Volume 29, Issue 2 -

An experimental investigation was carried out to study the effect of tab geometry on the flow characteristics of a jet issuing from a 2:1 rectangular nozzle using two-component hotwire anemometry. A pair of tabs of trapezoidal and square configurations (2% total blockage area) is placed on the minor-axis side of the rectangular nozzle and tested for two tab inclination angles of 135 ° and 45 ° , respectively. Tests were conducted for a nominal jet exit velocity of 20 ms -1 corresponding to a Reynolds number based on nozzle equivalent diameter of 5.02×10 4 . Relative to the plain jet, the potential-core length of the jet with trapezoidal tabs shows a reduction of 62% while the jet with square tabs shows a reduction of 75%, respectively. This is also accompanied by a significant upstream shift in peak centerline turbulence intensity (u'/U e ) . The tab inclination of 135 ° results in further enhanced jet characteristics in each case. The presence of each tab configuration is observed to inhibit the jet growth along the minor-axis plane thereby introducing large distortion in the jet cross-sectional development that ultimately leads to jet-core bifurcation along its major-axis. However, it is observed that while the jet with trapezoidal tabs switches its axis at some downstream location, the jet with square tabs does not switch axis at all. This indicates a strong influence of tab geometry on the overall jet flow development.

Servo-actuation system is one of the key executing subsystems of the flight control system of an aircraft. With the development of fly-by-wire control systems, redundant servo-actuation systems have been extensively applied. A servo-actuation system has a long life and high reliability, which results in the lack of failure information. In the mean time, available data is insufficient and imprecise during its design stage. In this paper, evidential networks (EN) are used to handle imprecise probabilities. The formulae of marginal belief mass for series system, parallel system, series-parallel system and parallel-series system are represented respectively. The basic reliability model and the mission reliability model of a three-redundancy servo-actuation system in an aircraft flight control system are analyzed using EN approach, respectively. The EN manage and quantify the imprecision of the servo-actuation effectively, and propagate imprecision from the root nodes to the top nodes representing the system reliability.

Compressor map interpolation is usually performed through the introduction of auxiliary coordinates ( β ). In this paper, a new analytical bivariate β function definition to be used in compressor map interpolation is studied. The function has user-defined parameters that must be adjusted to properly fit to a single map. The analytical nature of β allows for rapid calculations of the interpolation error estimation, which can be used as a quantitative measure of interpolation accuracy and also as a valid tool to compare traditional β function interpolation with new approaches (artificial neural networks, genetic algorithms, etc.). The quality of the method is analyzed by comparing the error output to the one of a well-known state-of-the-art methodology. This comparison is carried out for two different types of compressor and, in both cases, the error output using the method presented in this paper is found to be consistently lower. Moreover, an optimization routine able to locally minimize the interpolation error by shape variation of the β function is implemented. Further optimization introducing other important criteria is discussed.

Based on the derivation and calculation of the Corten-Dolan exponent d , a practical method of determining its value is proposed. This exponent depends not only upon the materials, but also upon the load spectrums. Therefore its value is obtained by a function which decreases with increasing stress amplitude. This exponent was investigated through analysis of fatigue damage evolution to determine its parameters. The proposed method has been effectively proved by experimental data from literature. Utilization of the modified Corten-Dolan's model significantly improves its life prediction capability when compared to the conventional model where that exponent was assumed to be constant.

Plumes from hydrocarbon-fueled rockets usually contain some amount of soot. In spite of the small amount, such soot particles can play a critical role in the characteristics of the infrared radiation emission since soot radiates a continuous, near-blackbody spectrum. The contribution of the soot to the plume radiation depends on the amount of soot, the physical properties of the particles, their concentration, and their temperature distribution in the flow field. The trajectories of solid particles and their temperatures can differ from those of the gas due to the particle mechanical and thermal inertia. CFD FLUENT code for solving two-phase Navier-Stokes equations coupled with chemical reactions and soot particle combustion was applied for exhaust plume simulations. Exhaust plumes with soot mass loading of 2% were simulated for three altitudes of 2 km, 8 km and 16 km. Radial distributions of the cloud particle density were obtained for different distances downstream the exhaust nozzle. As a result of the particle deceleration at the boundary layer inside the nozzle the particle concentration increased at the plume periphery. The particle temperature was higher than the gaseous temperature of the plume. The temperature difference between the soot particle and gas along corresponding trajectories was about 5-10%. The infrared radiation from the plumes with carbon soot was calculated. Its intensity was found to be dependent on the particle distribution in the plume.

Fault tree analysis (FTA), as one of the powerful tools in reliability engineering, has been widely used to enhance system quality attributes. In most fault tree analyses, precise values are adopted to represent the probabilities of occurrence of those events. Due to the lack of sufficient data or imprecision of existing data at the early stage of product design, it is often difficult to accurately estimate the failure rates of individual events or the probabilities of occurrence of the events. Therefore, such imprecision and uncertainty need to be taken into account in reliability analysis. In this paper, the evidential networks (EN) are employed to quantify and propagate the aforementioned uncertainty and imprecision in fault tree analysis. The detailed conversion processes of some logic gates to EN are described in fault tree (FT). The figures of the logic gates and the converted equivalent EN, together with the associated truth tables and the conditional belief mass tables, are also presented in this work. The new epistemic importance is proposed to describe the effect of ignorance degree of event. The fault tree of an aircraft engine damaged by oil filter plugs is presented to demonstrate the proposed method.

An air-breathing valveless PDE model was designed and manufactured, which was made up of subsonic inlet, mixing chamber, ignition chamber, detonation chamber. The total pressure recovery coefficient, flux coefficient and intake resistance with six different induction systems were measured by a semi free subsonic flow field. The proof-of-principle experiments of PDE model with different induction systems were all successfully carried out, by using liquid gasoline-air mixture with low-energy system (total stored energy less than 50 mJ). The measured detonation wave pressure ratio was very close to that of C-J detonation. The air-breathing PDE model was easy to initiate and worked in good condition. The deflagration to detonation transition (DDT) and operation frequency effect on pressure traces were also investigated by experiments. The results indicated the oscillation of pressure peak at P6 enhanced with the operation frequency increased. DDT accomplished before P6 and the DDT distance was about 0.9 m (from the ignitor).