fill this gap, discrete event simulation (DES) is used to develop an evaluation method for the contingent operational plan of a space station. First, DES is used to develop a model framework of the space station operations, and the launch delay of cargo vehicles is introduced into the integrated simulation procedure. Second, the precision of the results and the computational efficiency are improved using the variance reduction technique. The corresponding effect on the number of simulation trials is confirmed using four constraints and three measurable metrics. Finally, the proposed method is applied to a two-year space station operational plan. The results show that maintaining a short interval between the launch date of the cargo vehicles and the start of the launch windows can decrease prolonged duration after a launch delay. A statistical analysis can be used to determine a safe interval between the dates of the events and vehicle launch.