An aerodynamic analysis of a ducted fan for vertical take-off and landing unmanned aerial vehicles was performed using computational simulation. A commercial computational fluid dynamics tool was used to solve the Reynolds-averaged Navier-Stokes equations. The calculated results were validated in the subsonic wind tunnel at Hanyang University. The objective of this study is to investigate the aerodynamic performance of a ducted fan in crosswinds. The thrust, normal and side forces of the ducted fan are affected by the velocity magnitude and in-flow angle of the crosswind. The pitching moment of the ducted fan is significantly influenced by the crosswind due to asymmetric lift force created by the difference in suction velocity magnitude on the duct lip. Flow separation at the duct lip occurs under hovering and crosswind conditions. The inlet flow fields of the ducted fan are distorted by the duct lip separation. In conclusion, to improve the stability of the ducted fan, the pitching moment must be reduced.
The effect of the aspect ratio on the stall characteristics of low-aspect-ratio wings is experimentally investigated at a Reynolds number of 5.2×104. The aspect ratio ranges from 0.5 to 1.5 at intervals of 0.1. The aerodynamic coefficients of thin rectangular wings are measured in a low-speed wind tunnel. It is found that the aerodynamic characteristics of low-aspect-ratio wings are significantly sensitive to the aspect ratio. The maximum lift coefficient increases as the aspect ratio decreases toward unity. On the other hand, the difference in lift-to-drag ratio is negligible at large angles. Hysteresis during a stall occurs when the aspect ratio is within a narrow range. The span of the hysteresis loop has its peak at an aspect ratio of 1.0. The discussion also includes the analysis of lift components and flow visualization.
Experiments and numerical simulations were conducted to explore the interaction between an incident shockwave and a flame-holding region downstream of a ramp injector. It was found that when an incident shockwave was introduced downstream of the ramp injector, a flame-holding region with red chemiluminescence accompanying high-temperature water vapor formed, while no apparent flame-holding region was observed without it. OH-PLIF measurements were also performed. The concentration of OH radical increased downstream of the incident shockwave, indicating that combustion was enhanced in the downstream region. Results of wall pressure measurements and numerical simulations indicate that the pressure increase and enhanced mixing contribute to flame-holding downstream of the incident shockwave. The mainstream was compressed downstream of the shockwave, resulting in an increased reaction rate between the hydrogen and mainstream air, as well as enhanced mixing due to baroclinic torque generated by the pressure gradient.
Based on the space tether, the concept of a motorized momentum exchange tether (MMET) using the momentum exchange principle has been proposed for many years. In this paper, a new symmetric configuration of MMET, referred to as HEX-MMET, is proposed. HEX-MMET has some unique advantages, such as higher payload transfer efficiency and shorter orbit reboost period compared to the MMET. In order to study the dynamic characteristic of HEX-MMET, using traditional Lagrange methods, a two-dimensional (2D) dynamic model of HEX-MMET is presented. Then, the numerical solutions based on MATLAB were implemented. The simulation results show that external torque has little evident influence on the orbital parameters, such as the radius of HEX-MMET's center-of-mass (COM) and the true anomaly; however, there is an apparent effect on the attitude parameters, such as the angle between the line of two payloads and the line of local gravity gradient. Furthermore, the gravity gradient torque also effects the attitude of HEX-MMET periodically.
On the basis of previous analysis and control for two- to four-craft static Coulomb formations, this paper continues to further study the analytic solutions for an equal mass five-craft Coulomb formation under the Hill-frame formation. The paper analyzes the co-linear formation of one-dimension, planar formation of two-dimension and hexahedral formation of three-dimension for equal mass five-craft Coulomb formation flying. The authors discuss the conditions of an analytical solution to determine the net charges of craft in each case of static formation. Due to perturbations and disturbances in the GEO altitude, the static configuration can merely maintain its transient state. For the purpose of keeping it stable over the longtime, the Coulomb formation must be subjected to external control. This paper uses LQR control as an example. Finally, through the control applied, craft can stably maintain a static configuration around the static equilibrium point. Moreover, numerical simulation results are illustrated, and the paper discusses situations of Coulomb static configurations when the number of satellites is more than five.
A new homogenous observer-based finite-time convergent guidance law is proposed to intercept maneuvering targets without line-of-sight (LOS) angular rate information. The presented formulation is obtained via a combination of homogenous theory and second-order sliding mode (SOSM) method. Different from some existing observers, the proposed algorithm can estimate the lumped uncertainty and the LOS angular rate in an integrated manner. By virtue of the principle of SOSM, the undesired chattering is mitigated significantly without any sacrifice in performance. Detailed stability shows that the LOS angular rate can be stabilized in a small region around zero in finite time. Numerical simulations with some comparisons are carried out to demonstrate the superiority of the proposed method.
A one hundred hour test of a 100 mA class microwave discharge neutralizer was performed to investigate the erosion rate and deposition in the neutralizer. The mass loss of each component (antenna, discharge chamber wall, front yoke, back yoke, insulator and orifice) was measured in diode mode (contact voltage of 50 V) at a constant incident microwave power of 8 W and xenon mass flow rate of 49 µg/s. The measured erosion rates of the discharge chamber and the antenna were 180 µg/hr and 10 µg/hr, respectively. The element concentration of the deposited material on the back yoke and insulator were measured; the contamination on the insulator was found to contain 34% copper from the wall and 6% molybdenum from the antenna.